When you start using the topic branch workflow, you would merge topics often into a throw-away testing branch, and from time to time, end up performing the same conflict resolution over and over again. Git has a mechanism called rerere to help you in such a situation.

Even people who use rerere often do not realize one interesting aspect of the command, and this article is about showing off that part.

Let's pretend that you start from this base version.

$ mkdir /var/tmp/practice-rerere
$ cd /var/tmp/practice-rerere
$ git init
$ cat >hello.c <<\EOF
#include <stdio.h>
#include <string.h>

/*
 * Say the message
 */
void hello(char *message)
{
        printf("%s\n", message);
}

/*
 * Show greetings
 */
void greetings(char *message)
{
        hello(message);
        printf("Your message is %d bytes long\n",
               (int) strlen(message));
}

/*
 * Main program
 */
int main(int ac, char **av)
{
        greetings(av[1]);
        return 0;
}
EOF

$ cc -o hello ./hello.c && ./hello "hello world"
hello world
Your message is 11 bytes long

$ git add hello.c
$ git commit -m initial

$ git checkout -b upcase-first
$ git apply <<\EOF && git commit -a -m "upcase first word"
diff --git a/hello.c b/hello.c
index e0dbe10..7fcb4f3 100644
--- a/hello.c
+++ b/hello.c
@@ -1,11 +1,16 @@
+#include <ctype.h>
 #include <stdio.h>
 #include <string.h>

 /*
  * Say the message
  */
-void hello(char *message)
+void hello(char *message, int upcase_first)
 {
+       char *cp;
+       for (cp = message; *cp && !isspace(*cp); cp++)
+               *cp = toupper(*cp);
+
        printf("%s\n", message);
 }

@@ -14,7 +19,7 @@ void hello(char *message)
  */
 void greetings(char *message)
 {
-       hello(message);
+       hello(message, 1);
        printf("Your message is %d bytes long\n",
               (int) strlen(message));
 }
EOF
$ cc -o hello ./hello.c && ./hello "hello world"
HELLO world
Your message is 11 bytes long

$ git checkout -b const-fix master
$ git apply <<\EOF && git commit -a -m "constness fix"
diff --git a/hello.c b/hello.c
index e0dbe10..9970faa 100644
--- a/hello.c
+++ b/hello.c
@@ -4,7 +4,7 @@
 /*
  * Say the message
  */
-void hello(char *message)
+void hello(const char *message)
 {
        printf("%s\n", message);
 }
@@ -12,7 +12,7 @@ void hello(char *message)
 /*
  * Show greetings
  */
-void greetings(char *message)
+void greetings(const char *message)
 {
        hello(message);
        printf("Your message is %d bytes long\n",
@@ -22,7 +22,7 @@ void greetings(char *message)
 /*
  * Main program
  */
-int main(int ac, char **av)
+int main(int ac, const char **av)
 {
        greetings(av[1]);
        return 0;
EOF

$ git checkout upcase-first^0
Note: moving to 'upcase-first^0' which isn't a local branch
If you want to create a new branch from this checkout, you may do so
(now or later) by using -b with the checkout command again. Example:
  git checkout -b <new_branch_name>
HEAD is now at 362a9fb... upcase first

$ mkdir .git/rr-cache

$ git merge const-fix
Auto-merging hello.c
CONFLICT (content): Merge conflict in hello.c
Recorded preimage for 'hello.c'
Automatic merge failed; fix conflicts and then commit the result.

$ cat hello.c
#include <ctype.h>
#include <stdio.h>
#include <string.h>

/*
 * Say the message
 */
<<<<<<< HEAD
void hello(char *message, int upcase_first)
=======
void hello(const char *message)
>>>>>>> const-fix
{
        char *cp;
        for (cp = message; *cp && !isspace(*cp); cp++)
                *cp = toupper(*cp);

        printf("%s\n", message);
}

/*
 * Show greetings
...

<<<<<<< HEAD
void hello(char *message, int upcase_first) --- your changes
=======
void hello(const char *message) --- their changes
>>>>>>> const-fix

void hello(const char *message, int upcase_first)

void hello(const char *message, int upcase_first)
{
        const char *cp;
        for (cp = message; *cp && !isspace(*cp); cp++)
                putchar(toupper(*cp));
        message = cp;

        printf("%s\n", message);
}

$ cc -o hello ./hello.c && ./hello "hello world"
HELLO world
Your message is 11 bytes long

$ git commit -a -m 'test resolution'
Recorded resolution for 'hello.c'.
[detached HEAD 2fe010b] test resolution

$ git checkout upcase-first
Previous HEAD position was 2fe010b... test resolution
Switched to branch 'upcase-first'
$ git apply <<\EOF && git commit -a -m "add comment"
diff --git a/hello.c b/hello.c
index 9970faa..53abee2 100644
--- a/hello.c
+++ b/hello.c
@@ -20,7 +20,7 @@ void greetings(const char *message)
 }

 /*
- * Main program
+ * Main program - give greetings with the first word upcased.
  */
 int main(int ac, char **av)
 {
EOF
$ cc -o hello ./hello.c && ./hello "hello world"
HELLO world
Your message is 11 bytes long

$ git checkout HEAD^0
$ git merge const-fix
Auto-merging hello.c
CONFLICT (content): Merge conflict in hello.c
Resolved 'hello.c' using previous resolution.
Automatic merge failed; fix conflicts and then commit the result.

$ git checkout --conflict=merge hello.c
$ cat hello.c

• The branches produced the conflict exactly the same way. The hello() function has different sets of arguments.
• The body of the hello() function did not have any conflict; it merged cleanly at the textual level, but it is wrong as the merge result.

$ git rerere
Resolved 'hello.c' using previous resolution.

• Rerere remembers how you chose to resolve the conflicted regions;
• Rerere also remembers how you touched up outside the conflicted regions to adjust to semantic changes;
• Rerere can reuse previous resolution even though you were merging two branches with different contents than the one you resolved earlier.