snapshot: better virsh handling of missing current, parent

Previously, virsh 'snapshot-parent' and 'snapshot-current' were
completely silent in the case where the code conclusively proved
there was no parent or current snapshot, but differed in exit
status; this silence caused some confusion on whether the commands
worked.  Furthermore, commit d1be48f introduced a regression where
snapshot-parent would leak output about an unknown function, but
only on the first attempt, when talking to an older server that
lacks virDomainSnapshotGetParent.  This changes things to consistenly
report an error message and exit with status 1 when no snapshot
exists, and to avoid leaking unknown function warnings when using
fallbacks.

* tools/virsh.c (vshGetSnapshotParent): Alter signature, to
distinguish between real error and missing parent.  Don't pollute
last_error on success.
(cmdSnapshotParent): Adjust caller.  Always output message on
failure.
(cmdSnapshotList): Adjust caller.
(cmdSnapshotCurrent): Always output message on failure.

tools/virsh.c

@@ -12946,6 +12946,7 @@ cmdSnapshotCurrent(vshControl *ctl, const vshCmd *cmd)
     char *xml = NULL;
     const char *snapshotname = NULL;
     unsigned int flags = 0;
+    const char *domname;
 
     if (vshCommandOptBool(cmd, "security-info"))
         flags |= VIR_DOMAIN_XML_SECURE;
@@ -12953,7 +12954,7 @@ cmdSnapshotCurrent(vshControl *ctl, const vshCmd *cmd)
     if (!vshConnectionUsability(ctl, ctl->conn))
         goto cleanup;
 
-    dom = vshCommandOptDomain(ctl, cmd, NULL);
+    dom = vshCommandOptDomain(ctl, cmd, &domname);
     if (dom == NULL)
         goto cleanup;
 
@@ -12987,9 +12988,12 @@ cmdSnapshotCurrent(vshControl *ctl, const vshCmd *cmd)
     }
 
     current = virDomainHasCurrentSnapshot(dom, 0);
-    if (current < 0)
+    if (current < 0) {
         goto cleanup;
-    else if (current) {
+    } else if (!current) {
+        vshError(ctl, _("domain '%s' has no current snapshot"), domname);
+        goto cleanup;
+    } else {
         const char *name = NULL;
 
         if (!(snapshot = virDomainSnapshotCurrent(dom, 0)))
@@ -13011,6 +13015,8 @@ cmdSnapshotCurrent(vshControl *ctl, const vshCmd *cmd)
     ret = true;
 
 cleanup:
+    if (!ret)
+        virshReportError(ctl);
     VIR_FREE(xml);
     if (snapshot)
         virDomainSnapshotFree(snapshot);
@@ -13021,26 +13027,33 @@ cleanup:
 }
 
 /* Helper function to get the name of a snapshot's parent.  Caller
- * must free the result.  */
+ * must free the result.  Returns 0 on success (including when it was
-static char *
+ * proven no parent exists), and -1 on failure with error reported
-vshGetSnapshotParent(vshControl *ctl, virDomainSnapshotPtr snapshot)
+ * (such as no snapshot support or domain deleted in meantime).  */
+static int
+vshGetSnapshotParent(vshControl *ctl, virDomainSnapshotPtr snapshot,
+                     char **parent_name)
 {
     virDomainSnapshotPtr parent = NULL;
     char *xml = NULL;
     xmlDocPtr xmldoc = NULL;
     xmlXPathContextPtr ctxt = NULL;
-    char *parent_name = NULL;
+    int ret = -1;
+
+    *parent_name = NULL;
 
     /* Try new API, since it is faster. */
     if (!ctl->useSnapshotGetXML) {
         parent = virDomainSnapshotGetParent(snapshot, 0);
         if (parent) {
             /* API works, and virDomainSnapshotGetName will succeed */
-            parent_name = vshStrdup(ctl, virDomainSnapshotGetName(parent));
+            *parent_name = vshStrdup(ctl, virDomainSnapshotGetName(parent));
+            ret = 0;
             goto cleanup;
         }
         if (last_error->code == VIR_ERR_NO_DOMAIN_SNAPSHOT) {
             /* API works, and we found a root with no parent */
+            ret = 0;
             goto cleanup;
         }
         /* API didn't work, fall back to XML scraping. */
@@ -13055,15 +13068,23 @@ vshGetSnapshotParent(vshControl *ctl, virDomainSnapshotPtr snapshot)
     if (!xmldoc)
         goto cleanup;
 
-    parent_name = virXPathString("string(/domainsnapshot/parent/name)", ctxt);
+    *parent_name = virXPathString("string(/domainsnapshot/parent/name)", ctxt);
+    ret = 0;
 
 cleanup:
+    if (ret < 0) {
+        virshReportError(ctl);
+        vshError(ctl, "%s", _("unable to determine if snapshot has parent"));
+    } else {
+        virFreeError(last_error);
+        last_error = NULL;
+    }
     if (parent)
         virDomainSnapshotFree(parent);
     xmlXPathFreeContext(ctxt);
     xmlFreeDoc(xmldoc);
     VIR_FREE(xml);
-    return parent_name;
+    return ret;
 }
 
 /*
@@ -13188,13 +13209,13 @@ cmdSnapshotList(vshControl *ctl, const vshCmd *cmd)
             if (snapshot)
                 virDomainSnapshotFree(snapshot);
             snapshot = virDomainSnapshotLookupByName(dom, names[i], 0);
-            if (!snapshot) {
+            if (!snapshot ||
+                vshGetSnapshotParent(ctl, snapshot, &parents[i]) < 0) {
                 while (--i >= 0)
                     VIR_FREE(parents[i]);
                 VIR_FREE(parents);
                 goto cleanup;
             }
-            parents[i] = vshGetSnapshotParent(ctl, snapshot);
         }
         for (i = 0 ; i < actual ; i++) {
             memset(indentBuf, '\0', sizeof indentBuf);
@@ -13390,9 +13411,12 @@ cmdSnapshotParent(vshControl *ctl, const vshCmd *cmd)
     if (snapshot == NULL)
         goto cleanup;
 
-    parent = vshGetSnapshotParent(ctl, snapshot);
+    if (vshGetSnapshotParent(ctl, snapshot, &parent) < 0)
-    if (!parent)
         goto cleanup;
+    if (!parent) {
+        vshError(ctl, _("snapshot '%s' has no parent"), name);
+        goto cleanup;
+    }
 
     vshPrint(ctl, "%s", parent);