How does git track history overwrites?

How does git track history overwrites?

[Jens Tröger]

Hello,

I’m looking for details and some clarification on a `git fetch` behavior I observed, but can’t quite explain. More context is in this Github comment:

  https://github.com/jenstroeger/python-package-template/pull/1190#discussion_r3288253713

but it boils down to this:

  /tmp/bla > git -c protocol.version=2 fetch origin dda8db18cfc68df532abf33b185ecd12d5b7b326 --depth=1

It seems that sha dda8db1 (tag 1.20.0 previously pointed at it) was replaced due to a suspected history overwrite with fda7769 (tag 1.20.0 now points at it) and git figures that out:

  ...

  From https://github.com/adamchainz/blacken-docs
  * branch dda8db18cfc68df532abf33b185ecd12d5b7b326 -> FETCH_HEAD

And then:

  /tmp/bla > git checkout FETCH_HEAD
  Note: switching to 'FETCH_HEAD’

  ...

  HEAD is now at fda7769 Version 1.20.0

And:

  /tmp/bla > cat .git/HEAD 
  fda77690955e9b63c6687d8806bafd56a526e45f
  /tmp/bla > cat .git/FETCH_HEAD 
  dda8db18cfc68df532abf33b185ecd12d5b7b326 'dda8db18cfc68df532abf33b185ecd12d5b7b326' of https://github.com/adamchainz/blacken-docs

I’d like to understand the details some more, and how I could manually make that connection?

Thank you!
Jens

Re: How does git track history overwrites?

[Chris Torek]

On Sun, May 24, 2026 at 4:44 PM Jens Tröger <jens.troeger@light-speed.de> wrote:
> I’m looking for details and some clarification on a `git fetch` behavior I observed, but can’t quite explain. ...

This isn't really specific to "git fetch" at all, except for the
usage of FETCH_HEAD.

To really understand this properly, we need to understand
the root of a seeming contradiction:

1. Once saved in Git, no commit (in fact, no internal object of any sort)
   can ever be changed.
2. And yet, "git rebase" and force-push operations seem to rewrite
   history.

How can commits be immutable and yet rewrite-able? The trick here
lies in how we (humans) *find* commits.

Inside a Git repository, the "true name" of any commit (or indeed
any internal object) is its raw hash ID, such as your example of
dda8db18cfc68df532abf33b185ecd12d5b7b326. The hash ID (or
"object ID", though right now there are only two forms, a SHA1
hash or a SHA256 hash) is specific to that one object once it is
created, and forever more can never be used for any other object.
It will always mean that original object, as long as that object
exists.

Thus, as long as that commit exists, it's *that* commit, with *that*
ID, and no other.

But we (humans) don't *use* hash IDs. They're too cumbersome.
So Git provides us with the ability to translate a name to an ID:

> It seems that sha dda8db1 (tag 1.20.0 previously pointed at it)

The *name* refs/tags/1.20.0 used to produce the above ID.

> was replaced ... with fda7769 (tag 1.20.0 now points at it)

Some human directed Git to forcibly replace the hash ID associated
with the tag, in some repository or repositories.

(As the manuals note, this kind of forcible replacement of tags is
often a bad idea. It's usually better, once the tag has escaped the
confinement of a single repository anyway, to just admit that you
goofed up and make a new tag.)

If you use raw hash IDs, you can never be bitten by this kind of
tag replacement, but of course that's a bad idea for different
(and presumably obvious) reasons. I couldn't possibly name the
hash ID without using cut-and-paste here. I can *type* "1.20.0"
repeatedly without error though.

(There are additional considerations, having to do with how Git
cleans up unwanted leftover junk, via git gc / git maintenance. In
particular Git uses the human-readable names to figure out which
objects are useful, and which are unwanted junk. So you have to
identify *some* commits with names, or they'll eventually get
garbage-collected.)

[At this point, you ran git fetch with a raw hash ID, and:]

>   From https://github.com/adamchainz/blacken-docs
>   * branch dda8db18cfc68df532abf33b185ecd12d5b7b326 -> FETCH_HEAD

When git fetch obtains something from another different Git repository,
the new things have the same IDs in both repositories. Normally we do
this by *name* (branch or tag name), but for historical reasons, the fetch
operation deposits a hash ID (often along with additional information)
 in the file `.git/FETCH_HEAD`. This file then works as a pseudo-name
for the branch, tag, or commit(s) thus obtained:

> And then:
>
>   /tmp/bla > git checkout FETCH_HEAD
>   Note: switching to 'FETCH_HEAD’

This gives you a "detached HEAD" state, using the hash ID stored in
.git/FETCH_HEAD. That hash ID will be overwritten (thus lost) by the
*next* git fetch, so you're expected to save it in some more-permanent
name if you want it to stick around.

