Include parse error position in validate_expr response

Author: juliusv

distributor/http_server.go

@@ -1,6 +1,7 @@
 package distributor
 
 import (
+	"fmt"
 	"net/http"
 
 	"github.com/prometheus/common/log"
@@ -59,6 +60,39 @@ func (d *Distributor) UserStatsHandler(w http.ResponseWriter, r *http.Request) {
 // ValidateExprHandler validates a PromQL expression.
 func (d *Distributor) ValidateExprHandler(w http.ResponseWriter, r *http.Request) {
 	_, err := promql.ParseExpr(r.FormValue("expr"))
-	valid := (err == nil)
-	util.WriteJSONResponse(w, map[string]bool{"valid": valid})
+
+	// We mimick the response format of Prometheus's official API here for
+	// consistency, but unfortunately its private types (string consts etc.)
+	// aren't reusable.
+	if err == nil {
+		util.WriteJSONResponse(w, map[string]string{
+			"status": "success",
+		})
+		return
+	}
+
+	parseErr, ok := err.(*promql.ParseErr)
+	if !ok {
+		// This should always be a promql.ParseErr.
+		http.Error(w, fmt.Sprintf("unexpected error returned from PromQL parser: %v", err), http.StatusInternalServerError)
+		return
+	}
+
+	// If the parsing input was a single line, parseErr.Line is 0
+	// and the generated error string omits the line entirely. But we
+	// want to report line numbers consistently, no matter how many
+	// lines there are (starting at 1).
+	if parseErr.Line == 0 {
+		parseErr.Line = 1
+	}
+	w.WriteHeader(http.StatusBadRequest)
+	util.WriteJSONResponse(w, map[string]interface{}{
+		"status":    "error",
+		"errorType": "bad_data",
+		"error":     err.Error(),
+		"location": map[string]int{
+			"line": parseErr.Line,
+			"pos":  parseErr.Pos,
+		},
+	})
 }