fix(metrics): return 400 not 500 for empty or malformed GraphQL request body

pkg/assembler/server/server_badrequest_test.go

@@ -0,0 +1,85 @@
+//
+// Copyright 2024 The GUAC Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package server
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	_ "github.com/guacsec/guac/pkg/assembler/backends/keyvalue"
+
+	"github.com/guacsec/guac/internal/testing/stablememmap"
+	"github.com/guacsec/guac/pkg/assembler/backends"
+	"github.com/guacsec/guac/pkg/metrics"
+)
+
+// TestQueryEndpointBadRequest covers issue #2195: posting an empty or malformed
+// JSON body to the /query endpoint must return 400 Bad Request, not 500
+// Internal Server Error. The handler is exercised together with the Prometheus
+// metrics middleware because that middleware (not the GraphQL server) was the
+// source of the original 500 response.
+func TestQueryEndpointBadRequest(t *testing.T) {
+	ctx := metrics.WithMetrics(context.Background(), "guac_query_test")
+
+	store := stablememmap.GetStore()
+	backend, err := backends.Get("keyvalue", ctx, store)
+	if err != nil {
+		t.Fatalf("Error getting backend: %v", err)
+	}
+
+	srv := GetGraphqlServer(ctx, backend)
+	collector := metrics.FromContext(ctx, "guac_query_test")
+	handler := collector.MeasureGraphQLResponseDuration(srv)
+
+	tests := []struct {
+		name       string
+		body       string
+		wantStatus int
+	}{
+		{
+			name:       "empty body",
+			body:       "",
+			wantStatus: http.StatusBadRequest,
+		},
+		{
+			name:       "malformed json",
+			body:       "{not json",
+			wantStatus: http.StatusBadRequest,
+		},
+		{
+			name:       "valid query",
+			body:       `{"query":"{ __typename }"}`,
+			wantStatus: http.StatusOK,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			req := httptest.NewRequest(http.MethodPost, "/query", strings.NewReader(tc.body))
+			req.Header.Set("Content-Type", "application/json")
+			rr := httptest.NewRecorder()
+
+			handler.ServeHTTP(rr, req)
+
+			if rr.Code != tc.wantStatus {
+				t.Fatalf("POST /query with %s body: got status %d, want %d (body: %q)", tc.name, rr.Code, tc.wantStatus, rr.Body.String())
+			}
+		})
+	}
+}

pkg/metrics/measure_graphql_test.go

@@ -0,0 +1,77 @@
+//
+// Copyright 2024 The GUAC Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metrics
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// TestMeasureGraphQLResponseDurationDoesNotShortCircuit guards against a
+// regression of issue #2195: an empty or malformed request body used to make
+// this metrics middleware return 500 before the request ever reached the
+// GraphQL handler. The middleware must not decide request validity; it should
+// pass the request through to the downstream handler (which answers with the
+// correct status code for the payload) regardless of whether the body is valid
+// JSON.
+func TestMeasureGraphQLResponseDurationDoesNotShortCircuit(t *testing.T) {
+	ctx := WithMetrics(context.Background(), "guac_graphql_test")
+	collector := FromContext(ctx, "guac_graphql_test")
+
+	// A downstream handler that echoes the body it received so we can assert
+	// the middleware forwarded it unchanged.
+	var sawBody string
+	downstream := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		b, _ := io.ReadAll(r.Body)
+		sawBody = string(b)
+		w.WriteHeader(http.StatusOK)
+	})
+
+	handler := collector.MeasureGraphQLResponseDuration(downstream)
+
+	tests := []struct {
+		name string
+		body string
+	}{
+		{name: "empty body", body: ""},
+		{name: "malformed json", body: "{not json"},
+		{name: "valid json with operation name", body: `{"operationName":"Q","query":"{ __typename }"}`},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			sawBody = ""
+			req := httptest.NewRequest(http.MethodPost, "/query", strings.NewReader(tc.body))
+			req.Header.Set("Content-Type", "application/json")
+			rr := httptest.NewRecorder()
+
+			handler.ServeHTTP(rr, req)
+
+			// The middleware must reach the downstream handler instead of
+			// short-circuiting with a 500.
+			if rr.Code != http.StatusOK {
+				t.Fatalf("middleware short-circuited bad body: got status %d, want %d (request should reach downstream handler)", rr.Code, http.StatusOK)
+			}
+			if sawBody != tc.body {
+				t.Fatalf("downstream received body %q, want %q (body must be forwarded unchanged)", sawBody, tc.body)
+			}
+		})
+	}
+}

pkg/metrics/prometheus.go

@@ -276,26 +276,26 @@ func (pc *prometheusCollector) MeasureGraphQLResponseDuration(next http.Handler)
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		start := time.Now()
 
-		// Create a copy of the request body
+		// Create a copy of the request body so we can both inspect it here and
+		// still hand the original bytes to the downstream GraphQL handler.
 		body, err := io.ReadAll(r.Body)
 		if err != nil {
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
 		r.Body = io.NopCloser(bytes.NewBuffer(body)) // Replace the request body with a copy
 
-		// Create another copy for JSON unmarshalling
-		bodyCopy := make([]byte, len(body))
-		copy(bodyCopy, body)
-
-		// Parse the operation name from the request body copy
+		// Best-effort parse of the operation name for the metric label. An empty
+		// or malformed body must not be turned into an error here: validating the
+		// request payload is the GraphQL server's job, and it already responds
+		// with 400 Bad Request for bad input. Returning 500 from this metrics
+		// middleware short-circuited that and was misleading (see issue #2195),
+		// so we only record the operation name when the body parses and otherwise
+		// let the request flow through unchanged.
 		var graphqlRequest struct {
 			OperationName string `json:"operationName"`
 		}
-		if err := json.Unmarshal(bodyCopy, &graphqlRequest); err != nil {
-			http.Error(w, err.Error(), http.StatusInternalServerError)
-			return
-		}
+		_ = json.Unmarshal(body, &graphqlRequest)
 
 		rec := statusRecorder{w, http.StatusOK}