migration: add reporting of errors for outgoing migration

Currently if an application initiates an outgoing migration,
it may or may not, get an error reported back on failure. If
the error occurs synchronously to the 'migrate' command
execution, the client app will see the error message. This
is the case for DNS lookup failures. If the error occurs
asynchronously to the monitor command though, the error
will be thrown away and the client left guessing about
what went wrong. This is the case for failure to connect
to the TCP server (eg due to wrong port, or firewall
rules, or other similar errors).

In the future we'll be adding more scope for errors to
happen asynchronously with the TLS protocol handshake.
TLS errors are hard to diagnose even when they are well
reported, so discarding errors entirely will make it
impossible to debug TLS connection problems.

Management apps which do migration are already using
'query-migrate' / 'info migrate' to check up on progress
of background migration operations and to see their end
status. This is a fine place to also include the error
message when things go wrong.

This patch thus adds an 'error-desc' field to the
MigrationInfo struct, which will be populated when
the 'status' is set to 'failed':

(qemu) migrate -d tcp:localhost:9001
(qemu) info migrate
capabilities: xbzrle: off rdma-pin-all: off auto-converge: off zero-blocks: off compress: off events: off x-postcopy-ram: off
Migration status: failed (Error connecting to socket: Connection refused)
total time: 0 milliseconds

In the HMP, when doing non-detached migration, it is
also possible to display this error message directly
to the app.

(qemu) migrate tcp:localhost:9001
Error connecting to socket: Connection refused

Or with QMP

  {
    "execute": "query-migrate",
    "arguments": {}
  }
  {
    "return": {
      "status": "failed",
      "error-desc": "address resolution failed for myhost:9000: No address associated with hostname"
    }
  }

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <1461751518-12128-11-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>

hmp.c

@@ -35,6 +35,7 @@
 #include "block/qapi.h"
 #include "qemu-io.h"
 #include "qemu/cutils.h"
+#include "qemu/error-report.h"
 
 #ifdef CONFIG_SPICE
 #include <spice/enums.h>
@@ -168,8 +169,15 @@ void hmp_info_migrate(Monitor *mon, const QDict *qdict)
     }
 
     if (info->has_status) {
-        monitor_printf(mon, "Migration status: %s\n",
+        monitor_printf(mon, "Migration status: %s",
                        MigrationStatus_lookup[info->status]);
+        if (info->status == MIGRATION_STATUS_FAILED &&
+            info->has_error_desc) {
+            monitor_printf(mon, " (%s)\n", info->error_desc);
+        } else {
+            monitor_printf(mon, "\n");
+        }
+
         monitor_printf(mon, "total time: %" PRIu64 " milliseconds\n",
                        info->total_time);
         if (info->has_expected_downtime) {
@@ -1533,6 +1541,9 @@ static void hmp_migrate_status_cb(void *opaque)
         if (status->is_block_migration) {
             monitor_printf(status->mon, "\n");
         }
+        if (info->has_error_desc) {
+            error_report("%s", info->error_desc);
+        }
         monitor_resume(status->mon);
         timer_del(status->timer);
         g_free(status);

include/migration/migration.h

@@ -171,6 +171,9 @@ struct MigrationState
     QSIMPLEQ_HEAD(src_page_requests, MigrationSrcPageRequest) src_page_requests;
     /* The RAMBlock used in the last src_page_request */
     RAMBlock *last_req_rb;
+
+    /* The last error that occurred */
+    Error *error;
 };
 
 void migrate_set_state(int *state, int old_state, int new_state);
@@ -207,7 +210,7 @@ void rdma_start_outgoing_migration(void *opaque, const char *host_port, Error **
 
 void rdma_start_incoming_migration(const char *host_port, Error **errp);
 
-void migrate_fd_error(MigrationState *s);
+void migrate_fd_error(MigrationState *s, const Error *error);
 
 void migrate_fd_connect(MigrationState *s);
 

include/qapi/error.h

@@ -134,7 +134,7 @@ typedef enum ErrorClass {
 /*
  * Get @err's human-readable error message.
  */
-const char *error_get_pretty(Error *err);
+const char *error_get_pretty(const Error *err);
 
 /*
  * Get @err's error class.

migration/migration.c

@@ -691,6 +691,10 @@ MigrationInfo *qmp_query_migrate(Error **errp)
         break;
     case MIGRATION_STATUS_FAILED:
         info->has_status = true;
+        if (s->error) {
+            info->has_error_desc = true;
+            info->error_desc = g_strdup(error_get_pretty(s->error));
+        }
         break;
     case MIGRATION_STATUS_CANCELLED:
         info->has_status = true;
@@ -863,12 +867,15 @@ static void migrate_fd_cleanup(void *opaque)
     notifier_list_notify(&migration_state_notifiers, s);
 }
 
-void migrate_fd_error(MigrationState *s)
+void migrate_fd_error(MigrationState *s, const Error *error)
 {
-    trace_migrate_fd_error();
+    trace_migrate_fd_error(error ? error_get_pretty(error) : "");
     assert(s->to_dst_file == NULL);
     migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
                       MIGRATION_STATUS_FAILED);
+    if (!s->error) {
+        s->error = error_copy(error);
+    }
     notifier_list_notify(&migration_state_notifiers, s);
 }
 
