Follow redirect location as new referrers for nested module imports (#2031)

Fixes #1742
Fixes #2021

28 files changed, 813 insertions, 199 deletions

diff --git a/cli/compiler.rs b/cli/compiler.rs
index bd0a763c1..6816df2ee 100644
--- a/cli/compiler.rs
+++ b/cli/compiler.rs
@@ -104,6 +104,7 @@ impl WorkerBehavior for CompilerBehavior {
 #[derive(Debug, Clone)]
 pub struct ModuleMetaData {
   pub module_name: String,
+  pub module_redirect_source_name: Option<String>, // source of redirect
   pub filename: String,
   pub media_type: msg::MediaType,
   pub source_code: Vec<u8>,
@@ -250,6 +251,9 @@ pub fn compile_sync(
                   ) {
                     Ok(serde_json::Value::Object(map)) => ModuleMetaData {
                       module_name: module_meta_data_.module_name.clone(),
+                      module_redirect_source_name: module_meta_data_
+                        .module_redirect_source_name
+                        .clone(),
                       filename: module_meta_data_.filename.clone(),
                       media_type: module_meta_data_.media_type,
                       source_code: module_meta_data_.source_code.clone(),
@@ -342,6 +346,7 @@ mod tests {
 
       let mut out = ModuleMetaData {
         module_name: "xxx".to_owned(),
+        module_redirect_source_name: None,
         filename: "/tests/002_hello.ts".to_owned(),
         media_type: msg::MediaType::TypeScript,
         source_code: include_bytes!("../tests/002_hello.ts").to_vec(),
diff --git a/cli/deno_dir.rs b/cli/deno_dir.rs
index 703df43ad..1d69046d3 100644
--- a/cli/deno_dir.rs
+++ b/cli/deno_dir.rs
@@ -11,9 +11,11 @@ use crate::msg;
 use crate::tokio_util;
 use crate::version;
 use dirs;
-use futures::future::Either;
+use futures::future::{loop_fn, Either, Loop};
 use futures::Future;
+use http;
 use ring;
+use serde_json;
 use std;
 use std::fmt::Write;
 use std::fs;
@@ -34,6 +36,7 @@ fn extmap(ext: &str) -> msg::MediaType {
   }
 }
 
+#[derive(Clone)]
 pub struct DenoDir {
   // Example: /Users/rld/.deno/
   pub root: PathBuf,
@@ -154,66 +157,70 @@ impl DenoDir {
     let gen = self.gen.clone();
 
     Either::B(
-      get_source_code_async(module_name.as_str(), filename.as_str(), use_cache)
-        .then(move |result| {
-          let mut out = match result {
-            Ok(out) => out,
-            Err(err) => {
-              if err.kind() == ErrorKind::NotFound {
-                // For NotFound, change the message to something better.
-                return Err(errors::new(
-                  ErrorKind::NotFound,
-                  format!(
-                    "Cannot resolve module \"{}\" from \"{}\"",
-                    specifier, referrer
-                  ),
-                ));
-              } else {
-                return Err(err);
-              }
+      get_source_code_async(
+        self,
+        module_name.as_str(),
+        filename.as_str(),
+        use_cache,
+      ).then(move |result| {
+        let mut out = match result {
+          Ok(out) => out,
+          Err(err) => {
+            if err.kind() == ErrorKind::NotFound {
+              // For NotFound, change the message to something better.
+              return Err(errors::new(
+                ErrorKind::NotFound,
+                format!(
+                  "Cannot resolve module \"{}\" from \"{}\"",
+                  specifier, referrer
+                ),
+              ));
+            } else {
+              return Err(err);
             }
-          };
-
-          if out.source_code.starts_with(b"#!") {
-            out.source_code = filter_shebang(out.source_code);
           }
+        };
 
-          // If TypeScript we have to also load corresponding compile js and
-          // source maps (called output_code and output_source_map)
-          if out.media_type != msg::MediaType::TypeScript || !use_cache {
-            return Ok(out);
-          }
+        if out.source_code.starts_with(b"#!") {
+          out.source_code = filter_shebang(out.source_code);
+        }
 
-          let cache_key =
-            source_code_hash(&out.filename, &out.source_code, version::DENO);
-          let (output_code_filename, output_source_map_filename) = (
-            gen.join(cache_key.to_string() + ".js"),
-            gen.join(cache_key.to_string() + ".js.map"),
-          );
-
-          let result =
-            load_cache2(&output_code_filename, &output_source_map_filename);
-          match result {
-            Err(err) => {
-              if err.kind() == std::io::ErrorKind::NotFound {
-                // If there's no compiled JS or source map, that's ok, just
-                // return what we have.
-                Ok(out)
-              } else {
-                Err(err.into())
-              }
-            }
-            Ok((output_code, source_map)) => {
-              out.maybe_output_code = Some(output_code);
-              out.maybe_source_map = Some(source_map);
-              out.maybe_output_code_filename =
-                Some(output_code_filename.to_str().unwrap().to_string());
-              out.maybe_source_map_filename =
-                Some(output_source_map_filename.to_str().unwrap().to_string());
+        // If TypeScript we have to also load corresponding compile js and
+        // source maps (called output_code and output_source_map)
+        if out.media_type != msg::MediaType::TypeScript || !use_cache {
+          return Ok(out);
+        }
+
+        let cache_key =
+          source_code_hash(&out.filename, &out.source_code, version::DENO);
+        let (output_code_filename, output_source_map_filename) = (
+          gen.join(cache_key.to_string() + ".js"),
+          gen.join(cache_key.to_string() + ".js.map"),
+        );
+
+        let result =
+          load_cache2(&output_code_filename, &output_source_map_filename);
+        match result {
+          Err(err) => {
+            if err.kind() == std::io::ErrorKind::NotFound {
+              // If there's no compiled JS or source map, that's ok, just
+              // return what we have.
               Ok(out)
+            } else {
+              Err(err.into())
             }
           }
-        }),
+          Ok((output_code, source_map)) => {
+            out.maybe_output_code = Some(output_code);
+            out.maybe_source_map = Some(source_map);
+            out.maybe_output_code_filename =
+              Some(output_code_filename.to_str().unwrap().to_string());
+            out.maybe_source_map_filename =
+              Some(output_source_map_filename.to_str().unwrap().to_string());
+            Ok(out)
+          }
+        }
+      }),
     )
   }
 
@@ -318,7 +325,7 @@ impl DenoDir {
           get_cache_filename(self.deps_http.as_path(), &j).as_ref(),
         )
       }
-      // TODO(kevinkassimo): change this to support other protocols than http
+      // TODO(kevinkassimo): change this to support other protocols than http.
       _ => unimplemented!(),
     }
 
@@ -353,6 +360,7 @@ impl SourceMapGetter for DenoDir {
 /// download will be written to "filename". This happens no matter the value of
 /// use_cache.
 fn get_source_code_async(
+  deno_dir: &DenoDir,
   module_name: &str,
   filename: &str,
   use_cache: bool,
@@ -361,15 +369,15 @@ fn get_source_code_async(
   let module_name = module_name.to_string();
   let is_module_remote = is_remote(&module_name);
   // We try fetch local. Two cases:
-  // 1. This is a remote module and we're allowed to use cached downloads
-  // 2. This is a local module
+  // 1. This is a remote module and we're allowed to use cached downloads.
+  // 2. This is a local module.
   if !is_module_remote || use_cache {
     debug!(
       "fetch local or reload {} is_module_remote {}",
       module_name, is_module_remote
     );
     // Note that local fetch is done synchronously.
-    match fetch_local_source(&module_name, &filename) {
+    match fetch_local_source(deno_dir, &module_name, &filename, None) {
       Ok(Some(output)) => {
         debug!("found local source ");
         return Either::A(futures::future::ok(output));
@@ -396,27 +404,35 @@ fn get_source_code_async(
 
   debug!("is remote but didn't find module");
 
