From 07b7e56740958fd2544295e0676e57ed29e4b849 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Tue, 18 Apr 2023 13:08:30 -0700
Subject: [PATCH] Cache: fix multi-process race condition on macOS

This fixes `.INVAL => unreachable` being triggered by the cache system
on macOS when multiple processes race to create the same compilation.
The problem is that when two processes race to create a file, it
sometimes returns ENOENT even though that error code is nonsensical for
this situation.

Commit 2b0929929d67e222ca6a9523a3a594ed456c4a51 purportedly solved this,
but it did not open the file with write permissions, leading to the
EINVAL panic later on. This commit remedies the situation by introducing
a loop and simply retrying when the ENOENT occurs.
---
 lib/std/Build/Cache.zig | 41 +++++++++++++++++++++++------------------
 1 file changed, 23 insertions(+), 18 deletions(-)

diff --git a/lib/std/Build/Cache.zig b/lib/std/Build/Cache.zig
index f8c83451cb01..3b67f4b24cb7 100644
--- a/lib/std/Build/Cache.zig
+++ b/lib/std/Build/Cache.zig
@@ -434,24 +434,29 @@ pub const Manifest = struct {
                 }
             }
         } else {
-            if (self.cache.manifest_dir.createFile(&manifest_file_path, .{
-                .read = true,
-                .truncate = false,
-                .lock = .Exclusive,
-                .lock_nonblocking = self.want_shared_lock,
-            })) |manifest_file| {
-                self.manifest_file = manifest_file;
-                self.have_exclusive_lock = true;
-            } else |err| switch (err) {
-                // There are no dir components, so you would think that this was
-                // unreachable, however we have observed on macOS two processes racing
-                // to do openat() with O_CREAT manifest in ENOENT.
-                error.WouldBlock, error.FileNotFound => {
-                    self.manifest_file = try self.cache.manifest_dir.openFile(&manifest_file_path, .{
-                        .lock = .Shared,
-                    });
-                },
-                else => |e| return e,
+            while (true) {
+                if (self.cache.manifest_dir.createFile(&manifest_file_path, .{
+                    .read = true,
+                    .truncate = false,
+                    .lock = .Exclusive,
+                    .lock_nonblocking = self.want_shared_lock,
+                })) |manifest_file| {
+                    self.manifest_file = manifest_file;
+                    self.have_exclusive_lock = true;
+                    break;
+                } else |err| switch (err) {
+                    error.WouldBlock => {
+                        self.manifest_file = try self.cache.manifest_dir.openFile(&manifest_file_path, .{
+                            .lock = .Shared,
+                        });
+                        break;
+                    },
+                    // There are no dir components, so you would think that this was
+                    // unreachable, however we have observed on macOS two processes racing
+                    // to do openat() with O_CREAT manifest in ENOENT.
+                    error.FileNotFound => continue,
+                    else => |e| return e,
+                }
             }
         }