qemu: nicer error message if live disk snapshot unsupported

Without this patch, attempts to create a disk snapshot when qemu
is too old results in a cryptic message:

virsh # snapshot-create 23 --disk-only
error: operation failed: Failed to take snapshot: unknown command: 'snapshot_blkdev'

Now it reports:

virsh # snapshot-create 23 --disk-only
error: unsupported configuration: live disk snapshot not supported with this QEMU binary

All versions of qemu that support live disk snapshot also support
QMP (basically upstream qemu 1.1 and later, and backports to RHEL 6.2).

* src/qemu/qemu_capabilities.h (QEMU_CAPS_DISK_SNAPSHOT): New
capability.
* src/qemu/qemu_capabilities.c (qemuCaps): Track it.
(qemuCapsProbeQMPCommands): Set it.
* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateDiskActive): Use
it.
* src/qemu/qemu_monitor.c (qemuMonitorDiskSnapshot): Simplify.
* src/qemu/qemu_monitor_json.c (qemuMonitorJSONDiskSnapshot):
Likewise.
* src/qemu/qemu_monitor_text.h (qemuMonitorTextDiskSnapshot):
Delete.
* src/qemu/qemu_monitor_text.c (qemuMonitorTextDiskSnapshot):
Likewise.

src/qemu/qemu_capabilities.c

@@ -193,6 +193,7 @@ VIR_ENUM_IMPL(qemuCaps, QEMU_CAPS_LAST,
               "drive-mirror", /* 115 */
               "usb-redir.bootindex",
               "usb-host.bootindex",
+              "blockdev-snapshot-sync",
     );
 
 struct _qemuCaps {
@@ -1948,6 +1949,8 @@ qemuCapsProbeQMPCommands(qemuCapsPtr caps,
             qemuCapsSet(caps, QEMU_CAPS_VNC);
         else if (STREQ(name, "drive-mirror"))
             qemuCapsSet(caps, QEMU_CAPS_DRIVE_MIRROR);
+        else if (STREQ(name, "blockdev-snapshot-sync"))
+            qemuCapsSet(caps, QEMU_CAPS_DISK_SNAPSHOT);
         VIR_FREE(name);
     }
     VIR_FREE(commands);

src/qemu/qemu_capabilities.h

@@ -155,6 +155,7 @@ enum qemuCapsFlags {
     QEMU_CAPS_DRIVE_MIRROR       = 115, /* drive-mirror monitor command */
     QEMU_CAPS_USB_REDIR_BOOTINDEX = 116, /* usb-redir.bootindex */
     QEMU_CAPS_USB_HOST_BOOTINDEX = 117, /* usb-host.bootindex */
+    QEMU_CAPS_DISK_SNAPSHOT      = 118, /* blockdev-snapshot-sync command */
 
     QEMU_CAPS_LAST,                   /* this must always be the last item */
 };

src/qemu/qemu_driver.c

@@ -11281,6 +11281,11 @@ qemuDomainSnapshotCreateDiskActive(virQEMUDriverPtr driver,
             virReportOOMError();
             goto cleanup;
         }
+    } else if (!qemuCapsGet(priv->caps, QEMU_CAPS_DISK_SNAPSHOT)) {
+        virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
+                       _("live disk snapshot not supported with this "
+                         "QEMU binary"));
+        goto cleanup;
     }
 
     /* No way to roll back if first disk succeeds but later disks

src/qemu/qemu_monitor.c

@@ -2758,7 +2758,7 @@ qemuMonitorDiskSnapshot(qemuMonitorPtr mon, virJSONValuePtr actions,
                         const char *device, const char *file,
                         const char *format, bool reuse)
 {
-    int ret;
+    int ret = -1;
 
     VIR_DEBUG("mon=%p, actions=%p, device=%s, file=%s, format=%s, reuse=%d",
               mon, actions, device, file, format, reuse);
@@ -2769,17 +2769,12 @@ qemuMonitorDiskSnapshot(qemuMonitorPtr mon, virJSONValuePtr actions,
         return -1;
     }
 
-    if (mon->json) {
+    if (mon->json)
         ret = qemuMonitorJSONDiskSnapshot(mon, actions, device, file, format,
                                           reuse);
-    } else {
-        if (actions || STRNEQ(format, "qcow2") || reuse) {
-            virReportError(VIR_ERR_INVALID_ARG, "%s",
-                           _("text monitor lacks several snapshot features"));
-            return -1;
-        }
-        ret = qemuMonitorTextDiskSnapshot(mon, device, file);
-    }
+    else
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("disk snapshot requires JSON monitor"));
     return ret;
 }
 

src/qemu/qemu_monitor_json.c

@@ -2986,12 +2986,6 @@ qemuMonitorJSONDiskSnapshot(qemuMonitorPtr mon, virJSONValuePtr actions,
         if ((ret = qemuMonitorJSONCommand(mon, cmd, &reply)) < 0)
             goto cleanup;
 
-        if (qemuMonitorJSONHasError(reply, "CommandNotFound")) {
-            VIR_DEBUG("blockdev-snapshot-sync command not found, trying HMP");
-            ret = qemuMonitorTextDiskSnapshot(mon, device, file);
-            goto cleanup;
-        }
-
         ret = qemuMonitorJSONCheckError(cmd, reply);
     }
 

src/qemu/qemu_monitor_text.c

@@ -2958,43 +2958,6 @@ cleanup:
     return ret;
 }
 
-int
-qemuMonitorTextDiskSnapshot(qemuMonitorPtr mon, const char *device,
-                            const char *file)
-{
-    char *cmd = NULL;
-    char *reply = NULL;
-    int ret = -1;
-    char *safename;
-
-    if (!(safename = qemuMonitorEscapeArg(file)) ||
-        virAsprintf(&cmd, "snapshot_blkdev %s \"%s\"", device, safename) < 0) {
-        virReportOOMError();
-        goto cleanup;
-    }
-
-    if (qemuMonitorHMPCommand(mon, cmd, &reply))
-        goto cleanup;
-
-    if (strstr(reply, "error while creating qcow2") != NULL ||
-        strstr(reply, "unknown command:") != NULL) {
-        virReportError(VIR_ERR_OPERATION_FAILED,
-                       _("Failed to take snapshot: %s"), reply);
-        goto cleanup;
-    }
-
-    /* XXX Should we scrape 'info block' output for
-     * 'device:... file=name backing_file=oldname' to make sure the
-     * command succeeded?  */
-
-    ret = 0;
-
-cleanup:
-    VIR_FREE(safename);
-    VIR_FREE(cmd);
-    VIR_FREE(reply);
-    return ret;
-}
 
 int qemuMonitorTextArbitraryCommand(qemuMonitorPtr mon, const char *cmd,
                                     char **reply)

src/qemu/qemu_monitor_text.h

@@ -217,10 +217,6 @@ int qemuMonitorTextCreateSnapshot(qemuMonitorPtr mon, const char *name);
 int qemuMonitorTextLoadSnapshot(qemuMonitorPtr mon, const char *name);
 int qemuMonitorTextDeleteSnapshot(qemuMonitorPtr mon, const char *name);
 
-int qemuMonitorTextDiskSnapshot(qemuMonitorPtr mon,
-                                const char *device,
-                                const char *file);
-
 int qemuMonitorTextArbitraryCommand(qemuMonitorPtr mon, const char *cmd,
                                     char **reply);