-  // not cached/local, try remote
-  Either::B(fetch_remote_source_async(&module_name, &filename).and_then(
-    move |maybe_remote_source| match maybe_remote_source {
-      Some(output) => Ok(output),
-      None => Err(DenoError::from(std::io::Error::new(
-        std::io::ErrorKind::NotFound,
-        format!("cannot find remote file '{}'", &filename),
-      ))),
-    },
-  ))
+  // not cached/local, try remote.
+  Either::B(
+    fetch_remote_source_async(deno_dir, &module_name, &filename).and_then(
+      move |maybe_remote_source| match maybe_remote_source {
+        Some(output) => Ok(output),
+        None => Err(DenoError::from(std::io::Error::new(
+          std::io::ErrorKind::NotFound,
+          format!("cannot find remote file '{}'", &filename),
+        ))),
+      },
+    ),
+  )
 }
 
 #[cfg(test)]
 /// Synchronous version of get_source_code_async
 /// This function is deprecated.
 fn get_source_code(
+  deno_dir: &DenoDir,
   module_name: &str,
   filename: &str,
   use_cache: bool,
 ) -> DenoResult<ModuleMetaData> {
-  tokio_util::block_on(get_source_code_async(module_name, filename, use_cache))
+  tokio_util::block_on(get_source_code_async(
+    deno_dir,
+    module_name,
+    filename,
+    use_cache,
+  ))
 }
 
 fn get_cache_filename(basedir: &Path, url: &Url) -> PathBuf {
@@ -537,71 +553,176 @@ fn filter_shebang(bytes: Vec<u8>) -> Vec<u8> {
 /// Asynchronously fetch remote source file specified by the URL `module_name`
 /// and write it to disk at `filename`.
 fn fetch_remote_source_async(
+  deno_dir: &DenoDir,
   module_name: &str,
   filename: &str,
 ) -> impl Future<Item = Option<ModuleMetaData>, Error = DenoError> {
-  let filename = filename.to_string();
-  let module_name = module_name.to_string();
-  let p = PathBuf::from(filename.clone());
-  // We write a special ".mime" file into the `.deno/deps` directory along side the
-  // cached file, containing just the media type.
-  let media_type_filename = [&filename, ".mime"].concat();
-  let mt = PathBuf::from(&media_type_filename);
-  eprintln!("Downloading {}", &module_name);
-  http_util::fetch_string(&module_name).and_then(
-    move |(source, content_type)| {
-      match p.parent() {
-        Some(ref parent) => fs::create_dir_all(parent),
-        None => Ok(()),
-      }?;
-      deno_fs::write_file(&p, &source, 0o666)?;
-      // Remove possibly existing stale .mime file
-      // may not exist. DON'T unwrap
-      let _ = std::fs::remove_file(&media_type_filename);
-      // Create .mime file only when content type different from extension
-      let resolved_content_type = map_content_type(&p, Some(&content_type));
-      let ext = p
-        .extension()
-        .map(|x| x.to_str().unwrap_or(""))
-        .unwrap_or("");
-      let media_type = extmap(&ext);
-      if media_type == msg::MediaType::Unknown
-        || media_type != resolved_content_type
-      {
-        deno_fs::write_file(&mt, content_type.as_bytes(), 0o666)?
-      }
-      Ok(Some(ModuleMetaData {
-        module_name: module_name.to_string(),
-        filename: filename.to_string(),
-        media_type: map_content_type(&p, Some(&content_type)),
-        source_code: source.as_bytes().to_owned(),
-        maybe_output_code_filename: None,
-        maybe_output_code: None,
-        maybe_source_map_filename: None,
-        maybe_source_map: None,
-      }))
+  use crate::http_util::FetchOnceResult;
+  {
+    eprintln!("Downloading {}", module_name);
+  }
+
+  let filename = filename.to_owned();
+  let module_name = module_name.to_owned();
+
+  // We write a special ".headers.json" file into the `.deno/deps` directory along side the
+  // cached file, containing just the media type and possible redirect target (both are http headers).
+  // If redirect target is present, the file itself if not cached.
+  // In future resolutions, we would instead follow this redirect target ("redirect_to").
+  loop_fn(
+    (
+      deno_dir.clone(),
+      None,
+      None,
+      module_name.clone(),
+      filename.clone(),
+    ),
+    |(
+      dir,
+      maybe_initial_module_name,
+      maybe_initial_filename,
+      module_name,
+      filename,
+    )| {
+      let url = module_name.parse::<http::uri::Uri>().unwrap();
+      // Single pass fetch, either yields code or yields redirect.
+      http_util::fetch_string_once(url).and_then(move |fetch_once_result| {
+        match fetch_once_result {
+          FetchOnceResult::Redirect(url) => {
+            // If redirects, update module_name and filename for next looped call.
+            let resolve_result = dir
+              .resolve_module(&(url.to_string()), ".")
+              .map_err(DenoError::from);
+            match resolve_result {
+              Ok((new_module_name, new_filename)) => {
+                let mut maybe_initial_module_name = maybe_initial_module_name;
+                let mut maybe_initial_filename = maybe_initial_filename;
+                if maybe_initial_module_name.is_none() {
+                  maybe_initial_module_name = Some(module_name.clone());
+                  maybe_initial_filename = Some(filename.clone());
+                }
+                // Not yet completed. Follow the redirect and loop.
+                Ok(Loop::Continue((
+                  dir,
+                  maybe_initial_module_name,
+                  maybe_initial_filename,
+                  new_module_name,
+                  new_filename,
+                )))
+              }
+              Err(e) => Err(e),
+            }
+          }
+          FetchOnceResult::Code(source, maybe_content_type) => {
+            // We land on the code.
+            let p = PathBuf::from(filename.clone());
+            match p.parent() {
+              Some(ref parent) => fs::create_dir_all(parent),
+              None => Ok(()),
+            }?;
+            // Write file and create .headers.json for the file.
+            deno_fs::write_file(&p, &source, 0o666)?;
+            {
+              save_source_code_headers(&filename, maybe_content_type.clone(), None);
+            }
+            // Check if this file is downloaded due to some old redirect request.
+            if maybe_initial_filename.is_some() {
+              // If yes, record down the headers for redirect.
+              // Also create its containing folder.
+              let pp = PathBuf::from(filename.clone());
+              match pp.parent() {
+                Some(ref parent) => fs::create_dir_all(parent),
+                None => Ok(()),
+              }?;
+              {
+                save_source_code_headers(
+                  &maybe_initial_filename.clone().unwrap(),
+                  maybe_content_type.clone(),
+                  Some(module_name.clone()),
+                );
+              }
+            }
+            Ok(Loop::Break(Some(ModuleMetaData {
+              module_name: module_name.to_string(),
+              module_redirect_source_name: maybe_initial_module_name,
+              filename: filename.to_string(),
+              media_type: map_content_type(
+                &p,
+                maybe_content_type.as_ref().map(|s| s.as_str()),
+              ),
+              source_code: source.as_bytes().to_owned(),
+              maybe_output_code_filename: None,
+              maybe_output_code: None,
+              maybe_source_map_filename: None,
+              maybe_source_map: None,
+            })))
+          }
+        }
+      })
     },
   )
 }
 
-// Prototype https://github.com/denoland/deno/blob/golang/deno_dir.go#L37-L73
 /// Fetch remote source code.
 #[cfg(test)]
 fn fetch_remote_source(
+  deno_dir: &DenoDir,
   module_name: &str,
   filename: &str,
 ) -> DenoResult<Option<ModuleMetaData>> {
-  tokio_util::block_on(fetch_remote_source_async(module_name, filename))
+  tokio_util::block_on(fetch_remote_source_async(
+    deno_dir,
+    module_name,
+    filename,
+  ))
 }
 
