git rebase -i -autosquash conflict

Question

git is giving me a major headache when using --fixup and --autosquash. I would like to give two examples, one working perfectly fine and the other being a mess. (git version 2.6.2)

Working example:

First commit:

$ git init
$ printf '1\n' > test.file
$ git add test.file
$ git commit -m 'Insert 1 --> first line'  
$ cat test.file
1

Second commit (BUG):

$ printf 'This is\na BUG\n' >> test.file
$ git commit -am 'Added line 2 and 3 with BUG'
$ cat test.file
1  
This is  
a BUG

Third commit:

$ sed -i '2i 2' test.file
$ git commit -am 'Insert 2 --> second line'
$ cat test.file
1  
2  
This is  
a BUG

Fourth commit (fixup):

$ sed -i 's/a BUG/NOT a BUG/' test.file
$ git add test.file
$ git log --oneline
b021696 Insert 2 --> second line  
2e18b8d Added line 2 and 3 with BUG  
d7b60a1 Insert 1 --> first line  
$ git commit --fixup HEAD~
$ cat test.file
1  
2  
This is  
NOT a BUG

Rebase:

$ git log --oneline
fe99989 fixup! Added line 2 and 3 with BUG  
b021696 Insert 2 --> second line  
2e18b8d Added line 2 and 3 with BUG  
d7b60a1 Insert 1 --> first line

$ git rebase -i --autosquash HEAD~3

[detached HEAD 6660b0e] Added line 2 and 3 with BUG
Date: Tue Nov 3 13:28:07 2015 +0100
1 file changed, 2 insertions(+)
Successfully rebased and updated refs/heads/master.

---

Headache example: (Only difference is the BUGGY commit is a single line)

First commit:

$ git init
$ printf '1\n' > test.file
$ git add test.file
$ git commit -m 'Insert 1 --> first line'  
$ cat test.file
1

Second commit (BUG):

$ printf 'This is a BUG\n' >> test.file
$ git commit -am 'Added line 2 with BUG'
$ cat test.file
1
This is a BUG

Third commit:

$ sed -i '2i 2' test.file
$ git commit -am 'Insert 2 --> second line'
$ cat test.file
1  
2  
This is a BUG

Fourth commit (fixup):

$ sed -i 's/a BUG/NOT a BUG/' test.file
$ git add test.file
$ git log --oneline
2b83fe7 Insert 2 --> second line  
62cdd05 Added line 2 with BUG  
0ee3343 Insert 1 --> first line
$ git commit --fixup HEAD~
$ cat test.file
1  
2  
This is NOT a BUG

Rebase:

$ git log --oneline
c3d3db7 fixup! Added line 2 with BUG  
2b83fe7 Insert 2 --> second line  
62cdd05 Added line 2 with BUG  
0ee3343 Insert 1 --> first line
$ git rebase -i --autosquash HEAD~3
error: could not apply c3d3db7... fixup! Added line 2 with BUG

When you have resolved this problem, run "git rebase --continue".
If you prefer to skip this patch, run "git rebase --skip" instead.
To check out the original branch and stop rebasing, run "git rebase --abort".

Could not apply c3d3db78440e48c1bb637f78e0767520db65ea1e... fixup! Added line 2 with BUG

$ git status
interactive rebase in progress; onto 0ee3343
Last commands done (2 commands done):
   pick 62cdd05 Added line 2 with BUG
   fixup c3d3db7 fixup! Added line 2 with BUG
Next command to do (1 remaining command):
   pick 2b83fe7 Insert 2 --> second line
  (use "git rebase --edit-todo" to view and edit)
You are currently rebasing branch 'master' on '0ee3343'.
  (fix conflicts and then run "git rebase --continue")
  (use "git rebase --skip" to skip this patch)
  (use "git rebase --abort" to check out the original branch)

Unmerged paths:
  (use "git reset HEAD <file>..." to unstage)
  (use "git add <file>..." to mark resolution)

        both modified:   test.file

no changes added to commit (use "git add" and/or "git commit -a")  

$ cat test.file
1 
<<<<<<< HEAD
This is a BUG
======= 
2 
This is NOT a BUG
>>>>>>> c3d3db7... fixup! Added line 2 with BUG

---

Why does the fixup not apply cleanly?

Why does the fixup also contain "2" which should not be in the patch introduced by the fixup but in the patch of the former commit.

Answer 1

I think that the problem is the context of the changes. Look at this commit:

$ git show c3d3db7
diff --git a/test.file b/test.file
index 7a103db..8c8e69a 100644
--- a/test.file
+++ b/test.file
@@ -1,3 +1,3 @@
 1
 2
-This is a BUG
+This is NOT a BUG

And you want to apply this patch to the file with contents:

1
This is a BUG

See? The patch does not apply, because the context does not match. So a conflict arises and you have to fix it manually.

---

When you have the bugger line split in two, the patch is something like:

diff --git a/test.file b/test.file
--- a/test.file
+++ b/test.file
@@ -1,3 +1,3 @@
 1
 2
 This is
-a BUG
+NOT a BUG

And the file is:

1
This is
a BUG

Now, while the match is not perfect, at least the first unmodified line of the context matches, so the merge may continue.