From e2a266e744adaa7ee84a512fdf4cea44686ca0b6 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Thu, 8 Jan 2026 16:55:51 -0800
Subject: [PATCH] Revert "std.process.Child: rewrite using concurrent"

This reverts commit 76e1ba8f490812c6e2ebf6f6becd89a71275d21e.
---
 lib/std/process.zig       |  1 -
 lib/std/process/Child.zig | 81 +++++++++++++++++++++++++++------------
 2 files changed, 56 insertions(+), 26 deletions(-)

diff --git a/lib/std/process.zig b/lib/std/process.zig
index b203838e3f..4a021879a5 100644
--- a/lib/std/process.zig
+++ b/lib/std/process.zig
@@ -455,7 +455,6 @@ pub fn spawnPath(io: Io, dir: Io.Dir, options: SpawnOptions) SpawnError!Child {
 
 pub const RunError = CurrentPathError || posix.ReadError || SpawnError || posix.PollError || error{
     StreamTooLong,
-    ConcurrencyUnavailable,
 };
 
 pub const RunOptions = struct {
diff --git a/lib/std/process/Child.zig b/lib/std/process/Child.zig
index e64f6106fa..fc31014520 100644
--- a/lib/std/process/Child.zig
+++ b/lib/std/process/Child.zig
@@ -125,15 +125,14 @@ pub fn wait(child: *Child, io: Io) WaitError!Term {
     return io.vtable.childWait(io.userdata, child);
 }
 
-pub const CollectOutputError = error{
-    StreamTooLong,
-    ConcurrencyUnavailable,
-} || Allocator.Error || Io.File.Reader.Error;
+pub const CollectOutputError = error{StreamTooLong} || Allocator.Error || Io.File.Reader.Error;
 
 pub const CollectOutputOptions = struct {
     stdout: *std.ArrayList(u8),
     stderr: *std.ArrayList(u8),
-    allocator: Allocator,
+    /// Used for `stdout` and `stderr`. If not provided, only the existing
+    /// capacity will be used.
+    allocator: ?Allocator = null,
     stdout_limit: Io.Limit = .unlimited,
     stderr_limit: Io.Limit = .unlimited,
 };
@@ -145,24 +144,56 @@ pub const CollectOutputOptions = struct {
 /// The process must have been started with stdout and stderr set to
 /// `process.SpawnOptions.StdIo.pipe`.
 pub fn collectOutput(child: *const Child, io: Io, options: CollectOutputOptions) CollectOutputError!void {
-    var stdout = try io.concurrent(collectStream, .{
-        io, options.allocator, child.stdout.?, options.stdout, options.stdout_limit,
-    });
-    defer stdout.cancel(io) catch {};
-
-    var stderr = try io.concurrent(collectStream, .{
-        io, options.allocator, child.stderr.?, options.stderr, options.stderr_limit,
-    });
-    defer stderr.cancel(io) catch {};
-
-    try stdout.await(io);
-    try stderr.await(io);
-}
-
-fn collectStream(io: Io, gpa: Allocator, file: File, list: *std.ArrayList(u8), limit: Io.Limit) CollectOutputError!void {
-    var fr = file.readerStreaming(io, &.{});
-    fr.interface.appendRemaining(gpa, list, limit) catch |err| switch (err) {
-        error.ReadFailed => return fr.err.?,
-        else => |e| return e,
-    };
+    const files: [2]Io.File = .{ child.stdout.?, child.stderr.? };
+    const lists: [2]*std.ArrayList(u8) = .{ options.stdout, options.stderr };
+    const limits: [2]Io.Limit = .{ options.stdout_limit, options.stderr_limit };
+    var dones: [2]bool = .{ false, false };
+    var reads: [2]Io.Operation = undefined;
+    var vecs: [2][1][]u8 = undefined;
+    while (true) {
+        for (&reads, &lists, &files, dones, &vecs) |*read, list, file, done, *vec| {
+            if (done) {
+                read.* = .noop;
+                continue;
+            }
+            if (options.allocator) |gpa| try list.ensureUnusedCapacity(gpa, 1);
+            const cap = list.unusedCapacitySlice();
+            if (cap.len == 0) return error.StreamTooLong;
+            vec[0] = cap;
+            read.* = .{ .file_read_streaming = .{
+                .file = file,
+                .data = vec,
+                .nonblocking = true,
+                .result = undefined,
+            } };
+        }
+        var all_done = true;
+        var any_canceled = false;
+        var other_err: (error{StreamTooLong} || Io.File.Reader.Error)!void = {};
+        io.vtable.operate(io.userdata, &reads);
+        for (&reads, &lists, &limits, &dones) |*read, list, limit, *done| {
+            if (done.*) continue;
+            const n = read.file_read_streaming.result catch |err| switch (err) {
+                error.Canceled => {
+                    any_canceled = true;
+                    continue;
+                },
+                error.WouldBlock => continue,
+                else => |e| {
+                    other_err = e;
+                    continue;
+                },
+            };
+            if (n == 0) {
+                done.* = true;
+            } else {
+                all_done = false;
+            }
+            list.items.len += n;
+            if (list.items.len > @intFromEnum(limit)) other_err = error.StreamTooLong;
+        }
+        if (any_canceled) return error.Canceled;
+        try other_err;
+        if (all_done) return;
+    }
 }