[libvirt] [RFC PATCH] snapshot: better virsh handling of missing current, parent

[Eric Blake]

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.
(vshGetSnapshotParent): Adjust caller.
(cmdSnapshotCurrent): Always output message on failure.
---

This patch is an RFC because of backwards-compatibility concerns:

Currently, snapshot-current outputs nothing but has status 0 if
there is no current snapshot; but snapshot-parent outputs nothing
but has status 1 if there is no parent snapshot.  Either way, we
have an inconsistency that needs to be fixed, and gaining consistency
means breaking backwards compatibility with at least one command.

Approach 1 (this patch): change snapshot-parent and snapshot-current
to always output something, whether to stdout on success or to
stderr on failure, with lack of a snapshot being considered a
failure (where snapshot-current used to treat it as success).

Approach 2 (not written) since snapshot-current existed much longer,
makesnapshot-parent consistent by giving status 0 when it is silent.

Preferences?  (I guess mine is approach 1, by evidence of this patch).

 tools/virsh.c |   54 +++++++++++++++++++++++++++++++++++++++---------------
 1 files changed, 39 insertions(+), 15 deletions(-)

diff --git a/tools/virsh.c b/tools/virsh.c
index 1909dce..179cda5 100644
--- a/tools/virsh.c
+++ b/tools/virsh.c
@@ -12945,6 +12945,7 @@ 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;
@@ -12952,7 +12953,7 @@ 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;

@@ -12986,9 +12987,12 @@ cmdSnapshotCurrent(vshControl *ctl, const vshCmd *cmd)
     }

     current = virDomainHasCurrentSnapshot(dom, 0);
-    if (current < 0)
+    if (current < 0) {
+        goto cleanup;
+    } else if (!current) {
+        vshError(ctl, _("domain '%s' has no current snapshot"), domname);
         goto cleanup;
-    else if (current) {
+    } else {
         const char *name = NULL;

         if (!(snapshot = virDomainSnapshotCurrent(dom, 0)))
@@ -13010,6 +13014,8 @@ cmdSnapshotCurrent(vshControl *ctl, const vshCmd *cmd)
     ret = true;

 cleanup:
+    if (!ret)
+        virshReportError(ctl);
     VIR_FREE(xml);
     if (snapshot)
         virDomainSnapshotFree(snapshot);
@@ -13020,26 +13026,33 @@ cleanup:
 }

 /* Helper function to get the name of a snapshot's parent.  Caller
- * must free the result.  */
-static char *
-vshGetSnapshotParent(vshControl *ctl, virDomainSnapshotPtr snapshot)
+ * must free the result.  Returns 0 on success (including when it was
+ * proven no parent exists), and -1 on failure with error reported
+ * (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));
+            /* API works, and virDomainSnapshotGetName will be accurate */
+            *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. */
@@ -13054,15 +13067,23 @@ 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;
 }

 /*
@@ -13187,13 +13208,13 @@ 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);
@@ -13389,9 +13410,12 @@ cmdSnapshotParent(vshControl *ctl, const vshCmd *cmd)
     if (snapshot == NULL)
         goto cleanup;

-    parent = vshGetSnapshotParent(ctl, snapshot);
-    if (!parent)
+    if (vshGetSnapshotParent(ctl, snapshot, &parent) < 0)
         goto cleanup;
+    if (!parent) {
+        vshError(ctl, _("snapshot '%s' has no parent"), name);
+        goto cleanup;
+    }

     vshPrint(ctl, "%s", parent);

-- 
1.7.4.4