Commit 89f15c1362 for qemu.org

commit 89f15c1362970150bf5711cdb84b9742c0db0cd8
Author: Peter Xu <peterx@redhat.com>
Date:   Wed Aug 12 08:43:27 2026 -0400

    migration: Fix rare hang of migration_channel_read_peek()

    In an unlikely case, when a migration stream is attached to the destination
    QEMU and only send <4 bytes to the channel as magic, it's possible that
    migration_channel_read_peek() may spin forever.

    Fix it by adding a manual sleep for partial read.

    Since the path isn't attached to a coroutine, it means when partial read
    happens, there's yet not much we can do but hang the main thread, it will
    happen even for len==0 case.  It means monitors can hang due to this,
    either partial read or no data arrived (but connection established).

    Leave this for later, the hope is this is extremely rare in production.

    Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3889
    Reported-by: Feifan Qian <bea1e@proton.me>
    Cc: Daniel P. Berrangé <berrange@redhat.com>
    Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
    Link: https://lore.kernel.org/r/20260812124327.2572363-1-peterx@redhat.com
    Signed-off-by: Peter Xu <peterx@redhat.com>

diff --git a/migration/channel.c b/migration/channel.c
index 1e2935f926..266ae8f776 100644
--- a/migration/channel.c
+++ b/migration/channel.c
@@ -296,9 +296,16 @@ int migration_channel_read_peek(QIOChannel *ioc,

         if (len == buflen) {
             break;
+        } else if (len == QIO_CHANNEL_ERR_BLOCK) {
+            qio_channel_wait_cond(ioc, G_IO_IN);
+        } else {
+            /*
+             * When partially ready, we can't use qio_channel_wait_cond()
+             * because it will return immediately.  Apply a manual wait.
+             */
+            assert(!qemu_in_coroutine());
+            g_usleep(1000);
         }
-
-        qio_channel_wait_cond(ioc, G_IO_IN);
     }

     return 0;