g_source_destroy() only removes the GSource from the GMainContext it's
attached to, if any. It does not free it.

Use g_source_unref() instead so that the AioContext (which embeds a
GSource) is freed. There is no need to call g_source_destroy() in
aio_context_new() because the GSource isn't attached to a GMainContext
yet.

aio_ctx_finalize() expects everything to be set up already, so introduce
the new ctx->initialized boolean and do nothing when called with
!initialized. This also requires moving aio_context_setup() down after
event_notifier_init() since aio_ctx_finalize() won't release any
resources that aio_context_setup() acquired.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
v5:
- Added comments explaining how to clean up resources in error paths [Kevin]
v2:
- Fix spacing in aio_ctx_finalize() argument list [Eric]
---
 include/block/aio.h |  3 +++
 util/async.c        | 31 ++++++++++++++++++++++++++++---
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/include/block/aio.h b/include/block/aio.h
index 1657740a0e..2760f308f5 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -291,6 +291,9 @@ struct AioContext {
     gpointer epollfd_tag;
 
     const FDMonOps *fdmon_ops;
+
+    /* Was aio_context_new() successful? */
+    bool initialized;
 };
 
 /**
diff --git a/util/async.c b/util/async.c
index cb72ad3777..7d06ff98f3 100644
--- a/util/async.c
+++ b/util/async.c
@@ -366,12 +366,16 @@ aio_ctx_dispatch(GSource     *source,
 }
 
 static void
-aio_ctx_finalize(GSource     *source)
+aio_ctx_finalize(GSource *source)
 {
     AioContext *ctx = (AioContext *) source;
     QEMUBH *bh;
     unsigned flags;
 
+    if (!ctx->initialized) {
+        return;
+    }
+
     thread_pool_free_aio(ctx->thread_pool);
 
 #ifdef CONFIG_LINUX_AIO
@@ -579,16 +583,35 @@ AioContext *aio_context_new(Error **errp)
     int ret;
     AioContext *ctx;
 
+    /*
+     * ctx is freed by g_source_unref() (e.g. aio_context_unref()). ctx's
+     * resources are freed as follows:
+     *
+     * 1. By aio_ctx_finalize() after aio_context_new() has returned and set
+     *    ->initialized = true.
+     *
+     * 2. By manual cleanup code in this function's error paths before goto
+     *    fail.
+     *
+     * Be careful to free resources in both cases!
+     */
     ctx = (AioContext *) g_source_new(&aio_source_funcs, sizeof(AioContext));
     QSLIST_INIT(&ctx->bh_list);
     QSIMPLEQ_INIT(&ctx->bh_slice_list);
-    aio_context_setup(ctx);
 
     ret = event_notifier_init(&ctx->notifier, false);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Failed to initialize event notifier");
         goto fail;
     }
+
+    /*
+     * Resources cannot easily be freed manually after aio_context_setup(). If
+     * you add any new resources to AioContext, it's probably best to acquire
+     * them before aio_context_setup().
+     */
+    aio_context_setup(ctx);
+
     g_source_set_can_recurse(&ctx->source, true);
     qemu_lockcnt_init(&ctx->list_lock);
 
@@ -622,9 +645,11 @@ AioContext *aio_context_new(Error **errp)
 
     register_aiocontext(ctx);
 
+    ctx->initialized = true;
+
     return ctx;
 fail:
-    g_source_destroy(&ctx->source);
+    g_source_unref(&ctx->source);
     return NULL;
 }
 
-- 
2.51.1