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[RFC PATCH v2] fix error message in virMigrate3 if connection is broken



Changes from v1 [1]:
- return error value from VIR_DRV_SUPPORTS_FEATURE instead
  of setting error to out argument (decrease over enginering
  level on patch generator in other words :)

Currently virDomainMigrate3 reports "this function is not supported by the
connection driver: virDomainMigrate3" if connection to destination for example
is broken. This is a bit misleading.

This is a result of we treat errors as unsupported feature in
VIR_DRV_SUPPORTS_FEATURE macro. Let's return error from macro as is.

I guess all the other occurences of VIR_DRV_SUPPORTS_FEATURE need to be updated
as well so that we detect errors early and not have issues similar to
virDomainMigrate3. I'm going to fix the other places if this RFC is approved.

[1] https://www.redhat.com/archives/libvir-list/2020-September/msg01056.html
---
 src/driver.h         | 14 ++++++++
 src/libvirt-domain.c | 91 +++++++++++++++++++++++++++++++++++++---------------
 2 files changed, 79 insertions(+), 26 deletions(-)

diff --git a/src/driver.h b/src/driver.h
index 6278aa0..c29b2fa 100644
--- a/src/driver.h
+++ b/src/driver.h
@@ -60,6 +60,20 @@ typedef enum {
         (drv)->connectSupportsFeature((conn), (feature)) > 0 : 0)
 
 
+/*
+ * A little wrapper for connectSupportsFeature API to test that the API
+ * itself is available first. Return values are same as for API.
+ *
+ * Returns:
+ *  -1  on error
+ *   0  feature is not supported
+ *   1  feature is supported
+ */
+#define VIR_DRV_SUPPORTS_FEATURE2(drv, conn, feature) \
+    ((drv)->connectSupportsFeature ? \
+        (drv)->connectSupportsFeature((conn), (feature)) : 0)
+
+
 #define __VIR_DRIVER_H_INCLUDES___
 
 #include "driver-hypervisor.h"
diff --git a/src/libvirt-domain.c b/src/libvirt-domain.c
index cde86c7..03c357f 100644
--- a/src/libvirt-domain.c
+++ b/src/libvirt-domain.c
@@ -3846,6 +3846,9 @@ virDomainMigrate3(virDomainPtr domain,
     const char *dname = NULL;
     const char *dxml = NULL;
     unsigned long long bandwidth = 0;
+    int rc_src;
+    int rc_dst;
+    int rc;
 
     VIR_DOMAIN_DEBUG(domain, "dconn=%p, params=%p, nparms=%u flags=0x%x",
                      dconn, params, nparams, flags);
@@ -3878,15 +3881,21 @@ virDomainMigrate3(virDomainPtr domain,
     }
 
     if (flags & VIR_MIGRATE_OFFLINE) {
-        if (!VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
-                                      VIR_DRV_FEATURE_MIGRATION_OFFLINE)) {
+        rc = VIR_DRV_SUPPORTS_FEATURE2(domain->conn->driver, domain->conn,
+                                       VIR_DRV_FEATURE_MIGRATION_OFFLINE);
+        if (rc < 0) {
+            goto error;
+        } else if (rc == 0) {
             virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
                            _("offline migration is not supported by "
                              "the source host"));
             goto error;
         }
-        if (!VIR_DRV_SUPPORTS_FEATURE(dconn->driver, dconn,
-                                      VIR_DRV_FEATURE_MIGRATION_OFFLINE)) {
+        rc = VIR_DRV_SUPPORTS_FEATURE2(dconn->driver, dconn,
+                                       VIR_DRV_FEATURE_MIGRATION_OFFLINE);
+        if (rc < 0) {
+            goto error;
+        } else if (rc == 0) {
             virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
                            _("offline migration is not supported by "
                              "the destination host"));
@@ -3899,21 +3908,30 @@ virDomainMigrate3(virDomainPtr domain,
      * the flag for just the source side.  We mask it out to allow
      * migration from newer source to an older destination that
      * rejects the flag.  */
-    if (flags & VIR_MIGRATE_CHANGE_PROTECTION &&
-        !VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
-                                  VIR_DRV_FEATURE_MIGRATE_CHANGE_PROTECTION)) {
-        virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
-                       _("cannot enforce change protection"));
-        goto error;
+    if (flags & VIR_MIGRATE_CHANGE_PROTECTION) {
+        rc = VIR_DRV_SUPPORTS_FEATURE2(domain->conn->driver, domain->conn,
+                                       VIR_DRV_FEATURE_MIGRATE_CHANGE_PROTECTION);
+        if (rc < 0) {
+            goto error;
+        } else if (rc == 0) {
+            virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
+                           _("cannot enforce change protection"));
+            goto error;
+        }
     }
     flags &= ~VIR_MIGRATE_CHANGE_PROTECTION;
 
