From baf3123c1c849f7fb909d96c8ad0b1ead2bc4128 Mon Sep 17 00:00:00 2001
From: Zhao Zhili <zhilizhao@tencent.com>
Date: Mon, 24 Jun 2024 12:47:43 +0800
Subject: [PATCH] avutil/executor: Allowing thread_count be zero

Before the patch, disable threads support at configure/build time
was the only method to force zero thread in executor. However,
it's common practice for libavcodec to run on caller's thread when
user specify thread number to one. And for WASM environment, whether
threads are supported needs to be detected at runtime. So executor
should support zero thread at runtime.

A single thread executor can be useful, e.g., to handle network
protocol. So we can't take thread_count one as zero thread, which
disabled a valid usercase.

Other libraries take -threads 0 to mean auto. Executor as a low
level utils doesn't do cpu detect. So take thread_count zero as
zero thread, literally.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
---
 libavutil/executor.c | 28 ++++++++++++++++++----------
 libavutil/executor.h |  2 +-
 2 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/libavutil/executor.c b/libavutil/executor.c
index 26691fe157..fb20104b58 100644
--- a/libavutil/executor.c
+++ b/libavutil/executor.c
@@ -82,9 +82,11 @@ static int run_one_task(AVExecutor *e, void *lc)
         /* nothing */;
     if (*prev) {
         AVTask *t = remove_task(prev, *prev);
-        ff_mutex_unlock(&e->lock);
+        if (e->thread_count > 0)
+            ff_mutex_unlock(&e->lock);
         cb->run(t, lc, cb->user_data);
-        ff_mutex_lock(&e->lock);
+        if (e->thread_count > 0)
+            ff_mutex_lock(&e->lock);
         return 1;
     }
     return 0;
@@ -146,14 +148,17 @@ AVExecutor* av_executor_alloc(const AVTaskCallbacks *cb, int thread_count)
         return NULL;
     e->cb = *cb;
 
-    e->local_contexts = av_calloc(thread_count, e->cb.local_context_size);
+    e->local_contexts = av_calloc(FFMAX(thread_count, 1), e->cb.local_context_size);
     if (!e->local_contexts)
         goto free_executor;
 
-    e->threads = av_calloc(thread_count, sizeof(*e->threads));
+    e->threads = av_calloc(FFMAX(thread_count, 1), sizeof(*e->threads));
     if (!e->threads)
         goto free_executor;
 
+    if (!thread_count)
+        return e;
+
     has_lock = !ff_mutex_init(&e->lock, NULL);
     has_cond = !ff_cond_init(&e->cond, NULL);
 
@@ -175,9 +180,12 @@ free_executor:
 
 void av_executor_free(AVExecutor **executor)
 {
+    int thread_count;
+
     if (!executor || !*executor)
         return;
-    executor_free(*executor, 1, 1);
+    thread_count = (*executor)->thread_count;
+    executor_free(*executor, thread_count, thread_count);
     *executor = NULL;
 }
 
@@ -195,9 +203,9 @@ void av_executor_execute(AVExecutor *e, AVTask *t)
     ff_cond_signal(&e->cond);
     ff_mutex_unlock(&e->lock);
 
-#if !HAVE_THREADS
-    // We are running in a single-threaded environment, so we must handle all tasks ourselves
-    while (run_one_task(e, e->local_contexts))
-        /* nothing */;
-#endif
+    if (!e->thread_count || !HAVE_THREADS) {
+        // We are running in a single-threaded environment, so we must handle all tasks ourselves
+        while (run_one_task(e, e->local_contexts))
+            /* nothing */;
+    }
 }
diff --git a/libavutil/executor.h b/libavutil/executor.h
index c602bcb613..0eb21c10c8 100644
--- a/libavutil/executor.h
+++ b/libavutil/executor.h
@@ -46,7 +46,7 @@ typedef struct AVTaskCallbacks {
 /**
  * Alloc executor
  * @param callbacks callback structure for executor
- * @param thread_count worker thread number
+ * @param thread_count worker thread number, 0 for run on caller's thread directly
  * @return return the executor
  */
 AVExecutor* av_executor_alloc(const AVTaskCallbacks *callbacks, int thread_count);