 /// Fetch local or cached source code.
+/// This is a recursive operation if source file has redirection.
+/// It will keep reading filename.headers.json for information about redirection.
+/// module_initial_source_name would be None on first call,
+/// and becomes the name of the very first module that initiates the call
+/// in subsequent recursions.
+/// AKA if redirection occurs, module_initial_source_name is the source path
+/// that user provides, and the final module_name is the resolved path
+/// after following all redirections.
 fn fetch_local_source(
+  deno_dir: &DenoDir,
   module_name: &str,
   filename: &str,
+  module_initial_source_name: Option<String>,
 ) -> DenoResult<Option<ModuleMetaData>> {
   let p = Path::new(&filename);
-  let media_type_filename = [&filename, ".mime"].concat();
-  let mt = Path::new(&media_type_filename);
+  let source_code_headers = get_source_code_headers(&filename);
+  // If source code headers says that it would redirect elsewhere,
+  // (meaning that the source file might not exist; only .headers.json is present)
+  // Abort reading attempts to the cached source file and and follow the redirect.
+  if let Some(redirect_to) = source_code_headers.redirect_to {
+    // E.g.
+    // module_name https://import-meta.now.sh/redirect.js
+    // filename /Users/kun/Library/Caches/deno/deps/https/import-meta.now.sh/redirect.js
+    // redirect_to https://import-meta.now.sh/sub/final1.js
+    // real_filename /Users/kun/Library/Caches/deno/deps/https/import-meta.now.sh/sub/final1.js
+    // real_module_name = https://import-meta.now.sh/sub/final1.js
+    let (real_module_name, real_filename) =
+      deno_dir.resolve_module(&redirect_to, ".")?;
+    let mut module_initial_source_name = module_initial_source_name;
+    // If this is the first redirect attempt,
+    // then module_initial_source_name should be None.
+    // In that case, use current module name as module_initial_source_name.
+    if module_initial_source_name.is_none() {
+      module_initial_source_name = Some(module_name.to_owned());
+    }
+    // Recurse.
+    return fetch_local_source(
+      deno_dir,
+      &real_module_name,
+      &real_filename,
+      module_initial_source_name,
+    );
+  }
+  // No redirect needed or end of redirects.
+  // We can try read the file
   let source_code = match fs::read(p) {
     Err(e) => {
       if e.kind() == std::io::ErrorKind::NotFound {
@@ -612,16 +733,14 @@ fn fetch_local_source(
     }
     Ok(c) => c,
   };
-  // .mime file might not exists
-  // this is okay for local source: maybe_content_type_str will be None
-  let maybe_content_type_string = fs::read_to_string(&mt).ok();
-  // Option<String> -> Option<&str>
-  let maybe_content_type_str =
-    maybe_content_type_string.as_ref().map(String::as_str);
   Ok(Some(ModuleMetaData {
     module_name: module_name.to_string(),
+    module_redirect_source_name: module_initial_source_name,
     filename: filename.to_string(),
-    media_type: map_content_type(&p, maybe_content_type_str),
+    media_type: map_content_type(
+      &p,
+      source_code_headers.mime_type.as_ref().map(String::as_str),
+    ),
     source_code,
     maybe_output_code_filename: None,
     maybe_output_code: None,
@@ -630,6 +749,106 @@ fn fetch_local_source(
   }))
 }
 
+#[derive(Debug)]
+/// Header metadata associated with a particular "symbolic" source code file.
+/// (the associated source code file might not be cached, while remaining
+/// a user accessible entity through imports (due to redirects)).
+pub struct SourceCodeHeaders {
+  /// MIME type of the source code.
+  pub mime_type: Option<String>,
+  /// Where should we actually look for source code.
+  /// This should be an absolute path!
+  pub redirect_to: Option<String>,
+}
+
+static MIME_TYPE: &'static str = "mime_type";
+static REDIRECT_TO: &'static str = "redirect_to";
+
+fn source_code_headers_filename(filename: &str) -> String {
+  [&filename, ".headers.json"].concat()
+}
+
+/// Get header metadata associated with a single source code file.
+/// NOTICE: chances are that the source code itself is not downloaded due to redirects.
+/// In this case, the headers file provides info about where we should go and get
+/// the source code that redirect eventually points to (which should be cached).
+fn get_source_code_headers(filename: &str) -> SourceCodeHeaders {
+  let headers_filename = source_code_headers_filename(filename);
+  let hd = Path::new(&headers_filename);
+  // .headers.json file might not exists.
+  // This is okay for local source.
+  let maybe_headers_string = fs::read_to_string(&hd).ok();
+  if let Some(headers_string) = maybe_headers_string {
+    // TODO(kevinkassimo): consider introduce serde::Deserialize to make things simpler.
+    let maybe_headers: serde_json::Result<serde_json::Value> =
+      serde_json::from_str(&headers_string);
+    if let Ok(headers) = maybe_headers {
+      return SourceCodeHeaders {
+        mime_type: headers[MIME_TYPE].as_str().map(String::from),
+        redirect_to: headers[REDIRECT_TO].as_str().map(String::from),
+      };
+    }
+  }
+  SourceCodeHeaders {
+    mime_type: None,
+    redirect_to: None,
+  }
+}
+
+/// Save headers related to source filename to {filename}.headers.json file,
+/// only when there is actually something necessary to save.
+/// For example, if the extension ".js" already mean JS file and we have
+/// content type of "text/javascript", then we would not save the mime type.
+/// If nothing needs to be saved, the headers file is not created.
+fn save_source_code_headers(
+  filename: &str,
+  mime_type: Option<String>,
+  redirect_to: Option<String>,
+) {
+  let headers_filename = source_code_headers_filename(filename);
+  // Remove possibly existing stale .headers.json file.
+  // May not exist. DON'T unwrap.
+  let _ = std::fs::remove_file(&headers_filename);
+  let p = PathBuf::from(filename);
+  // TODO(kevinkassimo): consider introduce serde::Deserialize to make things simpler.
+  // This is super ugly at this moment...
+  // Had trouble to make serde_derive work: I'm unable to build proc-macro2.
+  let mut value_map = serde_json::map::Map::new();
+  if mime_type.is_some() {
+    let mime_type_string = mime_type.clone().unwrap();
+    let resolved_mime_type =
+      { map_content_type(Path::new(""), Some(mime_type_string.as_str())) };
+    let ext = p
+      .extension()
+      .map(|x| x.to_str().unwrap_or(""))
+      .unwrap_or("");
+    let ext_based_mime_type = extmap(&ext);
+    // Add mime to headers only when content type is different from extension.
+    if ext_based_mime_type == msg::MediaType::Unknown
+      || resolved_mime_type != ext_based_mime_type
+    {
+      value_map.insert(MIME_TYPE.to_string(), json!(mime_type_string));
+    }
+  }
+  if redirect_to.is_some() {
+    value_map.insert(REDIRECT_TO.to_string(), json!(redirect_to.unwrap()));
+  }
+  // Only save to file when there is actually data.
+  if value_map.len() > 0 {
+    let _ = serde_json::to_string(&value_map).map(|s| {
+      // It is possible that we need to create file
+      // with parent folders not yet created.
+      // (Due to .headers.json feature for redirection)
+      let hd = PathBuf::from(&headers_filename);
+      let _ = match hd.parent() {
+        Some(ref parent) => fs::create_dir_all(parent),
+        None => Ok(()),
+      };
+      let _ = deno_fs::write_file(&(hd.as_path()), s, 0o666);
+    });
+  }
+}
+
 #[cfg(test)]
 mod tests {
   use super::*;
@@ -709,6 +928,7 @@ mod tests {
       filename: filename.to_owned(),
       source_code: source_code[..].to_owned(),
       module_name: "hello.js".to_owned(),
+      module_redirect_source_name: None,
       media_type: msg::MediaType::TypeScript,
       maybe_output_code: Some(output_code[..].to_owned()),
       maybe_output_code_filename: None,
@@ -746,6 +966,35 @@ mod tests {
   }
 
