出现的问题
在《Pro Git》的第7章11节—— 合并子模块改动 中，输入git pull 命令，并没有出现如下书中的结果
$ git pull
remote: Counting objects: 2, done.
remote: Compressing objects: 100% (1/1), done.
remote: Total 2 (delta 1), reused 2 (delta 1)
Unpacking objects: 100% (2/2), done.
From https://github.com/chaconinc/MainProject
9a377d1..eb974f8 master -> origin/master
Fetching submodule DbConnector
warning: Failed to merge submodule DbConnector (merge following commits not found)
Auto-merging DbConnector
CONFLICT (submodule): Merge conflict in DbConnector
Automatic merge failed; fix conflicts and then commit the result.
而是出现下面的内容，
$ git pull
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 2 (delta 0), reused 2 (delta 0), pack-reused 0
Unpacking objects: 100% (2/2), done.
From https://github.com/username/MainProject
faf7642..f3561d9 master -> origin/master
Fetching submodule DbConnector
From https://github.com/username/DbConnector
42c390a..1a5f567 master -> origin/master
Updating faf7642..f3561d9
Fast-forward
DbConnector | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
此结果中并没有出现 merge following commits not found，再查看子模块，发现远程分支与本地分支并没有产生冲突，而是分叉了
$ graph
* a88a3be (HEAD -> master) add return 0;
| * 1a5f567 (origin/master, origin/HEAD) add return 1;
|/
* 42c390a first commit
解决的办法
出现这个问题的原因是，没有在主项目中，提交子模块的修改。下面就来具体演示一下如何复现书本上的内容。新建一个 MainProject 文件夹，在里面新建一个 main.c 文件，在该文件中输入
#inclue<stdio.h>
int main(){
}
然后把 MainProject 文件夹初始化为 git 项目
$ git init
Initialized empty Git repository in /Users/apple/Git/MainProject/.git/
$ git add main.c
$ git commit -m 'first commit'
[master (root-commit) 15a113b] first commit
1 file changed, 4 insertions(+)
create mode 100644 main.c
新建一个 DbConnector 文件夹，在里面新建一个 connector.c在该文件中输入
#include<stdio.h>
int main(){
printf("DbConnector");
}
初始化为 git 项目
$ git init
Initialized empty Git repository in /Users/apple/Git/DbConnector/.git/
$ git add connector.c
$ git commit -m 'first commit'
[master (root-commit) 42c390a] first commit
1 file changed, 4 insertions(+)
create mode 100644 main.c
在 Github 中新建一个 DbConnector 的仓库，把本地的内容推送上去
$ git remote add origin https://github.com/username/DbConnector.git
$ git push -u origin master
Enumerating objects: 3, done.
Counting objects: 100% (3/3), done.
Delta compression using up to 8 threads
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 262 bytes | 262.00 KiB/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To https://github.com/username/DbConnector.git
* [new branch] master -> master
Branch 'master' set up to track remote branch 'master' from 'origin'.
回到 MainProject，给它添加子模块，并提交
$ git submodule add https://github.com/username/DbConnector.git
Cloning into '/Users/apple/Documents/Git/MainProject/DbConnector'...
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 0), reused 3 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), done.
$ git commit -m 'add submodule'
[master faf7642] add submodule
2 files changed, 4 insertions(+)
create mode 100644 .gitmodules
create mode 160000 DbConnector
在 Github中新建一个 MainProject 仓库，把本地的内容推送上去
$ git remote add origin https://github.com/username/MainProject.git
$ git push -u origin master
Enumerating objects: 6, done.
Counting objects: 100% (6/6), done.
Delta compression using up to 8 threads
Compressing objects: 100% (4/4), done.
Writing objects: 100% (6/6), 595 bytes | 595.00 KiB/s, done.
Total 6 (delta 0), reused 0 (delta 0)
To https://github.com/username/MainProject.git
* [new branch] master -> master
Branch 'master' set up to track remote branch 'master' from 'origin'.
