From 46658257f458b7c3c95d7e10cbde85403f7bdb44 Mon Sep 17 00:00:00 2001
From: breakmit <breakmit@noreply.codeberg.org>
Date: Fri, 6 Mar 2026 04:51:28 +0100
Subject: [PATCH] Io.Threaded.spawnPosix: implement passing file descriptors as
 stdio (#31379)

`std.process.spawn`: remove the TODO for nonblocking file stdio and document the behavior.

Fix a bug in Io.Uring.dup2 where the function does not return on success

Reviewed-on: https://codeberg.org/ziglang/zig/pulls/31379
Reviewed-by: Andrew Kelley <andrew@ziglang.org>
Co-authored-by: breakmit <breakmit@noreply.codeberg.org>
Co-committed-by: breakmit <breakmit@noreply.codeberg.org>
---
 lib/std/Io/Dispatch.zig | 5 +----
 lib/std/Io/Threaded.zig | 2 +-
 lib/std/Io/Uring.zig    | 7 ++-----
 lib/std/process.zig     | 6 ++++++
 4 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/lib/std/Io/Dispatch.zig b/lib/std/Io/Dispatch.zig
index 1a1d644276..947e8e3070 100644
--- a/lib/std/Io/Dispatch.zig
+++ b/lib/std/Io/Dispatch.zig
@@ -4404,10 +4404,7 @@ fn setUpChildIo(
         .close => closeFd(std_fileno),
         .inherit => {},
         .ignore => try ev.dup2(dev_null_fd, std_fileno),
-        .file => |file| {
-            if (file.flags.nonblocking) @panic("TODO implement setUpChildIo when nonblocking file is used");
-            try ev.dup2(file.handle, std_fileno);
-        },
+        .file => |file| try ev.dup2(file.handle, std_fileno),
     }
 }
 
diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index 83bda2c865..b1804c4605 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -15659,7 +15659,7 @@ fn setUpChildIo(stdio: process.SpawnOptions.StdIo, pipe_fd: i32, std_fileno: i32
         .close => closeFd(std_fileno),
         .inherit => {},
         .ignore => try dup2(dev_null_fd, std_fileno),
-        .file => @panic("TODO implement setUpChildIo when file is used"),
+        .file => |file| try dup2(file.handle, std_fileno),
     }
 }
 
diff --git a/lib/std/Io/Uring.zig b/lib/std/Io/Uring.zig
index 0f4f64334f..6031b1b298 100644
--- a/lib/std/Io/Uring.zig
+++ b/lib/std/Io/Uring.zig
@@ -4550,10 +4550,7 @@ fn setUpChildIo(
         .close => _ = linux.close(std_fileno),
         .inherit => {},
         .ignore => try dup2(sync, dev_null_fd, std_fileno),
-        .file => |file| {
-            if (file.flags.nonblocking) @panic("TODO implement setUpChildIo when nonblocking file is used");
-            try dup2(sync, file.handle, std_fileno);
-        },
+        .file => |file| try dup2(sync, file.handle, std_fileno),
     }
 }
 
@@ -4565,7 +4562,7 @@ pub fn dup2(sync: *CancelRegion.Sync, old_fd: fd_t, new_fd: fd_t) DupError!void
     while (true) {
         try sync.cancel_region.await(.nothing);
         switch (linux.errno(linux.dup2(old_fd, new_fd))) {
-            .SUCCESS => {},
+            .SUCCESS => return,
             .BUSY, .INTR => {},
             .INVAL => |err| return errnoBug(err), // invalid parameters
             .BADF => |err| return errnoBug(err), // use after free
diff --git a/lib/std/process.zig b/lib/std/process.zig
index 1be0cd628d..701eb434b5 100644
--- a/lib/std/process.zig
+++ b/lib/std/process.zig
@@ -409,6 +409,12 @@ pub const SpawnOptions = struct {
         /// Inherit the corresponding stream from the parent process.
         inherit,
         /// Pass an already open file from the parent to the child.
+        ///
+        /// Nonblocking mode will be kept in the child process if present. This is
+        /// likely not supported by the child process. For example:
+        /// - Zig's std.Io.File.stdout() assumes blocking mode
+        /// - Rust explicity documents that nonblocking stdio may cause panics
+        /// - C++ standard streams do not support nonblocking file descriptors
         file: File,
         /// Pass a null stream to the child process by opening "/dev/null" on POSIX
         /// and "NUL" on Windows.