   #[test]
+  fn test_source_code_headers_get_and_save() {
+    let (temp_dir, _deno_dir) = test_setup();
+    let filename =
+      deno_fs::normalize_path(temp_dir.into_path().join("f.js").as_ref());
+    let headers_file_name = source_code_headers_filename(&filename);
+    assert_eq!(headers_file_name, [&filename, ".headers.json"].concat());
+    let _ = deno_fs::write_file(&PathBuf::from(&headers_file_name),
+      "{\"mime_type\":\"text/javascript\",\"redirect_to\":\"http://example.com/a.js\"}", 0o666);
+    let headers = get_source_code_headers(&filename);
+    assert_eq!(headers.mime_type.clone().unwrap(), "text/javascript");
+    assert_eq!(
+      headers.redirect_to.clone().unwrap(),
+      "http://example.com/a.js"
+    );
+
+    save_source_code_headers(
+      &filename,
+      Some("text/typescript".to_owned()),
+      Some("http://deno.land/a.js".to_owned()),
+    );
+    let headers2 = get_source_code_headers(&filename);
+    assert_eq!(headers2.mime_type.clone().unwrap(), "text/typescript");
+    assert_eq!(
+      headers2.redirect_to.clone().unwrap(),
+      "http://deno.land/a.js"
+    );
+  }
+
+  #[test]
   fn test_get_source_code_1() {
     let (_temp_dir, deno_dir) = test_setup();
     // http_util::fetch_sync_string requires tokio
@@ -757,9 +1006,9 @@ mod tests {
           .join("localhost_PORT4545/tests/subdir/mod2.ts")
           .as_ref(),
       );
-      let mime_file_name = format!("{}.mime", &filename);
+      let headers_file_name = source_code_headers_filename(&filename);
 
-      let result = get_source_code(module_name, &filename, true);
+      let result = get_source_code(&deno_dir, module_name, &filename, true);
       assert!(result.is_ok());
       let r = result.unwrap();
       assert_eq!(
@@ -767,12 +1016,13 @@ mod tests {
         "export { printHello } from \"./print_hello.ts\";\n".as_bytes()
       );
       assert_eq!(&(r.media_type), &msg::MediaType::TypeScript);
-      // Should not create .mime file due to matching ext
-      assert!(fs::read_to_string(&mime_file_name).is_err());
+      // Should not create .headers.json file due to matching ext
+      assert!(fs::read_to_string(&headers_file_name).is_err());
 
-      // Modify .mime
-      let _ = fs::write(&mime_file_name, "text/javascript");
-      let result2 = get_source_code(module_name, &filename, true);
+      // Modify .headers.json, write using fs write and read using save_source_code_headers
+      let _ =
+        fs::write(&headers_file_name, "{ \"mime_type\": \"text/javascript\" }");
+      let result2 = get_source_code(&deno_dir, module_name, &filename, true);
       assert!(result2.is_ok());
       let r2 = result2.unwrap();
       assert_eq!(
@@ -780,23 +1030,45 @@ mod tests {
         "export { printHello } from \"./print_hello.ts\";\n".as_bytes()
       );
       // If get_source_code does not call remote, this should be JavaScript
-      // as we modified before! (we do not overwrite .mime due to no http fetch)
+      // as we modified before! (we do not overwrite .headers.json due to no http fetch)
       assert_eq!(&(r2.media_type), &msg::MediaType::JavaScript);
       assert_eq!(
-        fs::read_to_string(&mime_file_name).unwrap(),
+        get_source_code_headers(&filename).mime_type.unwrap(),
         "text/javascript"
       );
 
-      // Don't use_cache
-      let result3 = get_source_code(module_name, &filename, false);
+      // Modify .headers.json again, but the other way around
+      save_source_code_headers(
+        &filename,
+        Some("application/json".to_owned()),
+        None,
+      );
+      let result3 = get_source_code(&deno_dir, module_name, &filename, true);
       assert!(result3.is_ok());
       let r3 = result3.unwrap();
-      let expected3 =
+      assert_eq!(
+        r3.source_code,
+        "export { printHello } from \"./print_hello.ts\";\n".as_bytes()
+      );
+      // If get_source_code does not call remote, this should be JavaScript
+      // as we modified before! (we do not overwrite .headers.json due to no http fetch)
+      assert_eq!(&(r3.media_type), &msg::MediaType::Json);
+      assert!(
+        fs::read_to_string(&headers_file_name)
+          .unwrap()
+          .contains("application/json")
+      );
+
+      // Don't use_cache
+      let result4 = get_source_code(&deno_dir, module_name, &filename, false);
+      assert!(result4.is_ok());
+      let r4 = result4.unwrap();
+      let expected4 =
         "export { printHello } from \"./print_hello.ts\";\n".as_bytes();
-      assert_eq!(r3.source_code, expected3);
-      // Now the old .mime file should have gone! Resolved back to TypeScript
-      assert_eq!(&(r3.media_type), &msg::MediaType::TypeScript);
-      assert!(fs::read_to_string(&mime_file_name).is_err());
+      assert_eq!(r4.source_code, expected4);
+      // Now the old .headers.json file should have gone! Resolved back to TypeScript
+      assert_eq!(&(r4.media_type), &msg::MediaType::TypeScript);
+      assert!(fs::read_to_string(&headers_file_name).is_err());
     });
   }
 
@@ -812,52 +1084,174 @@ mod tests {
           .join("localhost_PORT4545/tests/subdir/mismatch_ext.ts")
           .as_ref(),
       );
-      let mime_file_name = format!("{}.mime", &filename);
+      let headers_file_name = source_code_headers_filename(&filename);
 
-      let result = get_source_code(module_name, &filename, true);
+      let result = get_source_code(&deno_dir, module_name, &filename, true);
       assert!(result.is_ok());
       let r = result.unwrap();
       let expected = "export const loaded = true;\n".as_bytes();
       assert_eq!(r.source_code, expected);
-      // Mismatch ext with content type, create .mime
+      // Mismatch ext with content type, create .headers.json
       assert_eq!(&(r.media_type), &msg::MediaType::JavaScript);
       assert_eq!(
-        fs::read_to_string(&mime_file_name).unwrap(),
+        get_source_code_headers(&filename).mime_type.unwrap(),
         "text/javascript"
       );
 
-      // Modify .mime
-      let _ = fs::write(&mime_file_name, "text/typescript");
-      let result2 = get_source_code(module_name, &filename, true);
+      // Modify .headers.json
+      save_source_code_headers(
+        &filename,
+        Some("text/typescript".to_owned()),
+        None,
+      );
+      let result2 = get_source_code(&deno_dir, module_name, &filename, true);
       assert!(result2.is_ok());
       let r2 = result2.unwrap();
       let expected2 = "export const loaded = true;\n".as_bytes();
       assert_eq!(r2.source_code, expected2);
       // If get_source_code does not call remote, this should be TypeScript
-      // as we modified before! (we do not overwrite .mime due to no http fetch)
+      // as we modified before! (we do not overwrite .headers.json due to no http fetch)
       assert_eq!(&(r2.media_type), &msg::MediaType::TypeScript);
-      assert_eq!(
-        fs::read_to_string(&mime_file_name).unwrap(),
-        "text/typescript"
-      );
+      assert!(fs::read_to_string(&headers_file_name).is_err());
 
       // Don't use_cache
-      let result3 = get_source_code(module_name, &filename, false);
+      let result3 = get_source_code(&deno_dir, module_name, &filename, false);
       assert!(result3.is_ok());
       let r3 = result3.unwrap();
       let expected3 = "export const loaded = true;\n".as_bytes();
       assert_eq!(r3.source_code, expected3);
-      // Now the old .mime file should be overwritten back to JavaScript!
+      // Now the old .headers.json file should be overwritten back to JavaScript!
       // (due to http fetch)
       assert_eq!(&(r3.media_type), &msg::MediaType::JavaScript);
       assert_eq!(
-        fs::read_to_string(&mime_file_name).unwrap(),
+        get_source_code_headers(&filename).mime_type.unwrap(),
         "text/javascript"
       );
     });
   }
 
