[query] Fix incorrect content type in m3query/ error response

src/cmd/tools/dtest/docker/harness/carbon_test.go

@@ -32,7 +32,7 @@ import (
 )
 
 func findVerifier(expected string) resources.ResponseVerifier {
-	return func(status int, s string, err error) error {
+	return func(status int, _ map[string][]string, s string, err error) error {
 		if err != nil {
 			return err
 		}
@@ -55,7 +55,7 @@ func renderVerifier(v float64) resources.ResponseVerifier {
 		Datapoints [][]float64 `json:"datapoints"`
 	}
 
-	return func(status int, s string, err error) error {
+	return func(status int, _ map[string][]string, s string, err error) error {
 		if err != nil {
 			return err
 		}

src/cmd/tools/dtest/docker/harness/query_api_test.go

@@ -71,7 +71,7 @@ func testInvalidQueryReturns400(t *testing.T, tests []urlTest) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			assert.NoError(t, coord.RunQuery(verifyStatus(400), tt.url), "for query '%v'", tt.url)
+			assert.NoError(t, coord.RunQuery(verifyResponse(400), tt.url), "for query '%v'", tt.url)
 		})
 	}
 }
@@ -129,8 +129,8 @@ func queryString(params map[string]string) string {
 	return strings.Join(p, "&")
 }
 
-func verifyStatus(expectedStatus int) resources.ResponseVerifier {
-	return func(status int, resp string, err error) error {
+func verifyResponse(expectedStatus int) resources.ResponseVerifier {
+	return func(status int, headers map[string][]string, resp string, err error) error {
 		if err != nil {
 			return err
 		}
@@ -139,6 +139,12 @@ func verifyStatus(expectedStatus int) resources.ResponseVerifier {
 			return fmt.Errorf("expeceted %v status code, got %v", expectedStatus, status)
 		}
 
+		if contentType, ok := headers["Content-Type"]; !ok {
+			return fmt.Errorf("missing Content-Type header")
+		} else if len(contentType) != 1 || contentType[0] != "application/json" {
+			return fmt.Errorf("expected json content type, got %v", contentType)
+		}
+
 		return nil
 	}
 }

src/cmd/tools/dtest/docker/harness/resources/coordinator.go

@@ -55,7 +55,7 @@ var (
 )
 
 // ResponseVerifier is a function that checks if the query response is valid.
-type ResponseVerifier func(int, string, error) error
+type ResponseVerifier func(int, map[string][]string, string, error) error
 
 // Coordinator is a wrapper for a coordinator. It provides a wrapper on HTTP
 // endpoints that expose cluster management APIs as well as read and write
@@ -363,7 +363,7 @@ func (c *coordinator) query(
 	defer resp.Body.Close()
 	b, err := ioutil.ReadAll(resp.Body)
 
-	return verifier(resp.StatusCode, string(b), err)
+	return verifier(resp.StatusCode, resp.Header, string(b), err)
 }
 
 func (c *coordinator) RunQuery(

src/x/net/http/errors.go

@@ -92,25 +92,27 @@ func WriteError(w http.ResponseWriter, err error, opts ...WriteErrorOption) {
 		fn(&o)
 	}
 
+	statusCode := getStatusCode(err)
+	if o.response == nil {
+		w.Header().Set(HeaderContentType, ContentTypeJSON)
+		w.WriteHeader(statusCode)
+		json.NewEncoder(w).Encode(ErrorResponse{Error: err.Error()})
+	} else {
+		w.WriteHeader(statusCode)
+		w.Write(o.response)
+	}
+}
+
+func getStatusCode(err error) int {
 	switch v := err.(type) {
 	case Error:
-		w.WriteHeader(v.Code())
+		return v.Code()
 	case error:
 		if xerrors.IsInvalidParams(v) {
-			w.WriteHeader(http.StatusBadRequest)
+			return http.StatusBadRequest
 		} else if errors.Is(err, context.DeadlineExceeded) {
-			w.WriteHeader(http.StatusGatewayTimeout)
-		} else {
-			w.WriteHeader(http.StatusInternalServerError)
+			return http.StatusGatewayTimeout
 		}
-	default:
-		w.WriteHeader(http.StatusInternalServerError)
 	}
-
-	if o.response != nil {
-		w.Write(o.response)
-		return
-	}
-
-	json.NewEncoder(w).Encode(ErrorResponse{Error: err.Error()})
+	return http.StatusInternalServerError
 }