internal/mcp/jsonschema: generalize error wrapping

Add a wrapf function to simplify defers that wrap errors.

Change-Id: If4bedec7d3f56f609d3ca4620aebe7637a59f6bf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

jsonschema/json_pointer.go

@@ -69,11 +69,7 @@ func parseJSONPointer(ptr string) (segments []string, err error) {
 // This implementation suffices for JSON Schema: pointers are applied only to Schemas,
 // and refer only to Schemas.
 func dereferenceJSONPointer(s *Schema, sptr string) (_ *Schema, err error) {
-	defer func() {
+	defer wrapf(&err, "JSON Pointer %q", sptr)
-		if err != nil {
-			err = fmt.Errorf("JSON Pointer %q: %w", sptr, err)
-		}
-	}()
 
 	segments, err := parseJSONPointer(sptr)
 	if err != nil {

jsonschema/util.go

@@ -282,3 +282,10 @@ func assert(cond bool, msg string) {
 		panic("assertion failed: " + msg)
 	}
 }
+
+// wrapf wraps *errp with the given formatted message if *errp is not nil.
+func wrapf(errp *error, format string, args ...any) {
+	if *errp != nil {
+		*errp = fmt.Errorf("%s: %w", fmt.Sprintf(format, args...), *errp)
+	}
+}

jsonschema/validate.go

@@ -42,11 +42,7 @@ type state struct {
 
 // validate validates the reflected value of the instance.
 func (st *state) validate(instance reflect.Value, schema *Schema, callerAnns *annotations) (err error) {
-	defer func() {
+	defer wrapf(&err, "validating %s", schema)
-		if err != nil {
-			err = fmt.Errorf("%s: %w", schema, err)
-		}
-	}()
 
 	st.stack = append(st.stack, schema) // push
 	defer func() {