nvidia-settings: Don't query target relationships for nonexistent targets

Currently, nvidia-settings will ask the X driver about the relationships between
every type of target it knows about unconditionally.  When support for Canoas
targets in the X driver was removed (including the
NV_CTRL_BINARY_DATA_VCSCS_USED_BY_GPU attribute), this caused nvidia-settings to
trigger a BadMatch error and abort (on GTK2) or print an error about failing to
query relationships between targets (on GTK3).

This change updates nvidia-settings to not bother to query for relationships for
a target when no such targets exist in the system.  In addition to fixing the
BadMatch described above, it should reduce the startup latency due to fewer
server round-trips in the common case.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libXNVCtrlAttributes/NvCtrlAttributesUtils.c b/src/libXNVCtrlAttributes/NvCtrlAttributesUtils.c
index d358543..f20108d 100644
--- a/src/libXNVCtrlAttributes/NvCtrlAttributesUtils.c
+++ b/src/libXNVCtrlAttributes/NvCtrlAttributesUtils.c
@@ -491,6 +491,12 @@ static void add_target_relationships(CtrlTarget *target,
     int len;
     int i;
 
+    /* If no targets of this type exist in the system, don't bother querying
+     * the server about relationships. */
+    if (target->system->targets[target_type] == NULL) {
+        return;
+    }
+
     status = NvCtrlGetBinaryAttribute(target, 0, attr,
                                       (unsigned char **)(&pData), &len);
     if ((status != NvCtrlSuccess) || !pData) {