feat(cli): add `DENO_V8_FLAGS` env var (#17313)

Closes #5669

6 files changed, 63 insertions, 37 deletions

diff --git a/cli/args/flags.rs b/cli/args/flags.rs
index a5aff3d09..9f7b9206f 100644
--- a/cli/args/flags.rs
+++ b/cli/args/flags.rs
@@ -545,6 +545,7 @@ static ENV_VARIABLES_HELP: &str = r#"ENVIRONMENT VARIABLES:
                          (alternative to passing --no-prompt on invocation)
     DENO_NO_UPDATE_CHECK Set to disable checking if a newer Deno version is
                          available
+    DENO_V8_FLAGS        Set V8 command line options
     DENO_WEBGPU_TRACE    Directory to use for wgpu traces
     DENO_JOBS            Number of parallel workers used for the --parallel
                          flag with the test subcommand. Defaults to number
@@ -2115,7 +2116,8 @@ fn v8_flags_arg<'a>() -> Arg<'a> {
     .use_value_delimiter(true)
     .require_equals(true)
     .help("Set V8 command line options")
-    .long_help("To see a list of all available flags use --v8-flags=--help.")
+    .long_help("To see a list of all available flags use --v8-flags=--help.\
+    Any flags set with this flag are appended after the DENO_V8_FLAGS environmental variable")
 }
 
 fn seed_arg<'a>() -> Arg<'a> {
diff --git a/cli/main.rs b/cli/main.rs
index 08746042c..8c0dd0354 100644
--- a/cli/main.rs
+++ b/cli/main.rs
@@ -31,43 +31,19 @@ use crate::args::Flags;
 use crate::proc_state::ProcState;
 use crate::resolver::CliResolver;
 use crate::util::display;
+use crate::util::v8::get_v8_flags_from_env;
+use crate::util::v8::init_v8_flags;
 
 use args::CliOptions;
 use deno_core::anyhow::Context;
 use deno_core::error::AnyError;
 use deno_core::error::JsError;
-use deno_core::v8_set_flags;
 use deno_runtime::colors;
 use deno_runtime::fmt_errors::format_js_error;
 use deno_runtime::tokio_util::run_local;
 use std::env;
-use std::iter::once;
 use std::path::PathBuf;
 
-fn init_v8_flags(v8_flags: &[String]) {
-  let v8_flags_includes_help = v8_flags
-    .iter()
-    .any(|flag| flag == "-help" || flag == "--help");
-  // Keep in sync with `standalone.rs`.
-  let v8_flags = once("UNUSED_BUT_NECESSARY_ARG0".to_owned())
-    .chain(v8_flags.iter().cloned())
-    .collect::<Vec<_>>();
-  let unrecognized_v8_flags = v8_set_flags(v8_flags)
-    .into_iter()
-    .skip(1)
-    .collect::<Vec<_>>();
-  if !unrecognized_v8_flags.is_empty() {
-    for f in unrecognized_v8_flags {
-      eprintln!("error: V8 did not recognize flag '{}'", f);
-    }
-    eprintln!("\nFor a list of V8 flags, use '--v8-flags=--help'");
-    std::process::exit(1);
-  }
-  if v8_flags_includes_help {
-    std::process::exit(0);
-  }
-}
-
 async fn run_subcommand(flags: Flags) -> Result<i32, AnyError> {
   match flags.subcommand.clone() {
     DenoSubcommand::Bench(bench_flags) => {
@@ -285,9 +261,8 @@ pub fn main() {
       }
       Err(err) => unwrap_or_exit(Err(AnyError::from(err))),
     };
-    if !flags.v8_flags.is_empty() {
-      init_v8_flags(&flags.v8_flags);
-    }
+
+    init_v8_flags(&flags.v8_flags, get_v8_flags_from_env());
 
     util::logger::init(flags.log_level);
 
diff --git a/cli/standalone.rs b/cli/standalone.rs
index 829baa557..063ad7d33 100644
--- a/cli/standalone.rs
+++ b/cli/standalone.rs
@@ -6,6 +6,7 @@ use crate::colors;
 use crate::file_fetcher::get_source_from_data_url;
 use crate::ops;
 use crate::proc_state::ProcState;
+use crate::util::v8::construct_v8_flags;
 use crate::version;
 use crate::CliResolver;
 use deno_core::anyhow::Context;
@@ -38,7 +39,6 @@ use import_map::parse_from_json;
 use log::Level;
 use std::env::current_exe;
 use std::io::SeekFrom;
-use std::iter::once;
 use std::pin::Pin;
 use std::rc::Rc;
 use std::sync::Arc;
@@ -248,12 +248,7 @@ pub async fn run(
     todo!("Workers are currently not supported in standalone binaries");
   });
 
