windows: don't fail when concurrent threads/processes fail to rename file

On Windows, it seems that you can't rename a file if the target file
is open (Stebalien/tempfile#131). I think that's the reason for our
failing tests on Windows. This patch adds a simple wrapper around
`NamedTempFile::persist()` that returns the existing file instead of
failing, if there is one.

lib/src/file_util.rs

@@ -0,0 +1,73 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::fs::File;
+use std::path::Path;
+
+use tempfile::{NamedTempFile, PersistError};
+
+// Like NamedTempFile::persist(), but also succeeds if the target already
+// exists.
+pub fn persist_temp_file<P: AsRef<Path>>(
+    temp_file: NamedTempFile,
+    new_path: P,
+) -> Result<File, PersistError> {
+    match temp_file.persist(&new_path) {
+        Ok(file) => Ok(file),
+        Err(PersistError { error, file }) => {
+            if let Ok(existing_file) = File::open(new_path) {
+                Ok(existing_file)
+            } else {
+                Err(PersistError { error, file })
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use std::env::temp_dir;
+    use std::io::Write;
+
+    use test_case::test_case;
+
+    use super::*;
+
+    #[test]
+    fn test_persist_no_existing_file() {
+        let temp_dir = temp_dir();
+        let target = temp_dir.join("file");
+        let mut temp_file = NamedTempFile::new_in(&temp_dir).unwrap();
+        temp_file.write_all(b"contents").unwrap();
+        assert!(persist_temp_file(temp_file, &target).is_ok());
+    }
+
+    #[test_case(false ; "existing file open")]
+    #[test_case(true ; "existing file closed")]
+    fn test_persist_target_exists(existing_file_closed: bool) {
+        let temp_dir = temp_dir();
+        let target = temp_dir.join("file");
+        let mut temp_file = NamedTempFile::new_in(&temp_dir).unwrap();
+        temp_file.write_all(b"contents").unwrap();
+
+        let mut file = File::create(&target).unwrap();
+        file.write_all(b"contents").unwrap();
+        if existing_file_closed {
+            drop(file);
+        }
+
+        assert!(persist_temp_file(temp_file, &target).is_ok());
+    }
+}

lib/src/index.rs

@@ -31,6 +31,7 @@ use itertools::Itertools;
 use tempfile::NamedTempFile;
 
 use crate::commit::Commit;
+use crate::file_util::persist_temp_file;
 use crate::store::{ChangeId, CommitId};
 
 #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Hash)]
@@ -617,7 +618,7 @@ impl MutableIndex {
         let mut temp_file = NamedTempFile::new_in(&dir)?;
         let file = temp_file.as_file_mut();
         file.write_all(&buf).unwrap();
-        temp_file.persist(&index_file_path)?;
+        persist_temp_file(temp_file, &index_file_path)?;
 
         let mut cursor = Cursor::new(&buf);
         ReadonlyIndex::load_from(&mut cursor, dir, index_file_id_hex, hash_length)

lib/src/index_store.rs

@@ -24,6 +24,7 @@ use tempfile::NamedTempFile;
 
 use crate::commit::Commit;
 use crate::dag_walk;
+use crate::file_util::persist_temp_file;
 use crate::index::{MutableIndex, ReadonlyIndex};
 use crate::op_store::OperationId;
 use crate::operation::Operation;
@@ -151,7 +152,7 @@ impl IndexStore {
         let mut temp_file = NamedTempFile::new_in(&self.dir)?;
         let file = temp_file.as_file_mut();
         file.write_all(&index.name().as_bytes()).unwrap();
-        temp_file.persist(&self.dir.join("operations").join(op_id.hex()))?;
+        persist_temp_file(temp_file, &self.dir.join("operations").join(op_id.hex()))?;
         Ok(())
     }
 }

lib/src/lib.rs

@@ -30,6 +30,7 @@ pub mod conflicts;
 pub mod dag_walk;
 pub mod diff;
 pub mod evolution;
+pub mod file_util;
 pub mod files;
 pub mod git;
 pub mod git_store;

lib/src/local_store.rs

@@ -22,6 +22,7 @@ use blake2::{Blake2b, Digest};
 use protobuf::{Message, ProtobufError};
 use tempfile::{NamedTempFile, PersistError};
 
