migrate: add migration blockers

This lets different subsystems register an Error that is thrown whenever
migration is attempted.  This works nicely because it gracefully supports
things like hotplug.

Right now, if multiple errors are registered, only one of them is reported.
I expect that for 1.1, we'll extend query-migrate to return all of the reasons
why migration is disabled at any given point in time.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

migration.c

@@ -398,6 +398,18 @@ static MigrationState *migrate_init(Monitor *mon, int detach, int blk, int inc)
     return s;
 }
 
+static GSList *migration_blockers;
+
+void migrate_add_blocker(Error *reason)
+{
+    migration_blockers = g_slist_prepend(migration_blockers, reason);
+}
+
+void migrate_del_blocker(Error *reason)
+{
+    migration_blockers = g_slist_remove(migration_blockers, reason);
+}
+
 int do_migrate(Monitor *mon, const QDict *qdict, QObject **ret_data)
 {
     MigrationState *s = migrate_get_current();
@@ -417,6 +429,12 @@ int do_migrate(Monitor *mon, const QDict *qdict, QObject **ret_data)
         return -1;
     }
 
+    if (migration_blockers) {
+        Error *err = migration_blockers->data;
+        qerror_report_err(err);
+        return -1;
+    }
+
     s = migrate_init(mon, detach, blk, inc);
 
     if (strstart(uri, "tcp:", &p)) {

migration.h

@@ -17,6 +17,7 @@
 #include "qdict.h"
 #include "qemu-common.h"
 #include "notify.h"
+#include "error.h"
 
 typedef struct MigrationState MigrationState;
 
@@ -89,4 +90,18 @@ int ram_load(QEMUFile *f, void *opaque, int version_id);
 
 extern int incoming_expected;
 
+/**
+ * @migrate_add_blocker - prevent migration from proceeding
+ *
+ * @reason - an error to be returned whenever migration is attempted
+ */
+void migrate_add_blocker(Error *reason);
+
+/**
+ * @migrate_del_blocker - remove a blocking error from migration
+ *
+ * @reason - the error blocking migration
+ */
+void migrate_del_blocker(Error *reason);
+
 #endif