working copy: allow overwriting untracked files

Before this commit, we would crash when checking out a commit that has
a file that's currently ignored in the working copy.

lib/src/working_copy.rs

@@ -420,12 +420,14 @@ impl TreeState {
         executable: bool,
     ) -> FileState {
         create_parent_dirs(disk_path);
+        // TODO: Check that we're not overwriting an un-ignored file here (which might
+        // be created by a concurrent process).
         let mut file = OpenOptions::new()
             .write(true)
-            .create_new(true)
+            .create(true)
             .truncate(true)
             .open(disk_path)
-            .unwrap_or_else(|_| panic!("failed to open {:?} for write", &disk_path));
+            .unwrap_or_else(|err| panic!("failed to open {:?} for write: {:?}", &disk_path, err));
         let mut contents = self.store.read_file(path, id).unwrap();
         std::io::copy(&mut contents, &mut file).unwrap();
         self.set_executable(disk_path, executable);

lib/tests/test_working_copy.rs

@@ -12,8 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use std::fs::OpenOptions;
+use std::fs::{File, OpenOptions};
-use std::io::Write;
+use std::io::{Read, Write};
 #[cfg(unix)]
 use std::os::unix::fs::PermissionsExt;
 use std::sync::Arc;
@@ -339,3 +339,50 @@ fn test_gitignores(use_git: bool) {
         ]
     );
 }
+
+#[test_case(false ; "local store")]
+#[test_case(true ; "git store")]
+fn test_gitignores_checkout_overwrites_ignored(use_git: bool) {
+    // Tests that a .gitignore'd file gets overwritten if check out a commit where
+    // the file is tracked.
+
+    let _home_dir = testutils::new_user_home();
+    let settings = testutils::user_settings();
+    let (_temp_dir, repo) = testutils::init_repo(&settings, use_git);
+
+    // Write an ignored file called "modified" to disk
+    let gitignore_path = FileRepoPath::from(".gitignore");
+    testutils::write_working_copy_file(&repo, &gitignore_path, "modified\n");
+    let modified_path = FileRepoPath::from("modified");
+    testutils::write_working_copy_file(&repo, &modified_path, "garbage");
+
+    // Create a commit that adds the same file but with different contents
+    let mut tx = repo.start_transaction("test");
+    let mut tree_builder = repo
+        .store()
+        .tree_builder(repo.store().empty_tree_id().clone());
+    testutils::write_normal_file(&mut tree_builder, &modified_path, "contents");
+    let tree_id = tree_builder.write_tree();
+    let commit = CommitBuilder::for_new_commit(&settings, repo.store(), tree_id)
+        .set_open(true)
+        .set_description("add file".to_string())
+        .write_to_repo(tx.mut_repo());
+    let repo = tx.commit();
+
+    // Now check out the commit that adds the file "modified" with contents
+    // "contents". The exiting contents ("garbage") should be replaced in the
+    // working copy.
+    repo.working_copy_locked().check_out(commit).unwrap();
+
+    // Check that the new contents are in the working copy
+    let path = repo.working_copy_path().join("modified");
+    assert!(path.is_file());
+    let mut file = File::open(path).unwrap();
+    let mut buf = Vec::new();
+    file.read_to_end(&mut buf).unwrap();
+    assert_eq!(buf, b"contents");
+
+    // Check that the file is in the commit created by committing the working copy
+    let (_repo, new_commit) = repo.working_copy_locked().commit(&settings, repo.clone());
+    assert!(new_commit.tree().entry("modified").is_some());
+}