flake history: make operations atomic

pkgs/clan-cli/clan_cli/flakes/add.py

@@ -5,19 +5,22 @@ from pathlib import Path
 from clan_cli.dirs import user_history_file
 
 from ..async_cmd import CmdOut, runforcli
+from ..locked_open import locked_open
 
 
 async def add_flake(path: Path) -> dict[str, CmdOut]:
     user_history_file().parent.mkdir(parents=True, exist_ok=True)
     # append line to history file
     # TODO: Make this atomic
-    lines: set[str] = set()
-    if user_history_file().exists():
-        with open(user_history_file()) as f:
-            lines = set(f.readlines())
-    lines.add(str(path))
-    with open(user_history_file(), "w") as f:
-        f.writelines(lines)
+    lines: set = set()
+    old_lines = set()
+    with locked_open(user_history_file(), "w+") as f:
+        old_lines = set(f.readlines())
+        lines = old_lines | {str(path)}
+        if old_lines != lines:
+            f.seek(0)
+            f.writelines(lines)
+            f.truncate()
     return {}
 
 

pkgs/clan-cli/clan_cli/flakes/history.py

@@ -4,12 +4,14 @@ from pathlib import Path
 
 from clan_cli.dirs import user_history_file
 
+from ..locked_open import locked_open
+
 
 def list_history() -> list[Path]:
     if not user_history_file().exists():
         return []
     # read path lines from history file
-    with open(user_history_file()) as f:
+    with locked_open(user_history_file()) as f:
         lines = f.readlines()
     return [Path(line.strip()) for line in lines]
 

pkgs/clan-cli/clan_cli/locked_open.py

@@ -0,0 +1,15 @@
+import fcntl
+from collections.abc import Generator
+from contextlib import contextmanager
+from pathlib import Path
+
+
+@contextmanager
+def locked_open(filename: str | Path, mode: str = "r") -> Generator:
+    """
+    This is a context manager that provides an advisory write lock on the file specified by `filename` when entering the context, and releases the lock when leaving the context. The lock is acquired using the `fcntl` module's `LOCK_EX` flag, which applies an exclusive write lock to the file.
+    """
+    with open(filename, mode) as fd:
+        fcntl.flock(fd, fcntl.LOCK_EX)
+        yield fd
+        fcntl.flock(fd, fcntl.LOCK_UN)