fix: fallback behaviour added to atomic writes

crates/pixi_utils/src/atomic_write.rs

@@ -16,11 +16,17 @@ fn temp_file_for(path: &Path) -> std::io::Result<tempfile::NamedTempFile> {
         path.file_name().and_then(|n| n.to_str()).unwrap_or("tmp")
     );
 
-    tempfile::Builder::new().prefix(&prefix).tempfile_in(dir)
+    match tempfile::Builder::new().prefix(&prefix).tempfile_in(dir) {
+        Ok(file) => Ok(file),
+        Err(e) if e.kind() == std::io::ErrorKind::PermissionDenied => tempfile::Builder::new()
+            .prefix(&prefix)
+            .tempfile_in(std::env::temp_dir()),
+        Err(e) => Err(e),
+    }
 }
 
-/// Atomically write contents to a file by first writing to a temporary file in
-/// the same directory and then renaming it to the target path.
+/// Atomically write contents to a file by first writing to a temporary file and
+/// then renaming it to the target path.
 ///
 /// This ensures that the target file is never left in a partially-written state.
 /// If the write fails (e.g., due to disk full), the original file remains
@@ -49,3 +55,51 @@ pub fn atomic_write_sync(path: &Path, contents: impl AsRef<[u8]>) -> std::io::Re
 
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+
+    #[test]
+    fn test_temp_file_created_in_same_dir_when_writable() {
+        let dir = tempfile::tempdir().unwrap();
+        let target = dir.path().join("pixi.toml");
+
+        let temp = temp_file_for(&target).unwrap();
+
+        assert_eq!(temp.path().parent().unwrap(), dir.path());
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn test_temp_file_falls_back_to_tmp_when_parent_not_writable() {
+        use std::os::unix::fs::PermissionsExt;
+
+        let dir = tempfile::tempdir().unwrap();
+        let target = dir.path().join("pixi.toml");
+        fs::write(&target, b"[project]").unwrap();
+
+        fs::set_permissions(dir.path(), fs::Permissions::from_mode(0o555)).unwrap();
+
+        let temp = temp_file_for(&target).unwrap();
+
+        assert_eq!(temp.path().parent().unwrap(), std::env::temp_dir());
+        // resetting the permissions for cleanup
+        fs::set_permissions(dir.path(), fs::Permissions::from_mode(0o755)).unwrap();
+    }
+
+    #[test]
+    fn temp_file_has_correct_prefix() {
+        let dir = tempfile::tempdir().unwrap();
+        let target = dir.path().join("pixi.toml");
+
+        let temp = temp_file_for(&target).unwrap();
+        let name = temp.path().file_name().unwrap().to_str().unwrap();
+
+        assert!(
+            name.starts_with(".pixi.toml."),
+            "expected prefix `.pixi.toml.`, got `{name}`"
+        );
+    }
+}