@@ -967,6 +974,8 @@ MigrationState *migrate_init(const MigrationParams *params)
     s->postcopy_after_devices = false;
     s->migration_thread_running = false;
     s->last_req_rb = NULL;
+    error_free(s->error);
+    s->error = NULL;
 
     migrate_set_state(&s->state, MIGRATION_STATUS_NONE, MIGRATION_STATUS_SETUP);
 
@@ -1076,7 +1085,7 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
     }
 
     if (local_err) {
-        migrate_fd_error(s);
+        migrate_fd_error(s, local_err);
         error_propagate(errp, local_err);
         return;
     }

migration/rdma.c

@@ -3489,16 +3489,14 @@ void rdma_start_outgoing_migration(void *opaque,
                             const char *host_port, Error **errp)
 {
     MigrationState *s = opaque;
-    Error *local_err = NULL, **temp = &local_err;
-    RDMAContext *rdma = qemu_rdma_data_init(host_port, &local_err);
+    RDMAContext *rdma = qemu_rdma_data_init(host_port, errp);
     int ret = 0;
 
     if (rdma == NULL) {
-        ERROR(temp, "Failed to initialize RDMA data structures! %d", ret);
         goto err;
     }
 
-    ret = qemu_rdma_source_init(rdma, &local_err,
+    ret = qemu_rdma_source_init(rdma, errp,
         s->enabled_capabilities[MIGRATION_CAPABILITY_RDMA_PIN_ALL]);
 
     if (ret) {
@@ -3506,7 +3504,7 @@ void rdma_start_outgoing_migration(void *opaque,
     }
 
     trace_rdma_start_outgoing_migration_after_rdma_source_init();
-    ret = qemu_rdma_connect(rdma, &local_err);
+    ret = qemu_rdma_connect(rdma, errp);
 
     if (ret) {
         goto err;
@@ -3518,7 +3516,5 @@ void rdma_start_outgoing_migration(void *opaque,
     migrate_fd_connect(s);
     return;
 err:
-    error_propagate(errp, local_err);
     g_free(rdma);
-    migrate_fd_error(s);
 }

migration/tcp.c

@@ -40,7 +40,7 @@ static void tcp_wait_for_connect(int fd, Error *err, void *opaque)
     if (fd < 0) {
         DPRINTF("migrate connect error: %s\n", error_get_pretty(err));
         s->to_dst_file = NULL;
-        migrate_fd_error(s);
+        migrate_fd_error(s, err);
     } else {
         DPRINTF("migrate connect success\n");
         s->to_dst_file = qemu_fopen_socket(fd, "wb");

migration/unix.c

@@ -40,7 +40,7 @@ static void unix_wait_for_connect(int fd, Error *err, void *opaque)
     if (fd < 0) {
         DPRINTF("migrate connect error: %s\n", error_get_pretty(err));
         s->to_dst_file = NULL;
-        migrate_fd_error(s);
+        migrate_fd_error(s, err);
     } else {
         DPRINTF("migrate connect success\n");
         s->to_dst_file = qemu_fopen_socket(fd, "wb");

qapi-schema.json

@@ -484,6 +484,10 @@
 #        throttled during auto-converge. This is only present when auto-converge
 #        has started throttling guest cpus. (Since 2.7)
 #
+# @error-desc: #optional the human readable error description string, when
+#              @status is 'failed'. Clients should not attempt to parse the
+#              error strings. (Since 2.6)
+#
 # Since: 0.14.0
 ##
 { 'struct': 'MigrationInfo',
@@ -494,7 +498,8 @@
            '*expected-downtime': 'int',
            '*downtime': 'int',
            '*setup-time': 'int',
-           '*cpu-throttle-percentage': 'int'} }
+           '*cpu-throttle-percentage': 'int',
+           '*error-desc': 'str'} }
 
 ##
 # @query-migrate

trace-events

@@ -1481,7 +1481,7 @@ await_return_path_close_on_source_close(void) ""
 await_return_path_close_on_source_joining(void) ""
 migrate_set_state(int new_state) "new state %d"
 migrate_fd_cleanup(void) ""
-migrate_fd_error(void) ""
+migrate_fd_error(const char *error_desc) "error=%s"
 migrate_fd_cancel(void) ""
 migrate_handle_rp_req_pages(const char *rbname, size_t start, size_t len) "in %s at %zx len %zx"
 migrate_pending(uint64_t size, uint64_t max, uint64_t post, uint64_t nonpost) "pending size %" PRIu64 " max %" PRIu64 " (post=%" PRIu64 " nonpost=%" PRIu64 ")"

util/error.c

@@ -217,7 +217,7 @@ ErrorClass error_get_class(const Error *err)
     return err->err_class;
 }
 
-const char *error_get_pretty(Error *err)
+const char *error_get_pretty(const Error *err)
 {
     return err->msg;
 }