Azure DevOps流水线Git合并失败但本地正常，求排查原因

我有两个Git分支：source_branch与target_branch，二者从同一父分支创建，其中source_branch仅对单个文件有一处简单修改。我尝试通过Azure DevOps流水线的PowerShell任务将source_branch合并到target_branch，但执行合并指令时失败；而在本地执行相同操作时，合并与推送都能成功完成。

使用的PowerShell脚本如下：

git config --global user.email "build@azuredevops.com"
git config --global user.name "buildazuredevops"
git fetch
git checkout source_branch
git pull --allow-unrelated-histories
git checkout target_branch
git pull --allow-unrelated-histories
git checkout source_branch
git merge target_branch --allow-unrelated-histories
git checkout target_branch
git merge source_branch --allow-unrelated-histories
git push

执行日志：

Switched to a new branch 'source_branch'
branch 'source_branch' set up to track 'origin/source_branch'.
Already up to date.
Switched to a new branch 'target_branch'
branch 'target_branch' set up to track 'origin/target_branch'.
Already up to date.
Switched to branch 'source_branch'
Your branch is up to date with 'origin/source_branch'.
Auto-merging Apps/Admin/Admin.ControllerServices/Account/Query/AccountSetupControllerService.cs
CONFLICT (add/add): Merge conflict in Apps/Admin/Admin.ControllerServices/Account/Query/AccountSetupControllerService.cs
Automatic merge failed; fix conflicts and then commit the result.
error: you need to resolve your current index first
Apps/Admin/Admin.ControllerServices/Account/Query/AccountSetupControllerService.cs: needs merge
error: Merging is not possible because you have unmerged files.
hint: Fix them up in the work tree, and then use 'git add/rm <file>'
hint: as appropriate to mark resolution and make a commit.
fatal: Exiting because of an unresolved conflict.
Everything up-to-date

核心错误信息：

Switched to branch 'source_branch'
Your branch is up to date with 'origin/source_branch'.
Auto-merging Apps/Admin/Admin.ControllerServices/Account/Query/AccountSetupControllerService.cs
CONFLICT (add/add): Merge conflict in Apps/Admin/Admin.ControllerServices/Account/Query/AccountSetupControllerService.cs
Automatic merge failed; fix conflicts and then commit the result.
error: you need to resolve your current index first

1. 流水线脚本的合并逻辑完全错误 #

你的脚本执行了无意义的双向合并：先切到source_branch合并target_branch，这一步直接产生冲突后未处理，又切到target_branch尝试合并source_branch——但此时本地仓库处于未解决冲突的状态，Git直接终止操作。而你本地操作时应该是执行了正确的合并流程（比如直接切到target_branch合并source_branch），没有多余的反向合并步骤，所以不会触发冲突。

2. --allow-unrelated-histories参数滥用 #

两个分支来自同一父分支，根本不存在“无关历史”，全程加这个参数完全没必要。该参数会强制Git合并原本不相关的提交历史，反而可能干扰正常合并逻辑，导致本该自动合并的修改被判定为冲突。本地操作时你大概率没加这个参数，所以合并正常。

3. 流水线仓库的浅克隆导致历史不完整 #

Azure DevOps流水线的Git任务默认可能使用浅克隆（shallow clone），只拉取最近的提交，导致分支历史的完整性和本地仓库有差异，进而引发合并冲突。你可以在流水线的Git获取任务中设置浅克隆深度为0，拉取完整历史。

4. 分支保护机制的间接影响（可能性较低） #

如果target_branch在Azure DevOps中开启了分支保护，比如要求必须通过PR合并、有状态检查未通过，直接流水线推送可能被拦截。但从错误日志看，失败是合并冲突导致的，所以这个可能性较低，不过可以排查一下分支设置。

git config --global user.email "build@azuredevops.com"
git config --global user.name "buildazuredevops"
# 拉取完整仓库历史，避免浅克隆问题
git fetch --unshallow
git checkout target_branch
# 拉取最新的target_branch代码
git pull origin target_branch
# 合并source_branch到当前分支，--no-edit自动生成合并提交
git merge origin/source_branch --no-edit
# 推送到远程仓库
git push origin target_branch

内容的提问来源于stack exchange，提问作者Toni