scripts: Add zulip_tools.overwrite_symlink function to replace ln -nsf.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

scripts/lib/zulip_tools.py

@@ -11,6 +11,7 @@ import shlex
 import shutil
 import subprocess
 import sys
+import tempfile
 import time
 import json
 import uuid
@@ -38,6 +39,23 @@ BLUE = '\x1b[34m'
 MAGENTA = '\x1b[35m'
 CYAN = '\x1b[36m'
 
+def overwrite_symlink(src, dst):
+    # type: (str, str) -> None
+    while True:
+        tmp = tempfile.mktemp(
+            prefix='.' + os.path.basename(dst) + '.',
+            dir=os.path.dirname(dst))
+        try:
+            os.symlink(src, tmp)
+        except FileExistsError:
+            continue
+        break
+    try:
+        os.rename(tmp, dst)
+    except Exception:
+        os.remove(tmp)
+        raise
+
 def parse_cache_script_args(description):
     # type: (str) -> argparse.Namespace
     parser = argparse.ArgumentParser(description=description)