Re: [PATCH 0/1] trace2: fix tracing when NO_PTHREADS is defined

[Jeff King <peff@peff.net>]

On Wed, May 22, 2019 at 09:23:39AM -0400, Jeff Hostetler wrote:

> I was wondering about that too as the proper long term solution.
> We would need (as the discussion suggests [1]) to properly
> respect/represent the pthread_key_t argument.
> 
> For now, I've guarded my usage of pthread_getspecific() in the trace2
> (similar to what index-pack does), so its not urgent that we update it.
> And I'd rather we take this simple trace2 fix now and not try to combine
> it with fixes for the pthread macros.  Especially now as we're in the RC
> cycle for 2.22.

Yeah, I think that makes sense.

> I'll make a note to revisit the pthread code after 2.22.

For fun, here's a constant-time zero-allocation implementation that I
came up with. It passes t0211 with NO_PTHREADS, but I didn't test it
beyond that.

diff --git a/thread-utils.h b/thread-utils.h
index 4961487ed9..f466215742 100644
--- a/thread-utils.h
+++ b/thread-utils.h
@@ -18,7 +18,7 @@
 #define pthread_t int
 #define pthread_mutex_t int
 #define pthread_cond_t int
-#define pthread_key_t int
+#define pthread_key_t git_pthread_key_t
 
 #define pthread_mutex_init(mutex, attr) dummy_pthread_init(mutex)
 #define pthread_mutex_lock(mutex)
@@ -31,16 +31,49 @@
 #define pthread_cond_broadcast(cond)
 #define pthread_cond_destroy(cond)
 
-#define pthread_key_create(key, attr) dummy_pthread_init(key)
-#define pthread_key_delete(key)
+#define pthread_key_create(key, destroy) git_pthread_key_create(key, destroy)
+#define pthread_key_delete(key) git_pthread_key_delete(key)
 
 #define pthread_create(thread, attr, fn, data) \
 	dummy_pthread_create(thread, attr, fn, data)
 #define pthread_join(thread, retval) \
 	dummy_pthread_join(thread, retval)
 
-#define pthread_setspecific(key, data)
-#define pthread_getspecific(key) NULL
+#define pthread_setspecific(key, data) git_pthread_setspecific(key, data)
+#define pthread_getspecific(key) git_pthread_getspecific(key)
+
+typedef struct {
+	void *data;
+	/* extra indirection because setspecific is passed key by value */
+	void **vdata;
+} git_pthread_key_t;
+
+static inline int git_pthread_key_create(git_pthread_key_t *key,
+					 void (*destroy)(void *))
+{
+	key->data = NULL;
+	key->vdata = &key->data;
+	/* We don't use this; alternatively we could all via atexit(). */
+	if (destroy)
+		BUG("NO_PTHREADS does not support pthread key destructors");
+	return 0;
+}
+
+static inline int git_pthread_key_delete(git_pthread_key_t key)
+{
+	/* noop */
+	return 0;
+}
+
+static inline void git_pthread_setspecific(git_pthread_key_t key, void *data)
+{
+	*(key.vdata) = data;
+}
+
+static inline void *git_pthread_getspecific(git_pthread_key_t key)
+{
+	return key.data;
+}
 
 int dummy_pthread_create(pthread_t *pthread, const void *attr,
 			 void *(*fn)(void *), void *data);