qom: avoid reporting errors for NULL error object

When debugging QEMU it is often useful to put a breakpoint on the error_setg_internal method impl.

Unfortunately the object_property_add / object_class_property_add methods call object_property_find / object_class_property_find methods to check if a property exists already before adding the new property.

As a result there are a huge number of calls to error_setg_internal on startup of most QEMU commands, making it very painful to set a breakpoint on this method.

This puts a minor optimization on the code so that we avoid calling error_setg() when errp is NULL. Functionally there's no difference since error_setg() is a no-op when errp is NULL, but this lets us use breakpoints in GDB in a practical way.

Signed-off-by Daniel P. Berrangé <[email protected]>

---
 qom/object.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

qom/object.c +10 -3

index 547dcf97c3..ddd5e7a30e 100644
@@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name,
         return prop;
     }
 
-    error_setg(errp, "Property '.%s' not found", name);
+    /* Optimized to avoid calling error_setg if errp == NULL
+     * otherwise every property add call hits error_setg
+     * making it impratical to set breakpoints in GDB */
+    if (errp) {
+        error_setg(errp, "Property '.%s' not found", name);
+    }
     return NULL;
 }
 

@@ -1133,7 +1138,10 @@ ObjectProperty *object_class_property_find(ObjectClass *klass, const char *name,
     }
 
     prop = g_hash_table_lookup(klass->properties, name);
-    if (!prop) {
+    /* Optimized to avoid calling error_setg if errp == NULL
+     * otherwise every property add call hits error_setg
+     * making it impratical to set breakpoints in GDB */
+    if (!prop && errp) {
         error_setg(errp, "Property '.%s' not found", name);
     }
     return prop;
--

2.19.1

Hi

Quoted 17 lines · Daniel P. Berrangé

When debugging QEMU it is often useful to put a breakpoint on the error_setg_internal method impl.

Unfortunately the object_property_add / object_class_property_add methods call object_property_find / object_class_property_find methods to check if a property exists already before adding the new property.

As a result there are a huge number of calls to error_setg_internal on startup of most QEMU commands, making it very painful to set a breakpoint on this method.

This puts a minor optimization on the code so that we avoid calling error_setg() when errp is NULL. Functionally there's no difference since error_setg() is a no-op when errp is NULL, but this lets us use breakpoints in GDB in a practical way.

Quoted 1 line

Signed-off-by Daniel P. Berrangé <[email protected]>

Or maybe change the error_setg* macros? After all, if errp is NULL, it's an error qemu can handle.

Quoted 25 lines

---
 qom/object.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

qom/object.c +6 -1

index 547dcf97c3..ddd5e7a30e 100644
@@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name,
         return prop;
     }
 
-    error_setg(errp, "Property '.%s' not found", name);
+    /* Optimized to avoid calling error_setg if errp == NULL
+     * otherwise every property add call hits error_setg
+     * making it impratical to set breakpoints in GDB */
+    if (errp) {
+        error_setg(errp, "Property '.%s' not found", name);
+    }
     return NULL;
 }
 

@@ -1133,7 +1138,10 @@ ObjectProperty *object_class_property_find(ObjectClass *klass, const char *name,
     }
 

Quoted 13 lines

qom/object.c +4 -2

     prop = g_hash_table_lookup(klass->properties, name);
-    if (!prop) {
+    /* Optimized to avoid calling error_setg if errp == NULL
+     * otherwise every property add call hits error_setg
+     * making it impratical to set breakpoints in GDB */
+    if (!prop && errp) {
         error_setg(errp, "Property '.%s' not found", name);
     }
     return prop;
--

2.19.1

Signature · 1 line

Marc-André Lureau

When debugging QEMU it is often useful to put a breakpoint on the error_setg_internal method impl.

Unfortunately the object_property_add / object_class_property_add methods call object_property_find / object_class_property_find methods to check if a property exists already before adding the new property.

As a result there are a huge number of calls to error_setg_internal on startup of most QEMU commands, making it very painful to set a breakpoint on this method.

This puts a minor optimization on the code so that we avoid calling error_setg() when errp is NULL. Functionally there's no difference since error_setg() is a no-op when errp is NULL, but this lets us use breakpoints in GDB in a practical way.

Signed-off-by Daniel P. Berrangé <[email protected]>

---
 qom/object.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

qom/object.c +0 -0

index 547dcf97c3..ddd5e7a30e 100644
@@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name,
         return prop;
     }
 

Quoted 29 lines · Daniel P. Berrangé

When debugging QEMU it is often useful to put a breakpoint on the error_setg_internal method impl.

Unfortunately the object_property_add / object_class_property_add methods call object_property_find / object_class_property_find methods to check if a property exists already before adding the new property.

As a result there are a huge number of calls to error_setg_internal on startup of most QEMU commands, making it very painful to set a breakpoint on this method.

This puts a minor optimization on the code so that we avoid calling error_setg() when errp is NULL. Functionally there's no difference since error_setg() is a no-op when errp is NULL, but this lets us use breakpoints in GDB in a practical way.

Signed-off-by Daniel P. Berrangé <[email protected]>

---
 qom/object.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

qom/object.c +0 -0

index 547dcf97c3..ddd5e7a30e 100644
@@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, const char *name,
         return prop;
     }
 

qom/object.c ObjectProperty *object_property_find(Object *obj, const char *name, · line 1098

Quoted 10 lines

qom/object.c +6 -1

-    error_setg(errp, "Property '.%s' not found", name);
+    /* Optimized to avoid calling error_setg if errp == NULL
+     * otherwise every property add call hits error_setg
+     * making it impratical to set breakpoints in GDB */
+    if (errp) {
+        error_setg(errp, "Property '.%s' not found", name);
+    }
     return NULL;
 }
 

In my opinion, this function's design is awkward. Stress on *opinion*.

On success, it returns a (non-null) pointer.

On failure, it sets an error and returns null. Note that it has just one failure mode: "Property '.%s' not found". Setting an error is just a convenience for those callers that want to propagate exactly this error to their callers.

I count 30 callers. Only six pass a non-NULL argument to @errp.

I'd rather have a pair of functions similar to how Python has both .get() and .__getitem__(): the former doesn't fail, but returns None instead, and the latter does fail, throwing KeyError. In QEMU, we can't throw, so we set an error. Here's the obvious code:

    ObjectProperty *object_property_find(Object *obj, const char *name)
    {
        ObjectProperty *prop;
        ObjectClass *klass = object_get_class(obj);

        prop = object_class_property_find(klass, name, NULL);
        if (prop) {
            return prop;
        }

        prop = g_hash_table_lookup(obj->properties, name);
        if (prop) {
            return prop;
        }

        return NULL;
    }

    ObjectProperty *object_property_find_err(Object *obj, const char *name,
                                             Error **errp)

    {
        ObjectProperty *prop = object_property_find(obj, name);

        if (!prop) {
            error_setg(errp, "Property '.%s' not found", name);
        }
        return prop;
    }