[Bug] Diff preview (editor _preview) fails on Windows with "TerminateProcess: Access is denied"

[wbste]

Description

Hi all, I used Claude to help me with this (hope that's alright). Happy to provide additional details if it helps!

Gitea Version: 1.25.4

Git Version: 2.53.0.windows.1

Operating System: Windows Server (Build 26200)

Can you reproduce the bug? Always

Description

When editing a file in the web editor and triggering the "Preview Changes" tab, Gitea returns a 500 Internal Server Error. Normal diff views on committed changes work fine. Only the live diff preview (_preview endpoint) is affected.

Steps to Reproduce

1. Navigate to any file in a repository
2. Click the edit (pencil) icon
3. Make any change to the file content
4. Click the Preview Changes tab

Expected Behavior

A diff of the changes is shown.

Actual Behavior

500 Internal Server Error:

GetDiffPreview, unable to run diff-index pipeline in temporary repo: exec: canceling Cmd: TerminateProcess: Access is denied.

Debug Logs

git.Run(by:files.(*TemporaryUploadRepository).DiffIndex, repo:.../local-repo\upload.git274962278): "C:\\Program Files\\Git\\bin\\git.exe" diff-index --src-prefix=\a/ --dst-prefix=\b/ --cached -p HEAD
services/gitdiff/gitdiff.go:487:ParsePatch() [D] ParsePatch(1000, 5000, 100, ..., )
.../repository/files/temp_repo.go:413:(*TemporaryUploadRepository).DiffIndex() [E] Unable to diff-index in temporary repo user/test (C:\gitea\data\tmp\local-repo\upload.git274962278). Error: exec: canceling Cmd: TerminateProcess: Access is denied.
Stderr: 
.../repo/editor_preview.go:32:DiffPreviewPost() [E] GetDiffPreview: unable to run diff-index pipeline in temporary repo: exec: canceling Cmd: TerminateProcess: Access is denied.

Root Cause

In modules/repository/files/temp_repo.go, DiffIndex() runs git diff-index via a pipeline and reads stdout with ParsePatch(). After ParsePatch finishes reading, the pipeline cleanup cancels/kills the git subprocess. The code correctly tries to suppress this expected error:

if err != nil && !gitcmd.IsErrorCanceledOrKilled(err) {
    return nil, fmt.Errorf("unable to run diff-index pipeline in temporary repo: %w", err)
}

On Linux, killing a process that has already exited returns ESRCH, which IsErrorCanceledOrKilled handles gracefully. On Windows, calling TerminateProcess on an already-exited process returns ERROR_ACCESS_DENIED instead. IsErrorCanceledOrKilled in modules/git/gitcmd/error.go currently only checks for context.Canceled and signal: killed:

func IsErrorCanceledOrKilled(err error) bool {
    return errors.Is(err, context.Canceled) || IsErrorSignalKilled(err)
}

It does not recognise the Windows-specific TerminateProcess: Access is denied error string, so it returns false and the error propagates as a 500 — even though ParsePatch completed successfully and the diff data is available.

Suggested Fix

Add a Windows check to IsErrorCanceledOrKilled in modules/git/gitcmd/error.go. strings is already imported in that file:

func IsErrorCanceledOrKilled(err error) bool {
    // When "cancel()" a git command's context, the returned error of "Run()" could be one of them:
    // - context.Canceled
    // - *exec.ExitError: "signal: killed"
    // - On Windows: "TerminateProcess: Access is denied" (process already exited before kill)
    return errors.Is(err, context.Canceled) || IsErrorSignalKilled(err) || strings.Contains(err.Error(), "TerminateProcess: Access is denied")
}

This single change fixes all call sites at once (batch.go, githttp.go, gitdiff.go, temp_repo.go) since they all rely on this function.

Configuration

• Running as a Windows Service via WinSW (LocalSystem account)
• Git binary: C:\Program Files\Git\bin\git.exe
• Temp repo path: C:\gitea\data\tmp\local-repo
• SQLite database
• Reverse proxy: Caddy

How are you running Gitea?

Via binary on Windows Server (Build 26200)

Database

SQLite

[wbste]

Did a quick build and test (I couldn't get the official build to work so had to extract the assets from published binary). Above suggested fix seems to have worked, but would love a second pair of eyes from someone with expertise.

[wxiaoguang]

There should be no problem on main branch.

The logic has been improved by #36402

Could you try the nightly build?

• https://dl.gitea.com/gitea/main-nightly/
• https://hub.docker.com/r/gitea/gitea/tags?name=main-nightly

[wbste]

Downloaded: https://dl.gitea.com/gitea/main-nightly/gitea-main-nightly-windows-4.0-amd64.exe

Confirmed that will fix it! Thanks!