fix(posix): ensure nonblocking file descriptor is nonblocking (#10080)

src/bun.js/webcore/streams.zig

@@ -3562,7 +3562,7 @@ pub const FileReader = struct {
                     this.file_type = .nonblocking_pipe;
                 }
 
-                this.nonblocking = is_nonblocking_tty or (this.pollable and !(file.is_atty orelse false));
+                this.nonblocking = is_nonblocking_tty or bun.isNonBlocking(fd.int());
 
                 if (this.nonblocking and this.file_type == .pipe) {
                     this.file_type = .nonblocking_pipe;

src/bun.zig

@@ -515,6 +515,13 @@ pub fn ensureNonBlocking(fd: anytype) void {
     _ = std.os.fcntl(fd, std.os.F.SETFL, current | std.os.O.NONBLOCK) catch 0;
 }
 
+pub fn isNonBlocking(fd: anytype) bool {
+    if (Environment.isWindows) {
+        @compileError("isNonBlocking() is not supported on Windows");
+    }
+    return (std.os.fcntl(fd, std.os.F.GETFL, 0) catch 0 & std.os.O.NONBLOCK) == std.os.O.NONBLOCK;
+}
+
 const global_scope_log = sys.syslog;
 pub fn isReadable(fd: FileDescriptor) PollFlag {
     if (comptime Environment.isWindows) {

test/regression/issue/10080.test.ts

@@ -0,0 +1,33 @@
+import { test, expect } from "bun:test";
+import { bunEnv, bunExe, isPosix } from "harness";
+import { tmpdir } from "os";
+import { join } from "path";
+
+test.if(isPosix)(
+  "10080 - ensure blocking stdio is treated as such in FileReader",
+  async () => {
+    const expected = "foobar\n";
+    const filename = join(tmpdir(), "bun.test.stream." + Date.now() + ".js");
+    const contents = "for await (const line of console) {console.log(`foo${line}`)}";
+    await Bun.write(filename, contents);
+    const shellCommand = `exec &> >(${bunExe()} ${filename}); echo "bar"; while read -r line; do echo $line; done`;
+
+    const proc = Bun.spawn(["bash", "-c", shellCommand], {
+      stdin: "inherit",
+      stdout: "pipe",
+      stderr: "inherit",
+      env: bunEnv,
+    });
+    const { value } = await proc.stdout.getReader().read();
+    const output = new TextDecoder().decode(value);
+    if (output !== expected) {
+      expect(output).toEqual(expected);
+      throw new Error("Output didn't match!\n");
+    }
+
+    proc.kill(9);
+    await proc.exited;
+    expect(proc.killed).toBeTrue();
+  },
+  1000,
+);