internal/lsp: publish protocol.UnmarshalJSON

Missing or null arguments can be expressed in a variety of ways,
so provide a helper function to consolidate the necessary checks.

Change-Id: I595f71a6485d84883667f3c6adf5a87c41fbd6aa
Reviewed-on: https://go-review.googlesource.com/c/tools/+/556695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/cache/diagnostics.go

@@ -164,16 +164,13 @@ func bundleQuickFixes(sd *Diagnostic) bool {
 // BundledQuickFixes extracts any bundled codeActions from the
 // diag.Data field.
 func BundledQuickFixes(diag protocol.Diagnostic) []protocol.CodeAction {
-	// Clients may express "no fixes" in a variety of ways (#64503).
-	if diag.Data == nil ||
-		len(*diag.Data) == 0 ||
-		len(*diag.Data) == 4 && string(*diag.Data) == "null" {
-		return nil
-	}
 	var fix quickFixesJSON
-	if err := json.Unmarshal(*diag.Data, &fix); err != nil {
-		bug.Reportf("unmarshalling quick fix: %v", err)
-		return nil
+	if diag.Data != nil {
+		err := protocol.UnmarshalJSON(*diag.Data, &fix)
+		if err != nil {
+			bug.Reportf("unmarshalling quick fix: %v", err)
+			return nil
+		}
 	}
 
 	var actions []protocol.CodeAction

gopls/internal/protocol/generate/output.go

@@ -101,7 +101,7 @@ func genCase(method string, param, result *Type, dir string) {
 			nm = "ParamConfiguration" // gopls compatibility
 		}
 		fmt.Fprintf(out, "\t\tvar params %s\n", nm)
-		fmt.Fprintf(out, "\t\tif err := unmarshalParams(r.Params(), &params); err != nil {\n")
+		fmt.Fprintf(out, "\t\tif err := UnmarshalJSON(r.Params(), &params); err != nil {\n")
 		fmt.Fprintf(out, "\t\t\treturn true, sendParseError(ctx, reply, err)\n\t\t}\n")
 		p = ", &params"
 	}

gopls/internal/protocol/protocol.go

@@ -241,7 +241,7 @@ func CancelHandler(handler jsonrpc2.Handler) jsonrpc2.Handler {
 			return handler(ctx, replyWithDetachedContext, req)
 		}
 		var params CancelParams
-		if err := unmarshalParams(req.Params(), &params); err != nil {
+		if err := UnmarshalJSON(req.Params(), &params); err != nil {
 			return sendParseError(ctx, reply, err)
 		}
 		if n, ok := params.ID.(float64); ok {
@@ -271,16 +271,14 @@ func cancelCall(ctx context.Context, sender connSender, id jsonrpc2.ID) {
 	sender.Notify(ctx, "$/cancelRequest", &CancelParams{ID: &id})
 }
 
-// unmarshalParams unmarshals msg into the variable pointed to by
-// params. In JSONRPC, request.params is optional, so msg may may be
+// UnmarshalJSON unmarshals msg into the variable pointed to by
+// params. In JSONRPC, optional messages may be
 // "null", in which case it is a no-op.
-func unmarshalParams(msg json.RawMessage, params any) error {
-	if len(msg) > 0 && !bytes.Equal(msg, []byte("null")) {
-		if err := json.Unmarshal(msg, params); err != nil {
-			return err
-		}
+func UnmarshalJSON(msg json.RawMessage, v any) error {
+	if len(msg) == 0 || bytes.Equal(msg, []byte("null")) {
+		return nil
 	}
-	return nil
+	return json.Unmarshal(msg, v)
 }
 
 func sendParseError(ctx context.Context, reply jsonrpc2.Replier, err error) error {