executor: fix query panic on INFORMATION_SCHEMA.CLUSTER_SLOW_QUERY wi…

…th some slow query files (#54325) (#54935)

close #54324

executor/slow_query.go

@@ -1114,13 +1114,8 @@ func readLastLines(ctx context.Context, file *os.File, endCursor int64) ([]strin
 		lines = append(chars, lines...) // nozero
 
 		// find first '\n' or '\r'
-		for i := 0; i < len(chars); i++ {
-			// reach the line end
-			// the first newline may be in the line end at the first round
-			if i >= len(lines)-1 {
-				break
-			}
-			if (chars[i] == 10 || chars[i] == 13) && chars[i+1] != 10 && chars[i+1] != 13 {
+		for i := 0; i < len(chars)-1; i++ {
+			if (chars[i] == '\n' || chars[i] == '\r') && chars[i+1] != '\n' && chars[i+1] != '\r' {
 				firstNonNewlinePos = i + 1
 				break
 			}

executor/slow_query_test.go

@@ -720,3 +720,27 @@ func removeFiles(fileNames []string) {
 		os.Remove(fileName)
 	}
 }
+
+func TestIssue54324(t *testing.T) {
+	f, err := os.CreateTemp("", "test-tidb-slow-query-issue54324")
+	require.NoError(t, err)
+	defer os.Remove(f.Name()) // clean up
+
+	w := bufio.NewWriter(f)
+	for i := 0; i < 8191; i++ {
+		w.WriteByte('x')
+	}
+	w.WriteByte('\n')
+	for i := 0; i < 4096; i++ {
+		w.WriteByte('a')
+	}
+	require.NoError(t, w.Flush())
+
+	stat, err := f.Stat()
+	require.NoError(t, err)
+	endCursor := stat.Size()
+	lines, readBytes, err := readLastLines(context.Background(), f, endCursor)
+	require.NoError(t, err)
+	require.Len(t, lines, 2)
+	require.Equal(t, readBytes, 8192+4096)
+}