stdio: reset writable state after end() on file-backed process.stdout/stderr

src/js/builtins/ProcessObjectInternals.ts

@@ -65,6 +65,11 @@ export function getStdioWriteStream(
           // stdout/stderr don't produce readable data, so yield nothing
         })();
       };
+    } else {
+      // File-backed stdio: Node's SyncWriteStream runs end() -> finish ->
+      // destroy -> the _destroy override below -> _undestroy(), which resets
+      // writable state so later writes succeed. autoClose:false disabled that.
+      stream._writableState.autoDestroy = true;
     }
   }
 

test/js/node/process/process-stdout-write-after-end.test.ts

@@ -1,8 +1,9 @@
 import { expect, test } from "bun:test";
-import { bunEnv, bunExe } from "harness";
+import { bunEnv, bunExe, tempDir } from "harness";
+import fs from "node:fs";
 import path from "path";
 
-test.each(["stdout", "stderr"] as const)(
+test.concurrent.each(["stdout", "stderr"] as const)(
   "process.%s - write after end() errors and is not delivered (piped)",
   async which => {
     await using proc = Bun.spawn({
@@ -42,3 +43,50 @@ test.each(["stdout", "stderr"] as const)(
     expect(exitCode).toBe(0);
   },
 );
+
+test.concurrent.each(["stdout", "stderr"] as const)(
+  "process.%s - write after end() succeeds and is delivered (file)",
+  async which => {
+    using dir = tempDir("stdio-write-after-end-file", {});
+    const outPath = path.join(String(dir), "out.txt");
+    const fd = fs.openSync(outPath, "w");
+    try {
+      // Redirect the fixture's target stream to a regular file; the report
+      // stream stays piped so we can read the JSON facts.
+      await using proc = Bun.spawn({
+        cmd: [bunExe(), path.join(import.meta.dir, "process-stdout-write-after-end-file-fixture.mjs"), which],
+        stdout: which === "stdout" ? fd : "pipe",
+        stderr: which === "stderr" ? fd : "pipe",
+        stdin: "ignore",
+        env: bunEnv,
+      });
+
+      const reportStream = which === "stderr" ? proc.stdout : proc.stderr;
+      const [reportText, exitCode] = await Promise.all([reportStream.text(), proc.exited]);
+      const lines = reportText.trim().split("\n");
+      const report = JSON.parse(lines[lines.length - 1]);
+
+      // Node's file-backed stdio is never-closing: end() runs the finish ->
+      // destroy -> _undestroy cycle, which resets writable state, so a later
+      // write() succeeds with no error and writableEnded is false again.
+      expect(report).toEqual({
+        writableEnded: false,
+        writable: true,
+        ret: true,
+        cbErr: null,
+        ev: [],
+      });
+
+      const fileContents = fs.readFileSync(outPath, "latin1");
+      // stderr may carry benign ASAN/debug noise on fd 2; stdout is clean.
+      if (which === "stdout") {
+        expect(fileContents).toBe("ABCD\n");
+      } else {
+        expect(fileContents).toContain("ABCD\n");
+      }
+      expect(exitCode).toBe(0);
+    } finally {
+      fs.closeSync(fd);
+    }
+  },
+);