3 files changed, 102 insertions, 14 deletions

diff --git a/std/wasi/README.md b/std/wasi/README.md
index 70149c771..06dc36ff5 100644
--- a/std/wasi/README.md
+++ b/std/wasi/README.md
@@ -27,7 +27,7 @@ This module provides an implementation of the WebAssembly System Interface
 - [x] fd_prestat_dir_name
 - [x] fd_pwrite
 - [x] fd_read
-- [ ] fd_readdir
+- [x] fd_readdir
 - [x] fd_renumber
 - [x] fd_seek
 - [x] fd_sync
diff --git a/std/wasi/snapshot_preview1.ts b/std/wasi/snapshot_preview1.ts
index e42d7eec2..72ddd5d23 100644
--- a/std/wasi/snapshot_preview1.ts
+++ b/std/wasi/snapshot_preview1.ts
@@ -317,15 +317,12 @@ export default class Module {
 
     if (options.preopens) {
       for (const [vpath, path] of Object.entries(options.preopens)) {
-        const info = Deno.statSync(path);
-        if (!info.isDirectory) {
-          throw new TypeError(`${path} is not a directory`);
-        }
-
         const type = FILETYPE_DIRECTORY;
+        const entries = Array.from(Deno.readDirSync(path));
 
         const entry = {
           type,
+          entries,
           path,
           vpath,
         };
@@ -485,7 +482,10 @@ export default class Module {
           return ERRNO_BADF;
         }
 
-        entry.handle.close();
+        if (entry.handle) {
+          entry.handle.close();
+        }
+
         delete this.fds[fd];
 
         return ERRNO_SUCCESS;
@@ -797,7 +797,70 @@ export default class Module {
         cookie: bigint,
         bufused_out: number
       ): number => {
-        return ERRNO_NOSYS;
+        const entry = this.fds[fd];
+        if (!entry) {
+          return ERRNO_BADF;
+        }
+
+        const heap = new Uint8Array(this.memory.buffer);
+        const view = new DataView(this.memory.buffer);
+
+        let bufused = 0;
+
+        try {
+          const entries = Array.from(Deno.readDirSync(entry.path));
+          for (let i = Number(cookie); i < entries.length; i++) {
+            const name_data = new TextEncoder().encode(entries[i].name);
+
+            const entry_info = Deno.statSync(
+              resolve(entry.path, entries[i].name)
+            );
+            const entry_data = new Uint8Array(24 + name_data.byteLength);
+            const entry_view = new DataView(entry_data.buffer);
+
+            entry_view.setBigUint64(0, BigInt(i + 1), true);
+            entry_view.setBigUint64(
+              8,
+              BigInt(entry_info.ino ? entry_info.ino : 0),
+              true
+            );
+            entry_view.setUint32(16, name_data.byteLength, true);
+
+            switch (true) {
+              case entries[i].isFile:
+                var type = FILETYPE_REGULAR_FILE;
+                break;
+
+              case entries[i].isDirectory:
+                var type = FILETYPE_REGULAR_FILE;
+                break;
+
+              case entries[i].isSymlink:
+                var type = FILETYPE_SYMBOLIC_LINK;
+                break;
+
+              default:
+                var type = FILETYPE_REGULAR_FILE;
+                break;
+            }
+
+            entry_view.setUint8(20, type);
+            entry_data.set(name_data, 24);
+
+            const data = entry_data.slice(
+              0,
+              Math.min(entry_data.length, buf_len - bufused)
+            );
+            heap.set(data, buf_ptr + bufused);
+            bufused += data.byteLength;
+          }
+        } catch (err) {
+          return errno(err);
+        }
+
+        view.setUint32(bufused_out, bufused, true);
+
+        return ERRNO_SUCCESS;
       },
 
       fd_renumber: (fd: number, to: number): number => {
@@ -1126,6 +1189,28 @@ export default class Module {
         const data = new Uint8Array(this.memory.buffer, path_ptr, path_len);
         const path = resolve(entry.path, text.decode(data));
 
+        if ((oflags & OFLAGS_DIRECTORY) !== 0) {
+          // XXX (caspervonb) this isn't ideal as we can't get a rid for the
+          // directory this way so there's no native fstat but Deno.open
+          // doesn't work with directories on windows so we'll have to work
+          // around it for now.
+          try {
+            const entries = Array.from(Deno.readDirSync(path));
+            const opened_fd =
+              this.fds.push({
+                entries,
+                path,
+              }) - 1;
+
+            const view = new DataView(this.memory.buffer);
+            view.setUint32(opened_fd_out, opened_fd, true);
+          } catch (err) {
+            return errno(err);
+          }
+
+          return ERRNO_SUCCESS;
+        }
+
         const options = {
           read: false,
           write: false,
@@ -1140,12 +1225,6 @@ export default class Module {
           options.write = true;
         }
 
-        if ((oflags & OFLAGS_DIRECTORY) !== 0) {
-          // TODO (caspervonb) review if we can
-          // emulate this; unix supports opening
-          // directories, windows does not.
-        }
-
         if ((oflags & OFLAGS_EXCL) !== 0) {
           options.createNew = true;
         }
diff --git a/std/wasi/testdata/std_fs_read_dir.rs b/std/wasi/testdata/std_fs_read_dir.rs
new file mode 100644
index 000000000..ad8b70c72
--- /dev/null
+++ b/std/wasi/testdata/std_fs_read_dir.rs
@@ -0,0 +1,9 @@
+// { "preopens": { "/fixture": "fixture" } }
+
+fn main() {
+  let entries = std::fs::read_dir("/fixture").unwrap();
+  assert_eq!(entries.count(), 4);
+
+  let entries = std::fs::read_dir("/fixture/directory").unwrap();
+  assert_eq!(entries.count(), 2);
+}