console: retry on EAGAIN instead of dropping output when stdout is nonblocking

src/runtime/webcore/FileSink.rs

@@ -239,9 +239,12 @@
     {
         this.force_sync.set(true);
         // SAFETY(JsCell): single-field write; does not call into JS.
-        this.writer.with_mut(|w| w.force_sync = true);
-        if this.fd.get() != Fd::INVALID {
-            let _ = sys::update_nonblocking(this.fd.get(), false);
+        let fd = this.writer.with_mut(|w| {
+            w.force_sync = true;
+            w.get_fd()
+        });
+        if fd != Fd::INVALID {
+            let _ = sys::update_nonblocking(fd, false);
         }
     }
     #[cfg(windows)]

src/sys/lib.rs

@@ -9410,6 +9410,21 @@ fn fd_write_all_quiet(fd: Fd, mut bytes: &[u8]) -> bool {
         match write(fd, bytes) {
             Ok(0) => return false, // short write → give up
             Ok(n) => bytes = &bytes[n..],
+            #[cfg(unix)]
+            Err(e) if e.get_errno() == E::EINTR => continue,
+            #[cfg(unix)]
+            Err(e) if e.get_errno() == E::EAGAIN => {
+                // fd 1/2 may be O_NONBLOCK once `process.stdout`/`stderr` is
+                // materialized; block until writable instead of dropping bytes.
+                let mut pfd = [posix::PollFd {
+                    fd: fd.native() as core::ffi::c_int,
+                    events: posix::POLL_OUT,
+                    revents: 0,
+                }];
+                if posix::poll(&mut pfd, -1).is_err() {
+                    return false;
+                }
+            }
             Err(_) => return false,
         }
     }

test/js/node/process/process-stdio.test.ts

@@ -1,6 +1,6 @@
 import { spawn, spawnSync } from "bun";
 import { describe, expect, test } from "bun:test";
-import { bunEnv, bunExe } from "harness";
+import { bunEnv, bunExe, isWindows, tempDir } from "harness";
 import path from "path";
 import { isatty } from "tty";
 describe.concurrent("process-stdio", () => {
@@ -159,3 +159,102 @@ describe.concurrent("process-stdio", () => {
     );
   });
 });
+
+// Materializing process.stdout dups fd 1 and flips it to O_NONBLOCK. The native
+// console writer must not silently drop data on the resulting EAGAIN when the
+// pipe is full. Kept outside the concurrent block so the extra spawned children
+// don't push the already-slow stdin tests past their default timeout.
+describe.skipIf(isWindows)("console.log after process.stdout is materialized on a pipe", () => {
+  test("many lines survive a slow reader", async () => {
+    const N = 1500;
+    const pad = Buffer.alloc(500, "x").toString();
+    using dir = tempDir("stdout-nonblock-loss", {
+      "child.mjs": `
+        if (process.argv[2] === "touch") void process.stdout.writableHighWaterMark;
+        const pad = ${JSON.stringify(pad)};
+        for (let i = 0; i < ${N}; i++) console.log("O" + i + " " + pad);
+      `,
+    });
+    // A separate `cat` reader starts 400ms late behind a shell fifo, so the
+    // 64 KiB pipe fills and write(2) on the now-nonblocking fd 1 returns EAGAIN
+    // mid-run. The pipeline's exit status is cat's, not bun's, so the delivered
+    // count is what proves the regression is gone.
+    await using proc = Bun.spawn({
+      cmd: [
+        "/bin/sh",
+        "-c",
+        'exec "$0" "$1" touch | { sleep 0.4; exec cat; }',
+        bunExe(),
+        path.join(String(dir), "child.mjs"),
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "inherit",
+    });
+    const stdout = await proc.stdout.text();
+    const delivered = stdout.split("\n").filter(l => /^O\d+ x+$/.test(l)).length;
+    expect(delivered).toBe(N);
+  });
+
+  test("a single 1 MiB line is not truncated", async () => {
+    // A single 1 MiB write into even a fast `| wc -c` reader exceeds the 64 KiB
+    // pipe, so write(2) on the now-nonblocking fd 1 returns a partial count and
+    // then EAGAIN before the reader drains.
+    await using proc = Bun.spawn({
+      cmd: [
+        "/bin/sh",
+        "-c",
+        'exec "$0" -e "void process.stdout.isTTY; console.log(Buffer.alloc(1<<20, 65).toString())" | wc -c',
+        bunExe(),
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "inherit",
+    });
+    const stdout = (await proc.stdout.text()).trim();
+    expect(Number(stdout)).toBe((1 << 20) + 1);
+  });
+
+  test("parent console.log survives a bun child that touches inherited stdout", async () => {
+    // O_NONBLOCK lives on the shared open file description. A bun child with
+    // stdio:'inherit' that materializes its process.stdout flips the PARENT's
+    // fd 1 too; the parent's console writer must still deliver every line.
+    using dir = tempDir("stdout-nonblock-child-inherit", {
+      "parent.mjs": `
+        const { spawnSync } = require("node:child_process");
+        const r = spawnSync(process.execPath, ["-e", 'process.stdout.write("")'],
+                            { stdio: ["ignore", "inherit", "ignore"] });
+        if (r.error || r.status !== 0) throw new Error("child failed: " + (r.error ?? r.status));
+        const pad = Buffer.alloc(190, 120).toString();
+        for (let i = 0; i < 20000; i++) console.log("O" + i + " " + pad);
+      `,
+    });
+    await using proc = Bun.spawn({
+      cmd: ["/bin/sh", "-c", 'exec "$0" "$1" | { sleep 1; wc -l; }', bunExe(), path.join(String(dir), "parent.mjs")],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "inherit",
+    });
+    const stdout = (await proc.stdout.text()).trim();
+    expect(Number(stdout)).toBe(20000);
+  });
+
+  test("console.log survives a raw O_NONBLOCK on fd 1 (isolates the writer)", async () => {
+    // Bun.file(1).writer() goes through FileSink.setup (dup + O_NONBLOCK) but is
+    // NOT the process.stdout getter path, so ForceFileSinkToBeSynchronous never
+    // clears the flag. This isolates fd_write_all_quiet's EAGAIN handling.
+    await using proc = Bun.spawn({
+      cmd: [
+        "/bin/sh",
+        "-c",
+        'exec "$0" -e "void Bun.file(1).writer(); console.log(Buffer.alloc(1<<20, 65).toString())" | { sleep 0.4; exec wc -c; }',
+        bunExe(),
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "inherit",
+    });
+    const stdout = (await proc.stdout.text()).trim();
+    expect(Number(stdout)).toBe((1 << 20) + 1);
+  });
+});