Re: git replace: should it check for object type, and can it replace merges?

From: "Christian Couder" <redacted>

Hi,
On Sat, Aug 3, 2013 at 5:13 PM, Philip Oakley [off-list ref] 
wrote:

quoted

A recent comment http://stackoverflow.com/a/18027030/717355 on a 
question I
asked two years ago about 'grafts' and 'replace' indicates that users 
think
that 'git replace' can't replace a merge commit. The documentation 
doesn't
have any examples and gives the naive impression that one should only
replace a simple commit with another simple commit.

I am sorry if the documentation gives this impression.
I'd like to fix it, but I am not sure what should be changed.
Should adding an example be enough? Or do you want it to state that 
explicitely?

I did a quick markup which is shown below (an export of the commit from 
the gitk viewer)

quoted

Having looked at the code, I realised that anything can be replaced 
with
anything, which is perhaps not what was intended.

The documentation says in the "BUGS" section:

"And of course things may break if an object of one type is replaced
by an object of another type (for example a blob replaced by a
commit)."

I hadn't seen that part, being 'hidden' at the end of the paragraph 
(that is, it's easy to miss;-). If my update was acceptable then that 
sentence could probably be deleted (though it may require the checks to 
actually be in the code, and not just a "must" imperative in my 
documentation update).

So yes it is a know bug.

quoted

A simple thought is that
the replace should be like-for-like with regard to object type, 
though that
would not include replacing a sub-module for a tree (and vice versa).

Should 'git replace' check the object types to ensure they are 
sensible?

It would probably be a good idea to do that, yeah.
But I don't know much about submodules, so I can't say if replacing a
sub-module for a tree (and vice versa) should be allowed.
Or if there should be a --force-different-objects option for these
kinds of special cases.

An extra bit of thought made me realise that while a sub-module is 
represented as a special symbolic commit, it is still just an element of 
a tree object, so would still be a tree <-> tree replacement, so doesn't 
break the rule.

quoted

Would it be reasonable to add examples to indicate the range of
replacements, and how to prepare alternative merge commits, or is 
that a
hostage to fortune?

Yeah, adding examples would be a good idea. I don't understand what do
you mean with "range of replacements", though.

There were in two parts: 1) creating a replacement merge commit, and 2) 
creating a replacement tree,  possibly with a sub-module in it.

I am not sure preparing alternative commits or merge commits should be
an important part of the examples.

There are many cases that could be interesting to different users:

- replacing a non merge commit with a merge commit (if someone forgot
to use --no-ff when merging for example)
- replacing a merge commit with a non merge commit (if a rebase should
have been done)
- and of course replacing a non merge commit with a non merge commit,
or a merge commit with a merge commit

So I think explaining how another commit can be created from existing
commits belongs to some other parts of the git documentation.

Yes, I just haven't looked yet. I think there are some examples in the 
plumbing command man pages. They'd just need referencing.

Perhaps there could be such examples in the git hash-object and git
filter-branch documentation and we could just point to them.

Best,
Christian.
--

My quick markup, based on a local branch.
---8<---
commit c12c03462f8c65a593e702896b461f1c63d67ec5
Author: Philip Oakley [off-list ref]
Date:   Sat Aug 3 20:20:05 2013 +0100

    Doc: 'replace' the same object type, and mention merge commits

    Signed-off-by: Philip Oakley [off-list ref]

diff --git a/Documentation/git-replace.txt 

b/Documentation/git-replace.txt
index e0b4057..2ab451c 100644

--- a/Documentation/git-replace.txt
+++ b/Documentation/git-replace.txt

@@ -20,6 +20,10 @@ The name of the 'replace' reference is the SHA-1 of 

the object that is
 replaced. The content of the 'replace' reference is the SHA-1 of the
 replacement object.

+The type of the replacement object must be the same as that of the
+object it replaces. Merge commits can be replaced by non-merge commits
+and vice versa.
+
 Unless `-f` is given, the 'replace' reference must not yet exist.

 Replacement references will be used by default by all Git commands
--->8---

Philip