fix(posix): use fcntl to check if a fd is nonblocking (#10080)

The FileReader is responsible for deciding what sort of file descriptor
has been passed to it. On MacOS, when stdin is provided to it the
FileReader assumes that it is nonblocking even if the descriptor is not
as it passes the pollable and not a tty check, this then results in
hangs as the reader expects `EAGAIN`.

To fix this we now check the file descriptor to see if it is nonblocking
rather than using assumptions.

src/bun.js/webcore/streams.zig

@@ -3513,7 +3513,7 @@ 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

@@ -526,6 +526,10 @@ 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 {
+    (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 +1,29 @@
+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);