dotnet/runtime

Identify places where file handles are closed and very soon reopened again

Open

#64,288 opened on Jan 25, 2022

View on GitHub
 (1 comment) (2 reactions) (0 assignees)C# (5,445 forks)batch import
area-System.IOhelp wanted

Repository metrics

Stars
 (17,886 stars)
PR merge metrics
 (Avg merge 12d 11h) (661 merged PRs in 30d)

Description

As https://github.com/dotnet/runtime/pull/63912 has shown, there are places in BCL where files are being closed and very soon reopened (it's typically not obvious).

using (FileStream fs = new FileStream(destinationFileName, fMode, FileAccess.Write, FileShare.None, bufferSize: 0x1000, useAsync: false))
{
    using (Stream es = source.Open())
        es.CopyTo(fs);
} // the file is closed

File.SetLastWriteTime(destinationFileName, source.LastWriteTime.DateTime); // it's reopened

It causes two kinds of problems:

  • AV and other software can open the file in the meantime, lock the access and hence make the re-opening code fail (ERROR_SHARING_VIOLATION on Windows)
  • the perf is not ideal, as opening the files is expensive (especially on Unix where we perform more than 1 sys-call to open a file).

One of the potential solutions would be to use APIs that are accepting SafeFileHandle like https://github.com/dotnet/runtime/issues/20234

Contributor guide

Research direction
Search the runtime codebase for patterns where a FileStream is closed and then immediately a different file API (like File.SetLastWriteTime) is called on the same file. Consider using SafeFileHandle to avoid reopening.
Tech stack
csharp
Domain
backend
Issue type
Refactor
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C#
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60