promql: fix potential panic in the query logger (#6094)

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

promql/query_logger.go

@@ -41,12 +41,14 @@ const (
 	entrySize int = 1000
 )
 
-func parseBrokenJson(brokenJson []byte, logger log.Logger) (bool, string) {
+func parseBrokenJson(brokenJson []byte) (bool, string) {
 	queries := strings.ReplaceAll(string(brokenJson), "\x00", "")
-	queries = queries[:len(queries)-1] + "]"
+	if len(queries) > 0 {
+		queries = queries[:len(queries)-1] + "]"
+	}
 
 	// Conditional because of implementation detail: len() = 1 implies file consisted of a single char: '['.
-	if len(queries) == 1 {
+	if len(queries) <= 1 {
 		return false, "[]"
 	}
 
@@ -68,7 +70,7 @@ func logUnfinishedQueries(filename string, filesize int, logger log.Logger) {
 			return
 		}
 
-		queriesExist, queries := parseBrokenJson(brokenJson, logger)
+		queriesExist, queries := parseBrokenJson(brokenJson)
 		if !queriesExist {
 			return
 		}

promql/query_logger_test.go

@@ -136,3 +136,43 @@ func TestMMapFile(t *testing.T) {
 		t.Fatalf("Mmap failed")
 	}
 }
+
+func TestParseBrokenJson(t *testing.T) {
+	for _, tc := range []struct {
+		b []byte
+
+		ok  bool
+		out string
+	}{
+		{
+			b: []byte(""),
+		},
+		{
+			b: []byte("\x00\x00"),
+		},
+		{
+			b: []byte("\x00[\x00"),
+		},
+		{
+			b:   []byte("\x00[]\x00"),
+			ok:  true,
+			out: "[]",
+		},
+		{
+			b:   []byte("[\"up == 0\",\"rate(http_requests[2w]\"]\x00\x00\x00"),
+			ok:  true,
+			out: "[\"up == 0\",\"rate(http_requests[2w]\"]",
+		},
+	} {
+		t.Run("", func(t *testing.T) {
+			ok, out := parseBrokenJson(tc.b)
+			if tc.ok != ok {
+				t.Fatalf("expected %t, got %t", tc.ok, ok)
+				return
+			}
+			if ok && tc.out != out {
+				t.Fatalf("expected %s, got %s", tc.out, out)
+			}
+		})
+	}
+}