     /* Prefer extensible API but fall back to older migration APIs if params
      * only contains parameters which were supported by the older API. */
-    if (VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
-                                 VIR_DRV_FEATURE_MIGRATION_PARAMS) &&
-        VIR_DRV_SUPPORTS_FEATURE(dconn->driver, dconn,
-                                 VIR_DRV_FEATURE_MIGRATION_PARAMS)) {
+    rc_src = VIR_DRV_SUPPORTS_FEATURE2(domain->conn->driver, domain->conn,
+                                       VIR_DRV_FEATURE_MIGRATION_PARAMS);
+    if (rc_src < 0)
+        goto error;
+    rc_dst = VIR_DRV_SUPPORTS_FEATURE2(dconn->driver, dconn,
+                                       VIR_DRV_FEATURE_MIGRATION_PARAMS);
+    if (rc_dst < 0)
+        goto error;
+    if (rc_src && rc_dst) {
         VIR_DEBUG("Using migration protocol 3 with extensible parameters");
         ddomain = virDomainMigrateVersion3Params(domain, dconn, params,
                                                  nparams, flags);
@@ -3939,17 +3957,30 @@ virDomainMigrate3(virDomainPtr domain,
         goto error;
     }
 
-    if (VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
-                                 VIR_DRV_FEATURE_MIGRATION_V3) &&
-        VIR_DRV_SUPPORTS_FEATURE(dconn->driver, dconn,
-                                 VIR_DRV_FEATURE_MIGRATION_V3)) {
+    rc_src = VIR_DRV_SUPPORTS_FEATURE2(domain->conn->driver, domain->conn,
+                                       VIR_DRV_FEATURE_MIGRATION_V3);
+    if (rc_src < 0)
+        goto error;
+    rc_dst = VIR_DRV_SUPPORTS_FEATURE2(dconn->driver, dconn,
+                                       VIR_DRV_FEATURE_MIGRATION_V3);
+    if (rc_dst < 0)
+        goto error;
+    if (rc_src && rc_dst) {
         VIR_DEBUG("Using migration protocol 3");
         ddomain = virDomainMigrateVersion3(domain, dconn, dxml, flags,
                                            dname, uri, bandwidth);
-    } else if (VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
-                                        VIR_DRV_FEATURE_MIGRATION_V2) &&
-               VIR_DRV_SUPPORTS_FEATURE(dconn->driver, dconn,
-                                      VIR_DRV_FEATURE_MIGRATION_V2)) {
+        goto done;
+    }
+
+    rc_src = VIR_DRV_SUPPORTS_FEATURE2(domain->conn->driver, domain->conn,
+                                       VIR_DRV_FEATURE_MIGRATION_V2);
+    if (rc_src < 0)
+        goto error;
+    rc_dst = VIR_DRV_SUPPORTS_FEATURE2(dconn->driver, dconn,
+                                       VIR_DRV_FEATURE_MIGRATION_V2);
+    if (rc_dst < 0)
+        goto error;
+    if (rc_src && rc_dst) {
         VIR_DEBUG("Using migration protocol 2");
         if (dxml) {
             virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
@@ -3959,10 +3990,18 @@ virDomainMigrate3(virDomainPtr domain,
         }
         ddomain = virDomainMigrateVersion2(domain, dconn, flags,
                                            dname, uri, bandwidth);
-    } else if (VIR_DRV_SUPPORTS_FEATURE(domain->conn->driver, domain->conn,
-                                        VIR_DRV_FEATURE_MIGRATION_V1) &&
-               VIR_DRV_SUPPORTS_FEATURE(dconn->driver, dconn,
-                                        VIR_DRV_FEATURE_MIGRATION_V1)) {
+        goto done;
+    }
+
+    rc_src = VIR_DRV_SUPPORTS_FEATURE2(domain->conn->driver, domain->conn,
+                                       VIR_DRV_FEATURE_MIGRATION_V1);
+    if (rc_src < 0)
+        goto error;
+    rc_dst = VIR_DRV_SUPPORTS_FEATURE2(dconn->driver, dconn,
+                                       VIR_DRV_FEATURE_MIGRATION_V1);
+    if (rc_dst < 0)
+        goto error;
+    if (rc_src && rc_dst) {
         VIR_DEBUG("Using migration protocol 1");
         if (dxml) {
             virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
-- 
1.8.3.1


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