-  // Keep in sync with `main.rs`.
-  v8_set_flags(
-    once("UNUSED_BUT_NECESSARY_ARG0".to_owned())
-      .chain(metadata.v8_flags.iter().cloned())
-      .collect::<Vec<_>>(),
-  );
+  v8_set_flags(construct_v8_flags(&metadata.v8_flags, vec![]));
 
   let root_cert_store = ps.root_cert_store.clone();
 
diff --git a/cli/tests/integration/run_tests.rs b/cli/tests/integration/run_tests.rs
index d5a988bf4..2494df881 100644
--- a/cli/tests/integration/run_tests.rs
+++ b/cli/tests/integration/run_tests.rs
@@ -1154,6 +1154,12 @@ itest!(v8_flags_run {
   output: "run/v8_flags.js.out",
 });
 
+itest!(v8_flags_env_run {
+  envs: vec![("DENO_V8_FLAGS".to_string(), "--expose-gc".to_string())],
+  args: "run run/v8_flags.js",
+  output: "run/v8_flags.js.out",
+});
+
 itest!(v8_flags_unrecognized {
   args: "repl --v8-flags=--foo,bar,--trace-gc,-baz",
   output: "run/v8_flags_unrecognized.out",
diff --git a/cli/util/mod.rs b/cli/util/mod.rs
index d0b500e31..515bc64e9 100644
--- a/cli/util/mod.rs
+++ b/cli/util/mod.rs
@@ -13,4 +13,5 @@ pub mod path;
 pub mod progress_bar;
 pub mod text_encoding;
 pub mod unix;
+pub mod v8;
 pub mod windows;
diff --git a/cli/util/v8.rs b/cli/util/v8.rs
new file mode 100644
index 000000000..b6d6aa44e
--- /dev/null
+++ b/cli/util/v8.rs
@@ -0,0 +1,47 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+
+#[inline(always)]
+pub fn get_v8_flags_from_env() -> Vec<String> {
+  std::env::var("DENO_V8_FLAGS")
+    .ok()
+    .map(|flags| flags.split(',').map(String::from).collect::<Vec<String>>())
+    .unwrap_or_default()
+}
+
+#[inline(always)]
+pub fn construct_v8_flags(
+  v8_flags: &[String],
+  env_v8_flags: Vec<String>,
+) -> Vec<String> {
+  std::iter::once("UNUSED_BUT_NECESSARY_ARG0".to_owned())
+    .chain(env_v8_flags.into_iter())
+    .chain(v8_flags.iter().cloned())
+    .collect::<Vec<_>>()
+}
+
+pub fn init_v8_flags(v8_flags: &[String], env_v8_flags: Vec<String>) {
+  if v8_flags.is_empty() && env_v8_flags.is_empty() {
+    return;
+  }
+
+  let v8_flags_includes_help = env_v8_flags
+    .iter()
+    .chain(v8_flags)
+    .any(|flag| flag == "-help" || flag == "--help");
+  // Keep in sync with `standalone.rs`.
+  let v8_flags = construct_v8_flags(v8_flags, env_v8_flags);
+  let unrecognized_v8_flags = deno_core::v8_set_flags(v8_flags)
+    .into_iter()
+    .skip(1)
+    .collect::<Vec<_>>();
+  if !unrecognized_v8_flags.is_empty() {
+    for f in unrecognized_v8_flags {
+      eprintln!("error: V8 did not recognize flag '{}'", f);
+    }
+    eprintln!("\nFor a list of V8 flags, use '--v8-flags=--help'");
+    std::process::exit(1);
+  }
+  if v8_flags_includes_help {
+    std::process::exit(0);
+  }
+}