qemu: Don't report false error from MigrateFinish

virDomainMigrateFinish* APIs were unfortunately designed to return the
pointer to the domain on destination and NULL on error. This looks OK in
normal cases but the same API is also called when we know migration
failed and thus we expect Finish to return NULL even if it actually did
all it was supposed to do without any error. The call is defined to
return nonnull domain pointer over RPC, which means returning NULL will
always result in an error being send. If this was not in fact an error,
the API itself wouldn't set anything to the thread local virError, which
makes the RPC layer come up with it's own "Library function returned
error but did not set virError" error.

This is quite confusing and also hard to detect by the caller. This
patch adds a special error code which can be used to check that Finish
successfully aborted migration.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

include/libvirt/virterror.h

@@ -307,6 +307,7 @@ typedef enum {
     VIR_ERR_CPU_INCOMPATIBLE = 91,      /* given CPU is incompatible with host
                                            CPU*/
     VIR_ERR_XML_INVALID_SCHEMA = 92,    /* XML document doesn't validate against schema */
+    VIR_ERR_MIGRATE_FINISH_OK = 93,     /* Finish API succeeded but it is expected to return NULL */
 } virErrorNumber;
 
 /**

src/qemu/qemu_migration.c

@@ -5751,6 +5751,12 @@ qemuMigrationFinish(virQEMUDriverPtr driver,
     }
     virObjectUnref(caps);
     virObjectUnref(cfg);
+
+    /* Set a special error if Finish is expected to return NULL as a result of
+     * successful call with retcode != 0
+     */
+    if (retcode != 0 && !dom && !virGetLastError())
+        virReportError(VIR_ERR_MIGRATE_FINISH_OK, NULL);
     return dom;
 }
 

src/util/virerror.c

@@ -1369,6 +1369,9 @@ virErrorMsg(virErrorNumber error, const char *info)
             else
                 errmsg = _("XML document failed to validate against schema: %s");
             break;
+        case VIR_ERR_MIGRATE_FINISH_OK:
+            errmsg = _("migration successfully aborted");
+            break;
     }
     return errmsg;
 }