standalone: env var to disable the post-load madvise of embedded source

src/bun_core/env_var.rs

@@ -252,6 +252,7 @@ pub mod feature_flag {
     new_feature_flag!(pub BUN_FEATURE_FLAG_DISABLE_SOURCE_MAPS, "BUN_FEATURE_FLAG_DISABLE_SOURCE_MAPS", {});
     new_feature_flag!(pub BUN_FEATURE_FLAG_DISABLE_SPAWNSYNC_FAST_PATH, "BUN_FEATURE_FLAG_DISABLE_SPAWNSYNC_FAST_PATH", {});
     new_feature_flag!(pub BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING, "BUN_FEATURE_FLAG_DISABLE_SQL_AUTO_PIPELINING", {});
+    new_feature_flag!(pub BUN_FEATURE_FLAG_DISABLE_STANDALONE_MADVISE, "BUN_FEATURE_FLAG_DISABLE_STANDALONE_MADVISE", {});
     new_feature_flag!(pub BUN_DISABLE_TRANSPILED_SOURCE_CODE_PREVIEW, "BUN_DISABLE_TRANSPILED_SOURCE_CODE_PREVIEW", {});
     new_feature_flag!(pub BUN_FEATURE_FLAG_DISABLE_UV_FS_COPYFILE, "BUN_FEATURE_FLAG_DISABLE_UV_FS_COPYFILE", {});
     new_feature_flag!(pub BUN_DUMP_STATE_ON_CRASH, "BUN_DUMP_STATE_ON_CRASH", {});

src/standalone_graph/StandaloneModuleGraph.rs

@@ -2193,7 +2193,8 @@ impl StandaloneModuleGraph {
     /// The pages are clean file-backed COW, so any later read (lazy require,
     /// stack-trace source lookup) faults back in transparently from the
     /// executable on disk. Only applies when running as a compiled
-    /// standalone binary.
+    /// standalone binary; `BUN_FEATURE_FLAG_DISABLE_STANDALONE_MADVISE=1`
+    /// skips the hint.
     pub fn hint_source_pages_dont_need() {
         #[cfg(windows)]
         {
@@ -2225,7 +2226,11 @@ impl StandaloneModuleGraph {
 
             #[cfg(any(target_os = "macos", target_os = "linux", target_os = "android"))]
             {
-                if len == 0 {
+                if len == 0
+                    || bun_core::env_var::feature_flag::BUN_FEATURE_FLAG_DISABLE_STANDALONE_MADVISE
+                        .get()
+                        .unwrap_or(false)
+                {
                     return;
                 }
 

test/js/bun/compile/standalone-madvise-tla.test.ts

@@ -2,6 +2,8 @@
 // entrypoint has top-level await. loadEntryPoint() returns a promise without
 // blocking, so the call site at bun.js.zig:466 is hit synchronously before the
 // main event loop spins — TLA resolution happens later in that loop.
+// BUN_FEATURE_FLAG_DISABLE_STANDALONE_MADVISE, read by the compiled binary at
+// runtime, skips the hint.
 import { expect, test } from "bun:test";
 import { bunEnv, bunExe, isDebug, isWindows, tempDir } from "harness";
 import path from "node:path";
@@ -29,10 +31,22 @@ test.skipIf(isWindows || !isDebug)(
     expect(build.stderr.toString()).not.toContain("error:");
     expect(build.exitCode).toBe(0);
 
-    {
+    // BUN_FEATURE_FLAG_DISABLE_STANDALONE_MADVISE is read by the compiled
+    // executable at runtime (the binary above was built without it); a falsy
+    // value leaves the hint enabled. With the flag set, the function returns
+    // before logging anything, so the only evidence is the missing line.
+    for (const [flag, hinted] of [
+      [undefined, true],
+      ["0", true],
+      ["1", false],
+    ] as const) {
       await using proc = Bun.spawn({
         cmd: [out],
-        env: { ...bunEnv, BUN_DEBUG_StandaloneModuleGraph: "1" },
+        env: {
+          ...bunEnv,
+          BUN_DEBUG_StandaloneModuleGraph: "1",
+          BUN_FEATURE_FLAG_DISABLE_STANDALONE_MADVISE: flag,
+        },
         stdout: "pipe",
         stderr: "pipe",
       });
@@ -42,7 +56,11 @@ test.skipIf(isWindows || !isDebug)(
       expect(stdout).toContain("after-await");
       // Scoped loggers write to the debug-writer stream (stdout by default).
       // Either the success or failure variant proves the call site is reached.
-      expect(stdout).toContain("hintSourcePagesDontNeed:");
+      if (hinted) {
+        expect(stdout).toContain("hintSourcePagesDontNeed:");
+      } else {
+        expect(stdout).not.toContain("hintSourcePagesDontNeed:");
+      }
       expect(stderr).toBe("");
       expect(exitCode).toBe(0);
     }