1 files changed, 18 insertions, 3 deletions

diff --git a/src/gui/gui.zig b/src/gui/gui.zig
index d935b5fd..b34b314d 100644
--- a/src/gui/gui.zig
+++ b/src/gui/gui.zig
@@ -937,6 +937,8 @@ const Queue = struct {
     completion_len: usize = 0,
 
     fn lock(q: *Queue) void {
+        // unreachable failures: pthread calls on this queue's own initialised, non-recursive
+        // mutex and condition fail only on misuse (EINVAL, EDEADLK), never on input.
         std.debug.assert(libc.pthread_mutex_lock(&q.mutex) == .SUCCESS);
     }
 
@@ -1061,9 +1063,19 @@ const Queue = struct {
     }
 
     fn take(q: *Queue) MessageBatch {
+        var batch: MessageBatch = .{};
+        q.takeInto(&batch);
+        return batch;
+    }
+
+    /// `take` into a batch the caller keeps: an idle wakeup copies nothing
+    /// (the batch is some 20 KB, and was built and returned by value each
+    /// time).
+    fn takeInto(q: *Queue, batch: *MessageBatch) void {
+        batch.len = 0;
         q.lock();
         defer q.unlock();
-        var batch: MessageBatch = .{};
+        if (q.len == 0 and !q.files_changed and q.completion_len == 0) return;
         while (q.len > 0) {
             batch.items[batch.len] = q.items[q.head];
             batch.len += 1;
@@ -1080,7 +1092,6 @@ const Queue = struct {
         batch.len += q.completion_len;
         q.completion_len = 0;
         std.debug.assert(libc.pthread_cond_broadcast(&q.space) == .SUCCESS);
-        return batch;
     }
 
     fn discardCompletions(q: *Queue) void {
@@ -3999,6 +4010,8 @@ const Shell = struct {
     ptys: *[pardes.MAX_PANES]?Pty,
     gens: *[pardes.MAX_PANES]u32,
     queue: *Queue,
+    /// What drainQueue takes each wakeup, kept rather than rebuilt.
+    batch: MessageBatch = .{},
     pipe_tasks: *PipeTasks,
     inotify_fd: c_int,
     watches: *file_watch.Table,
@@ -4153,7 +4166,8 @@ const Shell = struct {
         s.reconcilePtys();
         // After this batch's output, which an exit is told behind.
         defer s.takeExits();
-        var msgs = s.queue.take();
+        s.queue.takeInto(&s.batch);
+        const msgs = &s.batch;
         var check_files = false;
         for (msgs.slice()) |m| switch (m) {
             .output => |o| {
@@ -8322,6 +8336,7 @@ fn captureFrame(g: *Gui, gpa: std.mem.Allocator, cmd: *c.SDL_GPUCommandBuffer, t
     const pixels: [*]const u8 = @ptrCast(mapped);
     try writeCapturePpm(g, gpa, pixels[0..size], sw, sh, "latest");
     var name: [24]u8 = undefined;
+    // unreachable below: "transition-" or "frame-" and a u32 fit 24
     if (transition) |frame| try writeCapturePpm(g, gpa, pixels[0..size], sw, sh, std.fmt.bufPrint(&name, "transition-{d}", .{frame}) catch unreachable);
     if (g.capture_series) try writeCapturePpm(g, gpa, pixels[0..size], sw, sh, std.fmt.bufPrint(&name, "frame-{d:0>5}", .{g.captured}) catch unreachable);
     g.captured +%= 1;