postcopy: Recover block devices on early failure

An early postcopy failure can be recovered from as long as we know
we haven't sent the command to run the destination.
We have to undo the bdrv_inactivate_all by calling
bdrv_invalidate_cache_all

Note that I'm not using ms->block_inactive because once we've
sent the postcopy package we dont want anything else to try
and recover the block storage on the source; the destination
might have started writing to it.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170202155909.31784-3-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/migration.c

@@ -1601,6 +1601,7 @@ static int postcopy_start(MigrationState *ms, bool *old_vm_running)
     QIOChannelBuffer *bioc;
     QEMUFile *fb;
     int64_t time_at_stop = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
+    bool restart_block = false;
     migrate_set_state(&ms->state, MIGRATION_STATUS_ACTIVE,
                       MIGRATION_STATUS_POSTCOPY_ACTIVE);
 
@@ -1620,6 +1621,7 @@ static int postcopy_start(MigrationState *ms, bool *old_vm_running)
     if (ret < 0) {
         goto fail;
     }
+    restart_block = true;
 
     /*
      * Cause any non-postcopiable, but iterative devices to
@@ -1676,6 +1678,18 @@ static int postcopy_start(MigrationState *ms, bool *old_vm_running)
 
     /* <><> end of stuff going into the package */
 
+    /* Last point of recovery; as soon as we send the package the destination
+     * can open devices and potentially start running.
+     * Lets just check again we've not got any errors.
+     */
+    ret = qemu_file_get_error(ms->to_dst_file);
+    if (ret) {
+        error_report("postcopy_start: Migration stream errored (pre package)");
+        goto fail_closefb;
+    }
+
+    restart_block = false;
+
     /* Now send that blob */
     if (qemu_savevm_send_packaged(ms->to_dst_file, bioc->data, bioc->usage)) {
         goto fail_closefb;
@@ -1713,6 +1727,17 @@ static int postcopy_start(MigrationState *ms, bool *old_vm_running)
 fail:
     migrate_set_state(&ms->state, MIGRATION_STATUS_POSTCOPY_ACTIVE,
                           MIGRATION_STATUS_FAILED);
+    if (restart_block) {
+        /* A failure happened early enough that we know the destination hasn't
+         * accessed block devices, so we're safe to recover.
+         */
+        Error *local_err = NULL;
+
+        bdrv_invalidate_cache_all(&local_err);
+        if (local_err) {
+            error_report_err(local_err);
+        }
+    }
     qemu_mutex_unlock_iothread();
     return -1;
 }