   #[test]
+  fn test_get_source_code_3() {
+    let (_temp_dir, deno_dir) = test_setup();
+    // Test basic follow and headers recording
+    tokio_util::init(|| {
+      let redirect_module_name =
+        "http://localhost:4546/tests/subdir/redirects/redirect1.js";
+      let redirect_source_filename = deno_fs::normalize_path(
+        deno_dir
+          .deps_http
+          .join("localhost_PORT4546/tests/subdir/redirects/redirect1.js")
+          .as_ref(),
+      );
+      let target_module_name =
+        "http://localhost:4545/tests/subdir/redirects/redirect1.js";
+      let redirect_target_filename = deno_fs::normalize_path(
+        deno_dir
+          .deps_http
+          .join("localhost_PORT4545/tests/subdir/redirects/redirect1.js")
+          .as_ref(),
+      );
+      let mod_meta = get_source_code(
+        &deno_dir,
+        redirect_module_name,
+        &redirect_source_filename,
+        true,
+      ).unwrap();
+      // File that requires redirection is not downloaded.
+      assert!(fs::read_to_string(&redirect_source_filename).is_err());
+      // ... but its .headers.json is created.
+      let redirect_source_headers =
+        get_source_code_headers(&redirect_source_filename);
+      assert_eq!(
+        redirect_source_headers.redirect_to.unwrap(),
+        "http://localhost:4545/tests/subdir/redirects/redirect1.js"
+      );
+      // The target of redirection is downloaded instead.
+      assert_eq!(
+        fs::read_to_string(&redirect_target_filename).unwrap(),
+        "export const redirect = 1;\n"
+      );
+      let redirect_target_headers =
+        get_source_code_headers(&redirect_target_filename);
+      assert!(redirect_target_headers.redirect_to.is_none());
+
+      // Examine the meta result.
+      assert_eq!(&mod_meta.module_name, target_module_name);
+      assert_eq!(
+        &mod_meta.module_redirect_source_name.clone().unwrap(),
+        redirect_module_name
+      );
+    });
+  }
+
+  #[test]
+  fn test_get_source_code_4() {
+    let (_temp_dir, deno_dir) = test_setup();
+    // Test double redirects and headers recording
+    tokio_util::init(|| {
+      let redirect_module_name =
+        "http://localhost:4548/tests/subdir/redirects/redirect1.js";
+      let redirect_source_filename = deno_fs::normalize_path(
+        deno_dir
+          .deps_http
+          .join("localhost_PORT4548/tests/subdir/redirects/redirect1.js")
+          .as_ref(),
+      );
+      let redirect_source_filename_intermediate = deno_fs::normalize_path(
+        deno_dir
+          .deps_http
+          .join("localhost_PORT4546/tests/subdir/redirects/redirect1.js")
+          .as_ref(),
+      );
+      let target_module_name =
+        "http://localhost:4545/tests/subdir/redirects/redirect1.js";
+      let redirect_target_filename = deno_fs::normalize_path(
+        deno_dir
+          .deps_http
+          .join("localhost_PORT4545/tests/subdir/redirects/redirect1.js")
+          .as_ref(),
+      );
+      let mod_meta = get_source_code(
+        &deno_dir,
+        redirect_module_name,
+        &redirect_source_filename,
+        true,
+      ).unwrap();
+
+      // File that requires redirection is not downloaded.
+      assert!(fs::read_to_string(&redirect_source_filename).is_err());
+      // ... but its .headers.json is created.
+      let redirect_source_headers =
+        get_source_code_headers(&redirect_source_filename);
+      assert_eq!(
+        redirect_source_headers.redirect_to.unwrap(),
+        target_module_name
+      );
+
+      // In the intermediate redirection step, file is also not downloaded.
+      assert!(
+        fs::read_to_string(&redirect_source_filename_intermediate).is_err()
+      );
+
+      // The target of redirection is downloaded instead.
+      assert_eq!(
+        fs::read_to_string(&redirect_target_filename).unwrap(),
+        "export const redirect = 1;\n"
+      );
+      let redirect_target_headers =
+        get_source_code_headers(&redirect_target_filename);
+      assert!(redirect_target_headers.redirect_to.is_none());
+
+      // Examine the meta result.
+      assert_eq!(&mod_meta.module_name, target_module_name);
+      assert_eq!(
+        &mod_meta.module_redirect_source_name.clone().unwrap(),
+        redirect_module_name
+      );
+    });
+  }
+
+  #[test]
   fn test_fetch_source_async_1() {
     use crate::tokio_util;
     // http_util::fetch_sync_string requires tokio
@@ -871,9 +1265,10 @@ mod tests {
           .join("127.0.0.1_PORT4545/tests/subdir/mt_video_mp2t.t3.ts")
           .as_ref(),
       );
-      let mime_file_name = format!("{}.mime", &filename);
+      let headers_file_name = source_code_headers_filename(&filename);
 
       let result = tokio_util::block_on(fetch_remote_source_async(
+        &deno_dir,
         &module_name,
         &filename,
       ));
@@ -881,16 +1276,21 @@ mod tests {
       let r = result.unwrap().unwrap();
       assert_eq!(r.source_code, b"export const loaded = true;\n");
       assert_eq!(&(r.media_type), &msg::MediaType::TypeScript);
-      // matching ext, no .mime file created
-      assert!(fs::read_to_string(&mime_file_name).is_err());
+      // matching ext, no .headers.json file created
+      assert!(fs::read_to_string(&headers_file_name).is_err());
 
-      // Modify .mime, make sure read from local
-      let _ = fs::write(&mime_file_name, "text/javascript");
-      let result2 = fetch_local_source(&module_name, &filename);
+      // Modify .headers.json, make sure read from local
+      save_source_code_headers(
+        &filename,
+        Some("text/javascript".to_owned()),
+        None,
+      );
+      let result2 =
+        fetch_local_source(&deno_dir, &module_name, &filename, None);
       assert!(result2.is_ok());
       let r2 = result2.unwrap().unwrap();
       assert_eq!(r2.source_code, b"export const loaded = true;\n");
-      // Not MediaType::TypeScript due to .mime modification
+      // Not MediaType::TypeScript due to .headers.json modification
       assert_eq!(&(r2.media_type), &msg::MediaType::JavaScript);
     });
   }
@@ -909,23 +1309,27 @@ mod tests {
           .join("localhost_PORT4545/tests/subdir/mt_video_mp2t.t3.ts")
           .as_ref(),
       );
-      let mime_file_name = format!("{}.mime", &filename);
+      let headers_file_name = source_code_headers_filename(&filename);
 
-      let result = fetch_remote_source(module_name, &filename);
+      let result = fetch_remote_source(&deno_dir, module_name, &filename);
       assert!(result.is_ok());
       let r = result.unwrap().unwrap();
       assert_eq!(r.source_code, "export const loaded = true;\n".as_bytes());
       assert_eq!(&(r.media_type), &msg::MediaType::TypeScript);
-      // matching ext, no .mime file created
-      assert!(fs::read_to_string(&mime_file_name).is_err());
+      // matching ext, no .headers.json file created
+      assert!(fs::read_to_string(&headers_file_name).is_err());
 
