多种 Git Pull 模式差异分析

背景

在执行 git pull 命令时,我们可能会看到这样的提示信息:

hint: Pulling without specifying how to reconcile divergent branches is
hint: discouraged. You can squelch this message by running one of the following
hint: commands sometime before your next pull:
hint: 
hint:   git config pull.rebase false  # merge (the default strategy)
hint:   git config pull.rebase true   # rebase
hint:   git config pull.ff only       # fast-forward only
hint: 
hint: You can replace "git config" with "git config --global" to set a default
hint: preference for all repositories. You can also pass --rebase, --no-rebase,
hint: or --ff-only on the command line to override the configured default per
hint: invocation.

显然 Git 希望我们指定一种 pull 的方式,它提供了三种模式:

  • merge
  • rebase
  • fast-forward only

区分

这三种模式都是为了指定 git pull 时如果本地和远端代码发生冲突时应该使用什么策略处理。

     A---B---C origin/master
    /
D---E---F---G master
    ^
    origin/master in your repository

Merge

Merge 模式会在发生冲突时,将远端分支上的改动 merge 到本地。官方文档中的实例说明了在触发 git pull --no-rebase (merge 模式)之后,变化如下:

          A---B---C origin/master
         /         \
    D---E---F---G---H master

可以看到会多出一个 commit H,这个结果可以理解为在本地 master 分支上,执行了 git merge origin/master1 将远端分支合并到了本地分支上。但是可以看到,远端的 master 分支和本地 master 分支还是不同步的。通常情况下这不符合我们的预期。

Rebase

Rebase 模式和 merge 模式类似,可以理解为在本地 master 分支上执行 git rebase origin/master1 将本地 master 分支基于远端 master 分支进行 rebase。执行完成后,本地的 master 分支会_超前于_远端的 master 分支。

         A---B---C origin/master
        /
    D---E---A---B---C---F'---G' master

Fast-Forward Only

To phrase that another way, when you try to merge one commit with a commit that can be reached by following the first commit’s history, Git simplifies things by moving the pointer forward because there is no divergent work to merge together — this is called a “fast-forward.”

首先我们需要了解 fast-forward 模式2,该模式在 git merge 中也有,该模式下,如果 Git 可以 resolve the merge,Git 只会移动指针,而不会新建 commit。

加入有下面这样两个分支:

* c5571a5 (HEAD -> master) feat: edit A on master branch
| * b56e7d6 (develop) feat: edit A in develop
|/  
* ea5e820 feat: edit A
* 3af1481 feat: add A

即使此时 master 和 develop 分支的结果是一样的,如果我们执行 git merge develop --ff-only,会报错:

fatal: Not possible to fast-forward, aborting.

我们回到 git pull,如果本地和远端代码如下,A 和 A' 中的实际内容是一致的:

     A---B---C origin/master
    /
D---E---A' master
    ^
    origin/master in your repository

在执行带有 --ff-onlygit pull 命令后,会发生报错。由于两地 commit 不一致,Git 无法通过 fast-forward 实现更新。

下面我们直接看文档来学习集中 fast-forward 模式:

With --ff, when possible resolve the merge as a fast-forward (only update the branch pointer to match the merged branch; do not create a merge commit). When not possible (when the merged-in history is not a descendant of the current history), create a merge commit.

With --no-ff, create a merge commit in all cases, even when the merge could instead be resolved as a fast-forward.

With --ff-only, resolve the merge as a fast-forward when possible. When not possible, refuse to merge and exit with a non-zero status.

可以看到,如果对于上面这种情况,我们使用 --ff,达到的效果和 git pull --merge 是一样的。


  1. 注意!只是打个比方,没有这种操作方式。 ↩︎

  2. 参考[官方文档](https://git-scm.com/book/en/v2/Git-Branching-Basic-Branching-and-Merging#:~:text=to phrase that another way%2C when you try to merge one commit with a commit that can be reached by following the first commit’s history%2C git simplifies things by moving the pointer forward because there is no divergent work to merge together — this is called a “fast-forward.")。 ↩︎

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