Add a test for Go flock implementation

tools/utils/filelock_test.go

@@ -0,0 +1,43 @@
+// License: GPLv3 Copyright: 2024, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package utils
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"testing"
+)
+
+var _ = fmt.Print
+
+func TestFileLock(t *testing.T) {
+	tdir := t.TempDir()
+
+	file_descriptor, err := os.Open(tdir)
+	if err != nil {
+		t.Fatalf("Initial open of %s failed with error: %s", tdir, err)
+	}
+	if err = LockFileExclusive(file_descriptor); err != nil {
+		file_descriptor.Close()
+		t.Fatalf("Initial lock of %s failed with error: %s", tdir, err)
+	}
+	defer func() {
+		_ = UnlockFile(file_descriptor)
+		file_descriptor.Close()
+	}()
+	cmd := exec.Command(KittyExe(), "+runpy", `
+import sys, os, fcntl
+fd = os.open(sys.argv[-1], os.O_RDONLY)
+try:
+	fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+except BlockingIOError:
+    sys.exit(0)
+else:
+	print("Lock unexpectedly succeeded", flush=True)
+	sys.exit(1)
+`, tdir)
+	if output, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("Lock test process failed with error: %s and output:\n%s", err, string(output))
+	}
+}