-      // Modify .mime, make sure read from local
-      let _ = fs::write(&mime_file_name, "text/javascript");
-      let result2 = fetch_local_source(module_name, &filename);
+      // Modify .headers.json, make sure read from local
+      save_source_code_headers(
+        &filename,
+        Some("text/javascript".to_owned()),
+        None,
+      );
+      let result2 = fetch_local_source(&deno_dir, module_name, &filename, None);
       assert!(result2.is_ok());
       let r2 = result2.unwrap().unwrap();
       assert_eq!(r2.source_code, "export const loaded = true;\n".as_bytes());
-      // Not MediaType::TypeScript due to .mime modification
+      // Not MediaType::TypeScript due to .headers.json modification
       assert_eq!(&(r2.media_type), &msg::MediaType::JavaScript);
     });
   }
@@ -943,15 +1347,14 @@ mod tests {
           .join("localhost_PORT4545/tests/subdir/no_ext")
           .as_ref(),
       );
-      let mime_file_name = format!("{}.mime", &filename);
-      let result = fetch_remote_source(module_name, &filename);
+      let result = fetch_remote_source(&deno_dir, module_name, &filename);
       assert!(result.is_ok());
       let r = result.unwrap().unwrap();
       assert_eq!(r.source_code, "export const loaded = true;\n".as_bytes());
       assert_eq!(&(r.media_type), &msg::MediaType::TypeScript);
-      // no ext, should create .mime file
+      // no ext, should create .headers.json file
       assert_eq!(
-        fs::read_to_string(&mime_file_name).unwrap(),
+        get_source_code_headers(&filename).mime_type.unwrap(),
         "text/typescript"
       );
 
@@ -962,15 +1365,14 @@ mod tests {
           .join("localhost_PORT4545/tests/subdir/mismatch_ext.ts")
           .as_ref(),
       );
-      let mime_file_name_2 = format!("{}.mime", &filename_2);
-      let result_2 = fetch_remote_source(module_name_2, &filename_2);
+      let result_2 = fetch_remote_source(&deno_dir, module_name_2, &filename_2);
       assert!(result_2.is_ok());
       let r2 = result_2.unwrap().unwrap();
       assert_eq!(r2.source_code, "export const loaded = true;\n".as_bytes());
       assert_eq!(&(r2.media_type), &msg::MediaType::JavaScript);
-      // mismatch ext, should create .mime file
+      // mismatch ext, should create .headers.json file
       assert_eq!(
-        fs::read_to_string(&mime_file_name_2).unwrap(),
+        get_source_code_headers(&filename_2).mime_type.unwrap(),
         "text/javascript"
       );
 
@@ -982,15 +1384,14 @@ mod tests {
           .join("localhost_PORT4545/tests/subdir/unknown_ext.deno")
           .as_ref(),
       );
-      let mime_file_name_3 = format!("{}.mime", &filename_3);
-      let result_3 = fetch_remote_source(module_name_3, &filename_3);
+      let result_3 = fetch_remote_source(&deno_dir, module_name_3, &filename_3);
       assert!(result_3.is_ok());
       let r3 = result_3.unwrap().unwrap();
       assert_eq!(r3.source_code, "export const loaded = true;\n".as_bytes());
       assert_eq!(&(r3.media_type), &msg::MediaType::TypeScript);
-      // unknown ext, should create .mime file
+      // unknown ext, should create .headers.json file
       assert_eq!(
-        fs::read_to_string(&mime_file_name_3).unwrap(),
+        get_source_code_headers(&filename_3).mime_type.unwrap(),
         "text/typescript"
       );
     });
@@ -999,14 +1400,14 @@ mod tests {
   #[test]
   fn test_fetch_source_3() {
     // only local, no http_util::fetch_sync_string called
-    let (_temp_dir, _deno_dir) = test_setup();
+    let (_temp_dir, deno_dir) = test_setup();
     let cwd = std::env::current_dir().unwrap();
     let cwd_string = cwd.to_str().unwrap();
     let module_name = "http://example.com/mt_text_typescript.t1.ts"; // not used
     let filename =
       format!("{}/tests/subdir/mt_text_typescript.t1.ts", &cwd_string);
 
-    let result = fetch_local_source(module_name, &filename);
+    let result = fetch_local_source(&deno_dir, module_name, &filename, None);
     assert!(result.is_ok());
     let r = result.unwrap().unwrap();
     assert_eq!(r.source_code, "export const loaded = true;\n".as_bytes());
diff --git a/cli/http_util.rs b/cli/http_util.rs
index 9c92c08eb..d04f18b1e 100644
--- a/cli/http_util.rs
+++ b/cli/http_util.rs
@@ -1,6 +1,7 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 use crate::errors;
 use crate::errors::DenoError;
+#[cfg(test)]
 use futures::future::{loop_fn, Loop};
 use futures::{future, Future, Stream};
 use hyper;
@@ -45,8 +46,13 @@ fn resolve_uri_from_location(base_uri: &Uri, location: &str) -> Uri {
   } else {
     // assuming path-noscheme | path-empty
     let mut new_uri_parts = base_uri.clone().into_parts();
-    new_uri_parts.path_and_query =
-      Some(format!("{}/{}", base_uri.path(), location).parse().unwrap());
+    let base_uri_path_str = base_uri.path().to_owned();
+    let segs: Vec<&str> = base_uri_path_str.rsplitn(2, "/").collect();
+    new_uri_parts.path_and_query = Some(
+      format!("{}/{}", segs.last().unwrap_or(&""), location)
+        .parse()
+        .unwrap(),
+    );
     Uri::from_parts(new_uri_parts).unwrap()
   }
 }
@@ -61,6 +67,74 @@ pub fn fetch_sync_string(module_name: &str) -> DenoResult<(String, String)> {
   tokio_util::block_on(fetch_string(module_name))
 }
 
+pub enum FetchOnceResult {
+  // (code, maybe_content_type)
+  Code(String, Option<String>),
+  Redirect(http::uri::Uri),
+}
+
+/// Asynchronously fetchs the given HTTP URL one pass only.
+/// If no redirect is present and no error occurs,
+/// yields Code(code, maybe_content_type).
+/// If redirect occurs, does not follow and
+/// yields Redirect(url).
+pub fn fetch_string_once(
+  url: http::uri::Uri,
+) -> impl Future<Item = FetchOnceResult, Error = DenoError> {
+  type FetchAttempt = (Option<String>, Option<String>, Option<FetchOnceResult>);
+  let client = get_client();
+  client
+    .get(url.clone())
+    .map_err(DenoError::from)
+    .and_then(move |response| -> Box<dyn Future<Item = FetchAttempt, Error = DenoError> + Send> {
+      if response.status().is_redirection() {
+        let location_string = response
+          .headers()
+          .get("location")
+          .expect("url redirection should provide 'location' header")
+          .to_str()
+          .unwrap()
+          .to_string();
+        debug!("Redirecting to {}...", &location_string);
+        let new_url = resolve_uri_from_location(&url, &location_string);
+        // Boxed trait object turns out to be the savior for 2+ types yielding same results.
+        return Box::new(
+          future::ok(None).join3(
+            future::ok(None),
+            future::ok(Some(FetchOnceResult::Redirect(new_url))
+          ))
+        );
+      } else if response.status().is_client_error() || response.status().is_server_error() {
+        return Box::new(future::err(
+          errors::new(errors::ErrorKind::Other,
+            format!("Import '{}' failed: {}", &url, response.status()))
+          ));
+      }
+      let content_type = response
+        .headers()
+        .get(CONTENT_TYPE)
+        .map(|content_type| content_type.to_str().unwrap().to_owned());
+      let body = response
+        .into_body()
+        .concat2()
+        .map(|body| String::from_utf8(body.to_vec()).ok())
+        .map_err(DenoError::from);
+      Box::new(body.join3(
+        future::ok(content_type),
+        future::ok(None)
+      ))
+    })
+    .and_then(move |(maybe_code, maybe_content_type, maybe_redirect)| {
+      if let Some(redirect) = maybe_redirect {
+        future::ok(redirect)
+      } else {
+        // maybe_code should always contain code here!
+        future::ok(FetchOnceResult::Code(maybe_code.unwrap(), maybe_content_type))
+      }
+    })
+}
+
+#[cfg(test)]
 /// Asynchronously fetchs the given HTTP URL. Returns (content, media_type).
 pub fn fetch_string(
   module_name: &str,
@@ -182,5 +256,5 @@ fn test_resolve_uri_from_location_relative_3() {
   let url = "http://deno.land/x".parse::<Uri>().unwrap();
   let new_uri = resolve_uri_from_location(&url, "z");
   assert_eq!(new_uri.host().unwrap(), "deno.land");
-  assert_eq!(new_uri.path(), "/x/z");
+  assert_eq!(new_uri.path(), "/z");
 }
diff --git a/cli/isolate.rs b/cli/isolate.rs
index ced1cb792..9dcf6e8f0 100644
--- a/cli/isolate.rs
+++ b/cli/isolate.rs
@@ -84,6 +84,15 @@ impl<B: DenoBehavior> Isolate<B> {
           &out.js_source(),
         )?;
 
