From e10eb8a2fd3b0e1cb325c353473a65a8bb04403a Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <m@mitchellh.com>
Date: Sat, 27 Sep 2025 12:34:57 -0700
Subject: [PATCH] build: limit cpu affinity to 32 cpus on Linux (#8925)

Related to #8924

Zig currenly has a bug where it crashes when compiling Ghostty on
systems with more than 32 cpus (See the linked issue for the gory
details). As a temporary hack, use `sched_setaffinity` on Linux systems
to limit the compile to the first 32 cores. Note that this affects the
build only. The resulting Ghostty executable is not limited in any way.

This is a more general fix than wrapping the Zig compiler with
`taskset`. First of all, it requires no action from the user or
packagers. Second, it will be easier for us to remove once the upstream
Zig bug is fixed.
---
 build.zig | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/build.zig b/build.zig
index 38cfd0e56..a2fb25e96 100644
--- a/build.zig
+++ b/build.zig
@@ -8,7 +8,13 @@ comptime {
 }
 
 pub fn build(b: *std.Build) !void {
-    // This defines all the available build options (e.g. `-D`).
+    // Works around a Zig but still present in 0.15.1. Remove when fixed.
+    // https://github.com/ghostty-org/ghostty/issues/8924
+    try limitCoresForZigBug();
+
+    // This defines all the available build options (e.g. `-D`). If you
+    // want to know what options are available, you can run `--help` or
+    // you can read `src/build/Config.zig`.
     const config = try buildpkg.Config.init(b);
     const test_filter = b.option(
         []const u8,
@@ -258,3 +264,13 @@ pub fn build(b: *std.Build) !void {
         try translations_step.addError("cannot update translations when i18n is disabled", .{});
     }
 }
+
+// WARNING: Remove this when https://github.com/ghostty-org/ghostty/issues/8924 is resolved!
+// Limit ourselves to 32 cpus on Linux because of an upstream Zig bug.
+fn limitCoresForZigBug() !void {
+    if (comptime builtin.os.tag != .linux) return;
+    const pid = std.os.linux.getpid();
+    var set: std.bit_set.ArrayBitSet(usize, std.os.linux.CPU_SETSIZE * 8) = .initEmpty();
+    for (0..32) |cpu| set.set(cpu);
+    try std.os.linux.sched_setaffinity(pid, &set.masks);
+}