block/curl: fix concurrent completion handling

curl_multi_check_completion would bail upon the first completed
transfer even if more completion messages were available thus leaving
some in flight IOs stuck.

Rework a bit the loop to make the iterations clearer and drop the breaks.

The original hang can be somewhat reproduced with the following command:

$ qemu-img convert -p -m 16 -O qcow2 -c --image-opts \
  'file.driver=https,file.url=https://scaleway.testdebit.info/10G.iso,file.readahead=1M' \
  /tmp/test.qcow2

Fixes: 1f2cead324 ("curl: Ensure all informationals are checked for completion")
Cc: qemu-stable@nongnu.org
Signed-off-by: Antoine Damhet <adamhet@scaleway.com>
Message-ID: <20260212162730.440855-2-adamhet@scaleway.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/curl.c

@@ -324,17 +324,11 @@ curl_find_buf(BDRVCURLState *s, uint64_t start, uint64_t len, CURLAIOCB *acb)
 static void curl_multi_check_completion(BDRVCURLState *s)
 {
     int msgs_in_queue;
+    CURLMsg *msg;
 
     /* Try to find done transfers, so we can free the easy
      * handle again. */
-    for (;;) {
-        CURLMsg *msg;
-        msg = curl_multi_info_read(s->multi, &msgs_in_queue);
-
-        /* Quit when there are no more completions */
-        if (!msg)
-            break;
-
+    while ((msg = curl_multi_info_read(s->multi, &msgs_in_queue))) {
         if (msg->msg == CURLMSG_DONE) {
             int i;
             CURLState *state = NULL;
@@ -397,7 +391,6 @@ static void curl_multi_check_completion(BDRVCURLState *s)
             }
 
             curl_clean_state(state);
-            break;
         }
     }
 }