qga: Clean up fragile use of error_is_set()

Using error_is_set(ERRP) to find out whether a function failed is
either wrong, fragile, or unnecessarily opaque.  It's wrong when ERRP
may be null, because errors go undetected when it is.  It's fragile
when proving ERRP non-null involves a non-local argument.  Else, it's
unnecessarily opaque (see commit 84d18f0).

The error_is_set(errp) in the guest agent command handler functions
are merely fragile, because all chall chains (do_qmp_dispatch() via
the generated marshalling functions) pass a non-null errp argument.

Make the code more robust and more obviously correct: receive the
error in a local variable, then propagate it through the parameter.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

1 files changed, 16 insertions, 6 deletions

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 6af974f..34ddba0 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -1067,8 +1067,11 @@ out:
 
 void qmp_guest_suspend_disk(Error **errp)
 {
-    bios_supports_mode("pm-is-supported", "--hibernate", "disk", errp);
-    if (error_is_set(errp)) {
+    Error *local_err = NULL;
+
+    bios_supports_mode("pm-is-supported", "--hibernate", "disk", &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
         return;
     }
 
@@ -1077,8 +1080,11 @@ void qmp_guest_suspend_disk(Error **errp)
 
 void qmp_guest_suspend_ram(Error **errp)
 {
-    bios_supports_mode("pm-is-supported", "--suspend", "mem", errp);
-    if (error_is_set(errp)) {
+    Error *local_err = NULL;
+
+    bios_supports_mode("pm-is-supported", "--suspend", "mem", &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
         return;
     }
 
@@ -1087,8 +1093,12 @@ void qmp_guest_suspend_ram(Error **errp)
 
 void qmp_guest_suspend_hybrid(Error **errp)
 {
-    bios_supports_mode("pm-is-supported", "--suspend-hybrid", NULL, errp);
-    if (error_is_set(errp)) {
+    Error *local_err = NULL;
+
+    bios_supports_mode("pm-is-supported", "--suspend-hybrid", NULL,
+                       &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
         return;
     }