+        // The resolved module is an alias to another module (due to redirects).
+        // Save such alias to the module map.
+        if out.module_redirect_source_name.is_some() {
+          self.mod_alias(
+            &out.module_redirect_source_name.clone().unwrap(),
+            &out.module_name,
+          );
+        }
+
         self.mod_load_deps(child_id)?;
       }
     }
@@ -117,10 +126,23 @@ impl<B: DenoBehavior> Isolate<B> {
     let out = fetch_module_meta_data_and_maybe_compile(&self.state, url, ".")
       .map_err(RustOrJsError::from)?;
 
+    // Be careful.
+    // url might not match the actual out.module_name
+    // due to the mechanism of redirection.
+
     let id = self
       .mod_new_and_register(true, &out.module_name.clone(), &out.js_source())
       .map_err(RustOrJsError::from)?;
 
+    // The resolved module is an alias to another module (due to redirects).
+    // Save such alias to the module map.
+    if out.module_redirect_source_name.is_some() {
+      self.mod_alias(
+        &out.module_redirect_source_name.clone().unwrap(),
+        &out.module_name,
+      );
+    }
+
     self.mod_load_deps(id)?;
 
     let state = self.state.clone();
@@ -153,6 +175,13 @@ impl<B: DenoBehavior> Isolate<B> {
     Ok(id)
   }
 
+  /// Create an alias for another module.
+  /// The alias could later be used to grab the module
+  /// which `target` points to.
+  fn mod_alias(&self, name: &str, target: &str) {
+    self.state.modules.lock().unwrap().alias(name, target);
+  }
+
   pub fn print_file_info(&self, module: &str) {
     let m = self.state.modules.lock().unwrap();
     m.print_file_info(&self.state.dir, module.to_string());
diff --git a/cli/modules.rs b/cli/modules.rs
index e2ded67b2..f40f5ca08 100644
--- a/cli/modules.rs
+++ b/cli/modules.rs
@@ -12,23 +12,78 @@ pub struct ModuleInfo {
   children: Vec<deno_mod>,
 }
 
+/// A symbolic module entity.
+pub enum SymbolicModule {
+  /// This module is an alias to another module.
+  /// This is useful such that multiple names could point to
+  /// the same underlying module (particularly due to redirects).
+  Alias(String),
+  /// This module associates with a V8 module by id.
+  Mod(deno_mod),
+}
+
+#[derive(Default)]
+/// Alias-able module name map
+pub struct ModuleNameMap {
+  inner: HashMap<String, SymbolicModule>,
+}
+
+impl ModuleNameMap {
+  pub fn new() -> Self {
+    ModuleNameMap {
+      inner: HashMap::new(),
+    }
+  }
+
+  /// Get the id of a module.
+  /// If this module is internally represented as an alias,
+  /// follow the alias chain to get the final module id.
+  pub fn get(&self, name: &str) -> Option<deno_mod> {
+    let mut mod_name = name;
+    loop {
+      let cond = self.inner.get(mod_name);
+      match cond {
+        Some(SymbolicModule::Alias(target)) => {
+          mod_name = target;
+        }
+        Some(SymbolicModule::Mod(mod_id)) => {
+          return Some(*mod_id);
+        }
+        _ => {
+          return None;
+        }
+      }
+    }
+  }
+
+  /// Insert a name assocated module id.
+  pub fn insert(&mut self, name: String, id: deno_mod) {
+    self.inner.insert(name, SymbolicModule::Mod(id));
+  }
+
+  /// Create an alias to another module.
+  pub fn alias(&mut self, name: String, target: String) {
+    self.inner.insert(name, SymbolicModule::Alias(target));
+  }
+}
+
 /// A collection of JS modules.
 #[derive(Default)]
 pub struct Modules {
   pub info: HashMap<deno_mod, ModuleInfo>,
-  pub by_name: HashMap<String, deno_mod>,
+  pub by_name: ModuleNameMap,
 }
 
 impl Modules {
   pub fn new() -> Modules {
     Self {
       info: HashMap::new(),
-      by_name: HashMap::new(),
+      by_name: ModuleNameMap::new(),
     }
   }
 
   pub fn get_id(&self, name: &str) -> Option<deno_mod> {
-    self.by_name.get(name).cloned()
+    self.by_name.get(name)
   }
 
   pub fn get_children(&self, id: deno_mod) -> Option<&Vec<deno_mod>> {
@@ -56,6 +111,10 @@ impl Modules {
     );
   }
 
+  pub fn alias(&mut self, name: &str, target: &str) {
+    self.by_name.alias(name.to_owned(), target.to_owned());
+  }
+
   pub fn resolve_cb(
     &mut self,
     deno_dir: &DenoDir,
@@ -78,8 +137,7 @@ impl Modules {
     }
     let (name, _local_filename) = r.unwrap();
 
