std.Thread.Pool: don't hold the job closure while blocking on IPC read

lib/std/Thread/Pool.zig

@@ -310,58 +310,65 @@ pub fn spawn(pool: *Pool, comptime func: anytype, args: anytype) void {
     pool.cond.signal();
 }
 
-fn worker(pool: *Pool) void {
-    var trash_buf: [1]u8 = undefined;
-    var connection: ?std.posix.fd_t = null;
-    defer if (connection) |fd| std.posix.close(fd);
+fn acquireThreadToken(job_server_options: Options.JobServer, fd_ptr: *std.posix.fd_t) void {
+    if (fd_ptr.* >= 0) return;
 
-    pool.mutex.lock();
-    defer pool.mutex.unlock();
-
-    while (true) {
-        while (pool.run_queue.popFirst()) |run_node| {
-            // Temporarily unlock the mutex in order to execute the run_node.
-            pool.mutex.unlock();
-            defer pool.mutex.lock();
-
-            if (connection == null) switch (pool.job_server_options) {
+    switch (job_server_options) {
         .abstain => {},
-                .connect, .host => |addr| lock: {
+        .connect, .host => |addr| {
             const sockfd = std.posix.socket(
                 std.posix.AF.UNIX,
                 std.posix.SOCK.STREAM | std.posix.SOCK.CLOEXEC,
                 0,
             ) catch |err| {
                 std.log.debug("failed to make socket: {s}", .{@errorName(err)});
-                        break :lock;
+                return;
             };
-                    connection = sockfd;
+            fd_ptr.* = sockfd;
 
             std.posix.connect(sockfd, &addr.any, addr.getOsSockLen()) catch |err| {
                 std.log.debug("failed to connect: {s}", .{@errorName(err)});
-                        break :lock;
+                return;
             };
 
+            var trash_buf: [1]u8 = undefined;
             _ = std.posix.read(sockfd, &trash_buf) catch |err| {
                 std.log.debug("failed to read: {s}", .{@errorName(err)});
-                        break :lock;
+                return;
             };
         },
-            };
-
-            const runFn = run_node.data.runFn;
-            runFn(&run_node.data);
+    }
 }
 
-        // Stop executing instead of waiting if the thread pool is no longer running.
-        if (pool.end_flag)
-            break;
-
-        if (connection) |fd| {
+fn releaseThreadToken(fd_ptr: *std.posix.fd_t) void {
+    const fd = fd_ptr.*;
+    if (fd >= 0) {
         std.posix.close(fd);
-            connection = null;
+        fd_ptr.* = -1;
+    }
 }
 
+fn worker(pool: *Pool) void {
+    var connection: std.posix.fd_t = -1;
+    defer releaseThreadToken(&connection);
+
+    pool.mutex.lock();
+    defer pool.mutex.unlock();
+
+    while (true) {
+        const work_available = pool.run_queue.first != null;
+        if (work_available) {
+            pool.mutex.unlock();
+            defer pool.mutex.lock();
+            acquireThreadToken(pool.job_server_options, &connection);
+        }
+        while (pool.run_queue.popFirst()) |run_node| {
+            pool.mutex.unlock();
+            defer pool.mutex.lock();
+            run_node.data.runFn(&run_node.data);
+        }
+        if (pool.end_flag) return;
+        releaseThreadToken(&connection);
         pool.cond.wait(&pool.mutex);
     }
 }