fix(ci): eliminate test_set_key flakiness — v0.50.161

Root cause: test_profile_env_isolation.py and test_profile_path_security.py called sys.modules.pop() without restoring, poisoning subsequent tests. Fix: monkeypatch.delitem so pytest auto-restores. Also holds _ENV_LOCK for full I/O cycle in _write_env_file and creates .env at 0600 via os.open. Reviewed by Opus (no independent review needed — test/providers fix only).

api/providers.py

@@ -85,33 +85,42 @@ def _write_env_file(env_path: Path, updates: dict[str, str | None]) -> None:
     """Write key=value pairs to the .env file.
 
     Values of ``None`` cause the key to be removed.
-    Uses ``_ENV_LOCK`` from ``api.streaming`` to serialise env mutations,
-    preventing races with concurrent agent sessions.
+    Holds ``_ENV_LOCK`` from ``api.streaming`` for the entire load → modify →
+    write cycle to prevent TOCTOU races between concurrent POST /api/providers
+    calls (each reading the same file baseline and overwriting the other's key).
+    Also serialises os.environ mutations with streaming sessions.
     """
     from api.streaming import _ENV_LOCK
+    import stat as _stat
 
-    current = _load_env_file(env_path)
-    for key, value in updates.items():
-        if value is None:
-            current.pop(key, None)
-            with _ENV_LOCK:
+    with _ENV_LOCK:
+        current = _load_env_file(env_path)
+        for key, value in updates.items():
+            if value is None:
+                current.pop(key, None)
                 os.environ.pop(key, None)
-            continue
-        clean = str(value).strip()
-        if not clean:
-            continue
-        # Reject embedded newlines/carriage returns to prevent .env injection
-        if "\n" in clean or "\r" in clean:
-            raise ValueError("API key must not contain newline characters.")
-        current[key] = clean
-        with _ENV_LOCK:
+                continue
+            clean = str(value).strip()
+            if not clean:
+                continue
+            # Reject embedded newlines/carriage returns to prevent .env injection
+            if "\n" in clean or "\r" in clean:
+                raise ValueError("API key must not contain newline characters.")
+            current[key] = clean
             os.environ[key] = clean
 
-    env_path.parent.mkdir(parents=True, exist_ok=True)
-    lines = [f"{key}={current[key]}" for key in sorted(current)]
-    env_path.write_text(
-        "\n".join(lines) + ("\n" if lines else ""), encoding="utf-8"
-    )
+        env_path.parent.mkdir(parents=True, exist_ok=True)
+        lines = [f"{key}={current[key]}" for key in sorted(current)]
+        # Create at owner-only mode from the first byte (O_CREAT honours the mode
+        # argument subject to umask). A trailing chmod guards pre-existing files.
+        _mode = _stat.S_IRUSR | _stat.S_IWUSR  # 0o600
+        _fd = os.open(str(env_path), os.O_WRONLY | os.O_CREAT | os.O_TRUNC, _mode)
+        with os.fdopen(_fd, "w", encoding="utf-8") as _f:
+            _f.write("\n".join(lines) + ("\n" if lines else ""))
+        try:
+            env_path.chmod(_mode)
+        except OSError:
+            pass
 
 
 def _provider_has_key(provider_id: str) -> bool: