chore(format_query): Change response type to application/json

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * [8953](https://github.com/grafana/loki/pull/8953) **dannykopping**: Querier: block queries by hash.
 * [8851](https://github.com/grafana/loki/pull/8851) **jeschkies**: Introduce limit to require a set of labels for selecting streams.
+* [9016](https://github.com/grafana/loki/pull/9016) **kavirajk**: Change response type of `format_query` handler to `application/json`
 
 ##### Fixes
 

docs/sources/api/_index.md

@@ -729,6 +729,15 @@ Params:
 
 The `/loki/api/v1/format_query` endpoint allows to format LogQL queries. It returns an error if the passed LogQL is invalid. It is exposed by all Loki components and helps to improve readability and the debugging experience of LogQL queries.
 
+The following example formats the expression LogQL `{foo=   "bar"}` into
+
+```json
+{
+   "status" : "success",
+   "data" : "{foo=\"bar\"}"
+}
+```
+
 ## List series
 
 The Series API is available under the following:

pkg/loki/format_query_handler.go

@@ -1,24 +1,51 @@
 package loki
 
 import (
-	"fmt"
+	"encoding/json"
 	"net/http"
 
 	"github.com/grafana/loki/pkg/logql/syntax"
-	serverutil "github.com/grafana/loki/pkg/util/server"
+	"github.com/grafana/loki/pkg/util/server"
 )
 
 func formatQueryHandler() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
+		var (
+			statusCode = http.StatusOK
+			status     = "success"
+			formatted  string
+			errStr     string
+		)
+
 		expr, err := syntax.ParseExpr(r.FormValue("query"))
 		if err != nil {
-			serverutil.WriteError(err, w)
-			return
+			statusCode = http.StatusBadRequest
+			status = "invalid-query"
+			errStr = err.Error()
+		}
+
+		if err == nil {
+			formatted = syntax.Prettify(expr)
+		}
+
+		resp := FormatQueryResponse{
+			Status: status,
+			Data:   formatted,
+			Err:    errStr,
 		}
 
-		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
-		w.WriteHeader(http.StatusOK)
+		w.Header().Set("Content-Type", "application/json; charset=utf-8")
+		w.WriteHeader(statusCode)
+
+		if err := json.NewEncoder(w).Encode(resp); err != nil {
+			server.WriteError(err, w)
+		}
 
-		fmt.Fprintf(w, "%s", syntax.Prettify(expr))
 	}
 }
+
+type FormatQueryResponse struct {
+	Status string `json:"status"`
+	Data   string `json:"data,omitempty"`
+	Err    string `json:"error,omitempty"`
+}

pkg/loki/format_query_handler_test.go

@@ -0,0 +1,55 @@
+package loki
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_formatQueryHandlerResponse(t *testing.T) {
+	cases := []struct {
+		name     string
+		query    string
+		expected FormatQueryResponse
+	}{
+		{
+			name:  "happy-path",
+			query: `{foo="bar"}`,
+			expected: FormatQueryResponse{
+				Status: "success",
+				Data:   `{foo="bar"}`,
+			},
+		},
+		{
+			name:  "invalid-query",
+			query: `{foo="bar}`,
+			expected: FormatQueryResponse{
+				Status: "invalid-query",
+				Err:    "parse error at line 1, col 6: literal not terminated",
+			},
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			req, err := http.NewRequest("GET", fmt.Sprintf("http://localhost:808?query=%s", tc.query), nil)
+			require.NoError(t, err)
+
+			w := httptest.NewRecorder()
+
+			formatQueryHandler()(w, req)
+
+			var got FormatQueryResponse
+
+			err = json.NewDecoder(w.Body).Decode(&got)
+			require.NoError(t, err)
+
+			assert.Equal(t, tc.expected, got)
+		})
+	}
+}