[PATCH 4/5] 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 without yielding in the main
thread causing two unwanted consequences:

- CPU will spin 100% waiting for the rest bytes until it reaches 4
- (more importantly..) Main thread is stuck during this process as the qio
  operation won't really yield the coroutine

Fix it by consuming the bytes that arrived.

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3889
Cc: Daniel P. Berrangé <[email protected]>
Reported-by: Feifan Qian <[email protected]>
Signed-off-by: Peter Xu <[email protected]>
---
 migration/channel.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/migration/channel.c b/migration/channel.c
index 1e2935f926..28fe1d2906 100644
--- a/migration/channel.c
+++ b/migration/channel.c
@@ -294,11 +294,31 @@ int migration_channel_read_peek(QIOChannel *ioc,
             return -1;
         }
 
-        if (len == buflen) {
+        if (len == iov.iov_len) {
             break;
-        }
+        } else if (len == 0) {
+            qio_channel_wait_cond(ioc, G_IO_IN);
+        } else {
+            ssize_t received = len;
 
-        qio_channel_wait_cond(ioc, G_IO_IN);
+            /*
+             * Partially arrived, read out to make qio_channel_wait_cond()
+             * won't return immediately, causing an unwanted spin on this
+             * CPU.
+             */
+            iov.iov_len = len;
+            len = qio_channel_readv_full(ioc, &iov, 1, NULL, NULL, 0, errp);
+            /*
+             * QIO_CHANNEL_ERR_BLOCK also shouldn't happen, due to the
+             * prior peek just happened.  We should be pretty sure we will
+             * read what we peeked, or the channel was broken.
+             */
+            if (len != received) {
+                return -1;
+            }
+            iov.iov_base += received;
+            iov.iov_len = buflen - received;
+        }
     }
 
     return 0;
-- 
2.54.0