+use crate::file_util::persist_temp_file;
 use crate::repo_path::{RepoPath, RepoPathComponent};
 use crate::store::{
     ChangeId, Commit, CommitId, Conflict, ConflictId, ConflictPart, FileId, MillisSinceEpoch,
@@ -140,7 +141,7 @@ impl Store for LocalStore {
         encoder.finish()?;
         let id = FileId(hasher.finalize().to_vec());
 
-        temp_file.persist(self.file_path(&id))?;
+        persist_temp_file(temp_file, self.file_path(&id))?;
         Ok(id)
     }
 
@@ -159,7 +160,7 @@ impl Store for LocalStore {
         hasher.update(&target.as_bytes());
         let id = SymlinkId(hasher.finalize().to_vec());
 
-        temp_file.persist(self.symlink_path(&id))?;
+        persist_temp_file(temp_file, self.symlink_path(&id))?;
         Ok(id)
     }
 
@@ -186,7 +187,7 @@ impl Store for LocalStore {
 
         let id = TreeId(Blake2b::digest(&proto_bytes).to_vec());
 
-        temp_file.persist(self.tree_path(&id))?;
+        persist_temp_file(temp_file, self.tree_path(&id))?;
         Ok(id)
     }
 
@@ -209,7 +210,7 @@ impl Store for LocalStore {
 
         let id = CommitId(Blake2b::digest(&proto_bytes).to_vec());
 
-        temp_file.persist(self.commit_path(&id))?;
+        persist_temp_file(temp_file, self.commit_path(&id))?;
         Ok(id)
     }
 
@@ -232,7 +233,7 @@ impl Store for LocalStore {
 
         let id = ConflictId(Blake2b::digest(&proto_bytes).to_vec());
 
-        temp_file.persist(self.conflict_path(&id))?;
+        persist_temp_file(temp_file, self.conflict_path(&id))?;
         Ok(id)
     }
 }

lib/src/simple_op_store.rs

@@ -22,6 +22,7 @@ use blake2::{Blake2b, Digest};
 use protobuf::{Message, ProtobufError};
 use tempfile::{NamedTempFile, PersistError};
 
+use crate::file_util::persist_temp_file;
 use crate::op_store::{
     OpStore, OpStoreError, OpStoreResult, Operation, OperationId, OperationMetadata, View, ViewId,
 };
@@ -98,7 +99,7 @@ impl OpStore for SimpleOpStore {
 
         let id = ViewId(Blake2b::digest(&proto_bytes).to_vec());
 
-        temp_file.persist(self.view_path(&id))?;
+        persist_temp_file(temp_file, self.view_path(&id))?;
         Ok(id)
     }
 
@@ -121,7 +122,7 @@ impl OpStore for SimpleOpStore {
 
         let id = OperationId(Blake2b::digest(&proto_bytes).to_vec());
 
-        temp_file.persist(self.operation_path(&id))?;
+        persist_temp_file(temp_file, self.operation_path(&id))?;
         Ok(id)
     }
 }

lib/src/working_copy.rs

@@ -33,6 +33,7 @@ use thiserror::Error;
 
 use crate::commit::Commit;
 use crate::commit_builder::CommitBuilder;
+use crate::file_util::persist_temp_file;
 use crate::gitignore::GitIgnoreFile;
 use crate::lock::FileLock;
 use crate::matchers::EverythingMatcher;
@@ -237,9 +238,7 @@ impl TreeState {
         // there is no unknown data in it
         self.update_read_time();
         proto.write_to_writer(temp_file.as_file_mut()).unwrap();
-        temp_file
-            .persist(self.state_path.join("tree_state"))
-            .unwrap();
+        persist_temp_file(temp_file, self.state_path.join("tree_state")).unwrap();
     }
 
     fn file_state(&self, path: &Path) -> Option<FileState> {
@@ -643,7 +642,7 @@ impl WorkingCopy {
     fn write_proto(&self, proto: crate::protos::working_copy::Checkout) {
         let mut temp_file = NamedTempFile::new_in(&self.state_path).unwrap();
         proto.write_to_writer(temp_file.as_file_mut()).unwrap();
-        temp_file.persist(self.state_path.join("checkout")).unwrap();
+        persist_temp_file(temp_file, self.state_path.join("checkout")).unwrap();
     }
 
     fn read_proto(&self) -> crate::protos::working_copy::Checkout {