在 本机的其它文件夹 或 别的计算机 中克隆 MainProject
$ git clone --recursive https://github.com/username/MainProject.git
Cloning into 'MainProject'...
remote: Enumerating objects: 6, done.
remote: Counting objects: 100% (6/6), done.
remote: Compressing objects: 100% (4/4), done.
remote: Total 6 (delta 0), reused 6 (delta 0), pack-reused 0
Unpacking objects: 100% (6/6), done.
Submodule 'DbConnector' (https://github.com/username/DbConnector.git) registered for path 'DbConnector'
Cloning into '/Users/apple/Downloads/MainProject/DbConnector'...
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 0), reused 3 (delta 0), pack-reused 0
Submodule path 'DbConnector': checked out '42c390a15689a128dd017f1ca38b267af1c206c8'
现在我们有了两个 MainProject，一个是原本的，一个是克隆的，目前两个仓库里的内容是一模一样的，现在让它们的分支产生分叉。来到克隆过后的 MainProject 里的子模块 DbConnector
git checkout master
在代码中添加return 0;，如下
#include<stdio.h>
int main(){
printf("DbConnector");
return 0;
}
并在子模块中提交
$ git commit -am 'add return 0;'
[master 24cfc12] add return 0;
1 file changed, 1 insertion(+)
下面是解决问题的关键！关键！关键！重要的事情说三遍！首先切换回克隆的主项目目录
$ cd ..
运行git status我们可以看到我们的修改
$ git status
On branch master
Your branch is up to date with 'origin/master'.
Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git checkout -- <file>..." to discard changes in working directory)
modified: DbConnector (new commits)
no changes added to commit (use "git add" and/or "git commit -a")
我们必须把在子项目修改的内容在主项目再提交一次
$ git commit -am 'add return 0;'
[master febe029] add return 0;
1 file changed, 1 insertion(+), 1 deletion(-)
如果没有提交，当下面使用 git pull命令时，提示信息就不会出现 merge following commits not found,就不会输出和书上一样的结果。现在回到原本的 MainProject 中的子模块，添加与克隆的项目的子模块不一样的提交，如下
#include<stdio.h>
int main(){
printf("DbConnector");
return 1;
}
在子模块中提交该修改
$ git commit -am 'add return 1;'
[master 1a5f567] add return 1;
1 file changed, 1 insertion(+)
回到原本的 MainProject 中，把当前的主项目和子模块的提交推向远程分支，使远程分支与克隆过后的分支产生分叉。在 connector.c 文件中添加 return 1;
$ cd ..
$ git commit -am 'add return 1;'
[master 524639a] add return1;
1 file changed, 1 insertion(+)
$ git push --recurse-submodules=on-demand
Pushing submodule 'DbConnector'
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 303 bytes | 303.00 KiB/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To https://github.com/username/DbConnector.git
42c390a..1a5f567 master -> master
Enumerating objects: 3, done.
Counting objects: 100% (3/3), done.
Delta compression using up to 8 threads
Compressing objects: 100% (2/2), done.
Writing objects: 100% (2/2), 302 bytes | 302.00 KiB/s, done.
Total 2 (delta 0), reused 0 (delta 0)
To https://github.com/username/MainProject.git
faf7642..f3561d9 master -> master
然后在克隆的 MainProject 中拉取远程服务器的修改
$ git pull
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 2 (delta 0), reused 2 (delta 0), pack-reused 0
Unpacking objects: 100% (2/2), done.
From https://github.com/username/MainProject
faf7642..f3561d9 master -> origin/master
Fetching submodule DbConnector
From https://github.com/username/DbConnector
42c390a..1a5f567 master -> origin/master
Failed to merge submodule DbConnector (merge following commits not found)
Auto-merging DbConnector
CONFLICT (submodule): Merge conflict in DbConnector
Automatic merge failed; fix conflicts and then commit the result.
这样就出现和书中一样的结果了。