-    if let Some(id) = self.by_name.get(&name) {
-      let child_id = *id;
+    if let Some(child_id) = self.by_name.get(&name) {
       info.children.push(child_id);
       return child_id;
     } else {
diff --git a/tests/023_no_ext_with_mime b/tests/023_no_ext_with_headers
index 87951d835..87951d835 100644
--- a/tests/023_no_ext_with_mime
+++ b/tests/023_no_ext_with_headers
diff --git a/tests/023_no_ext_with_headers.headers.json b/tests/023_no_ext_with_headers.headers.json
new file mode 100644
index 000000000..5b6f09aeb
--- /dev/null
+++ b/tests/023_no_ext_with_headers.headers.json
@@ -0,0 +1 @@
+{ "mime_type": "application/javascript" }
diff --git a/tests/023_no_ext_with_mime.out b/tests/023_no_ext_with_headers.out
index e427984d4..e427984d4 100644
--- a/tests/023_no_ext_with_mime.out
+++ b/tests/023_no_ext_with_headers.out
diff --git a/tests/023_no_ext_with_headers.test b/tests/023_no_ext_with_headers.test
new file mode 100644
index 000000000..5be189af0
--- /dev/null
+++ b/tests/023_no_ext_with_headers.test
@@ -0,0 +1,2 @@
+args: tests/023_no_ext_with_headers --reload
+output: tests/023_no_ext_with_headers.out
diff --git a/tests/023_no_ext_with_mime.mime b/tests/023_no_ext_with_mime.mime
deleted file mode 100644
index d7482e1bc..000000000
--- a/tests/023_no_ext_with_mime.mime
+++ /dev/null
@@ -1 +0,0 @@
-application/javascript
\ No newline at end of file
diff --git a/tests/023_no_ext_with_mime.test b/tests/023_no_ext_with_mime.test
deleted file mode 100644
index 5943f6526..000000000
--- a/tests/023_no_ext_with_mime.test
+++ /dev/null
@@ -1,2 +0,0 @@
-args: tests/023_no_ext_with_mime --reload
-output: tests/023_no_ext_with_mime.out
diff --git a/tests/024_import_no_ext_with_headers.test b/tests/024_import_no_ext_with_headers.test
new file mode 100644
index 000000000..572158f12
--- /dev/null
+++ b/tests/024_import_no_ext_with_headers.test
@@ -0,0 +1,2 @@
+args: tests/024_import_no_ext_with_headers.ts --reload
+output: tests/024_import_no_ext_with_headers.ts.out
diff --git a/tests/024_import_no_ext_with_headers.ts b/tests/024_import_no_ext_with_headers.ts
new file mode 100644
index 000000000..c8621d0e6
--- /dev/null
+++ b/tests/024_import_no_ext_with_headers.ts
@@ -0,0 +1 @@
+import "./023_no_ext_with_headers";
diff --git a/tests/024_import_no_ext_with_mime.ts.out b/tests/024_import_no_ext_with_headers.ts.out
index e427984d4..e427984d4 100644
--- a/tests/024_import_no_ext_with_mime.ts.out
+++ b/tests/024_import_no_ext_with_headers.ts.out
diff --git a/tests/024_import_no_ext_with_mime.test b/tests/024_import_no_ext_with_mime.test
deleted file mode 100644
index 2ffe4810f..000000000
--- a/tests/024_import_no_ext_with_mime.test
+++ /dev/null
@@ -1,2 +0,0 @@
-args: tests/024_import_no_ext_with_mime.ts --reload
-output: tests/024_import_no_ext_with_mime.ts.out
diff --git a/tests/024_import_no_ext_with_mime.ts b/tests/024_import_no_ext_with_mime.ts
deleted file mode 100644
index d5140de66..000000000
--- a/tests/024_import_no_ext_with_mime.ts
+++ /dev/null
@@ -1 +0,0 @@
-import "./023_no_ext_with_mime";
diff --git a/tests/026_redirect_javascript.js b/tests/026_redirect_javascript.js
new file mode 100644
index 000000000..226a6b622
--- /dev/null
+++ b/tests/026_redirect_javascript.js
@@ -0,0 +1,2 @@
+import { value } from "http://localhost:4547/redirects/redirect3.js";
+console.log(value);
diff --git a/tests/026_redirect_javascript.js.out b/tests/026_redirect_javascript.js.out
new file mode 100644
index 000000000..290864299
--- /dev/null
+++ b/tests/026_redirect_javascript.js.out
@@ -0,0 +1 @@
+3 imports 1
diff --git a/tests/026_redirect_javascript.js.test b/tests/026_redirect_javascript.js.test
new file mode 100644
index 000000000..a66cb0ea0
--- /dev/null
+++ b/tests/026_redirect_javascript.js.test
@@ -0,0 +1,2 @@
+args: tests/026_redirect_javascript.js --reload
+output: tests/026_redirect_javascript.js.out
diff --git a/tests/027_redirect_typescript.ts b/tests/027_redirect_typescript.ts
new file mode 100644
index 000000000..584341975
--- /dev/null
+++ b/tests/027_redirect_typescript.ts
@@ -0,0 +1,2 @@
+import { value } from "http://localhost:4547/redirects/redirect4.ts";
+console.log(value);
diff --git a/tests/027_redirect_typescript.ts.out b/tests/027_redirect_typescript.ts.out
new file mode 100644
index 000000000..480d4e8ca
--- /dev/null
+++ b/tests/027_redirect_typescript.ts.out
@@ -0,0 +1 @@
+4 imports 1
diff --git a/tests/027_redirect_typescript.ts.test b/tests/027_redirect_typescript.ts.test
new file mode 100644
index 000000000..8abfbc616
--- /dev/null
+++ b/tests/027_redirect_typescript.ts.test
@@ -0,0 +1,2 @@
+args: tests/027_redirect_typescript.ts --reload
+output: tests/027_redirect_typescript.ts.out
\ No newline at end of file
diff --git a/tests/subdir/redirects/redirect1.js b/tests/subdir/redirects/redirect1.js
new file mode 100644
index 000000000..d674be88c
--- /dev/null
+++ b/tests/subdir/redirects/redirect1.js
@@ -0,0 +1 @@
+export const redirect = 1;
diff --git a/tests/subdir/redirects/redirect1.ts b/tests/subdir/redirects/redirect1.ts
new file mode 100644
index 000000000..d674be88c
--- /dev/null
+++ b/tests/subdir/redirects/redirect1.ts
@@ -0,0 +1 @@
+export const redirect = 1;
diff --git a/tests/subdir/redirects/redirect2.js b/tests/subdir/redirects/redirect2.js
new file mode 100644
index 000000000..e4244f638
--- /dev/null
+++ b/tests/subdir/redirects/redirect2.js
@@ -0,0 +1 @@
+import "./redirect1.js";
diff --git a/tests/subdir/redirects/redirect3.js b/tests/subdir/redirects/redirect3.js
new file mode 100644
index 000000000..e24f2af32
--- /dev/null
+++ b/tests/subdir/redirects/redirect3.js
@@ -0,0 +1,2 @@
+import { redirect } from "./redirect1.js";
+export const value = `3 imports ${redirect}`;
diff --git a/tests/subdir/redirects/redirect4.ts b/tests/subdir/redirects/redirect4.ts
new file mode 100644
index 000000000..f31ad886e
--- /dev/null
+++ b/tests/subdir/redirects/redirect4.ts
@@ -0,0 +1,2 @@
+import { redirect } from "./redirect1.ts";
+export const value: string = `4 imports ${redirect}`;
diff --git a/tools/http_server.py b/tools/http_server.py
index 4df0e7690..e44a86fc7 100755
--- a/tools/http_server.py
+++ b/tools/http_server.py
@@ -12,6 +12,8 @@ from time import sleep
 
 PORT = 4545
 REDIRECT_PORT = 4546
+ANOTHER_REDIRECT_PORT = 4547
+DOUBLE_REDIRECTS_PORT = 4548
 
 
 class ContentTypeHandler(SimpleHTTPServer.SimpleHTTPRequestHandler):
@@ -99,24 +101,42 @@ def server():
     return s
 
 
-def redirect_server():
+def base_redirect_server(host_port, target_port, extra_path_segment=""):
     os.chdir(root_path)
-    target_host = "http://localhost:%d" % PORT
+    target_host = "http://localhost:%d" % target_port
 
     class RedirectHandler(SimpleHTTPServer.SimpleHTTPRequestHandler):
         def do_GET(self):
             self.send_response(301)
-            self.send_header('Location', target_host + self.path)
+            self.send_header('Location',
+                             target_host + extra_path_segment + self.path)
             self.end_headers()
 
     Handler = RedirectHandler
     SocketServer.TCPServer.allow_reuse_address = True
-    s = SocketServer.TCPServer(("", REDIRECT_PORT), Handler)
+    s = SocketServer.TCPServer(("", host_port), Handler)
     print "redirect server http://localhost:%d/ -> http://localhost:%d/" % (
-        REDIRECT_PORT, PORT)
+        host_port, target_port)
     return s
 
 
+# redirect server
+def redirect_server():
+    return base_redirect_server(REDIRECT_PORT, PORT)
+
+
+# another redirect server pointing to the same port as the one above
+# BUT with an extra subdir path
+def another_redirect_server():
+    return base_redirect_server(
+        ANOTHER_REDIRECT_PORT, PORT, extra_path_segment="/tests/subdir")
+
+
+# redirect server that points to another redirect server
+def double_redirects_server():
+    return base_redirect_server(DOUBLE_REDIRECTS_PORT, REDIRECT_PORT)
+
+
 def spawn():
     # Main http server
     s = server()
@@ -128,6 +148,16 @@ def spawn():
     r_thread = Thread(target=rs.serve_forever)
     r_thread.daemon = True
     r_thread.start()
+    # Another redirect server
+    ars = another_redirect_server()
+    ar_thread = Thread(target=ars.serve_forever)
+    ar_thread.daemon = True
+    ar_thread.start()
+    # Double redirects server
+    drs = double_redirects_server()
+    dr_thread = Thread(target=drs.serve_forever)
+    dr_thread.daemon = True
+    dr_thread.start()
     sleep(1)  # TODO I'm too lazy to figure out how to do this properly.
     return thread