The key difference between a branch name and a tag name is that
branch names are *expected* to map to different hash IDs over time,
with updates adding new commits to the branch causing the branch
name to remember the latest commit's ID. Each commit in turn
remembers the IDs of its parent commit or commits, so knowing
the *last* one suffices to allow Git to find *every* one.

Rewriting history with rebase consists of copying old (presumably
bad) commits to new (presumably good/better) ones, whose backwards
links to each previous commit chain through the new-and-improved
commits until you reach the point where the rewrite joins existing
history. Then we update the branch name to remember the latest
of the new-and-improved commits, and it *seems* that we've changed
history. The old history is still in there, and will stick around for quite
a while (at least a month by default, in standard clones) "just in case".

Tag names are not supposed to move, and whether someone else's tag
update to their clone changes your own clone's tags is something
you can control to some extent. It's not a good idea to depend on
other people's clones to follow tag changes, but it's also not a
good idea to depend on your own or other people's clones *not* to
follow such changes, since both behaviors are possible.

Chris

Re: How does git track history overwrites?

[Junio C Hamano]

Jens Tröger <jens.troeger@light-speed.de> writes:

> Hello,
>
> I’m looking for details and some clarification on a `git fetch` behavior I observed, but can’t quite explain. More context is in this Github comment:
>
>   https://github.com/jenstroeger/python-package-template/pull/1190#discussion_r3288253713
>
> but it boils down to this:
>
>   /tmp/bla > git -c protocol.version=2 fetch origin dda8db18cfc68df532abf33b185ecd12d5b7b326 --depth=1
>
> It seems that sha dda8db1 (tag 1.20.0 previously pointed at it) was replaced due to a suspected history overwrite with fda7769 (tag 1.20.0 now points at it) and git figures that out:
>
>   ...
>
>   From https://github.com/adamchainz/blacken-docs
>   * branch dda8db18cfc68df532abf33b185ecd12d5b7b326 -> FETCH_HEAD
>
> And then:
>
>   /tmp/bla > git checkout FETCH_HEAD
>   Note: switching to 'FETCH_HEAD’
>
>   ...
>
>   HEAD is now at fda7769 Version 1.20.0
>
> And:
>
>   /tmp/bla > cat .git/HEAD 
>   fda77690955e9b63c6687d8806bafd56a526e45f
>   /tmp/bla > cat .git/FETCH_HEAD 
>   dda8db18cfc68df532abf33b185ecd12d5b7b326 'dda8db18cfc68df532abf33b185ecd12d5b7b326' of https://github.com/adamchainz/blacken-docs
>
> I’d like to understand the details some more, and how I could manually make that connection?

Where does this line in your discussion page at GitHub (which is
omitted from the post to this list) come from?

    commit fda77690955e9b63c6687d8806bafd56a526e45f (grafted, HEAD)

Are you doing anything funky with .git/info/grafts by any chance?

Re: How does git track history overwrites?

[Jens Tröger]

Thank you Chris and Junio!


> [Junio] Where does this line in your discussion page at GitHub (which is
> omitted from the post to this list) come from?
> 
>    commit fda77690955e9b63c6687d8806bafd56a526e45f (grafted, HEAD)
> 
> Are you doing anything funky with .git/info/grafts by any chance?

That line is the result of a `git log` after the `git fetch` I mentioned in my initial email.


> [Chris] To really understand this properly, we need to understand
> the root of a seeming contradiction:
> 
> [...]

Thank you for your elaborate explanation, Chris, that all makes a lot of sense. A few follow-up questions:

 •  Is all the object information stored with a repo clone locally as well, or does some/most/all of it stay on the remote server repo?
 •  How exactly does git connect the dots between commit dda8db1 and fda7769, how does it “know” the former was superseded by the latter (i.e. I fetch the former and Git uses the latter for head)?
 •  Based on the previous question, can I manually find such a connection between two commit objects too?

It sounds like reading some internals would be helpful. I’m noodling through https://git-scm.com/book/en/v2/Git-Internals-Git-Objects but perhaps you have some more recommendations?

With many greetings,
Jens

Re: How does git track history overwrites?

[Junio C Hamano]

Jens Tröger <jens.troeger@light-speed.de> writes:

> Thank you Chris and Junio!
>
>
>> [Junio] Where does this line in your discussion page at GitHub (which is
>> omitted from the post to this list) come from?
>> 
>>    commit fda77690955e9b63c6687d8806bafd56a526e45f (grafted, HEAD)
>> 
>> Are you doing anything funky with .git/info/grafts by any chance?
>
> That line is the result of a `git log` after the `git fetch` I mentioned in my initial email.

Sorry, I may have been unclear.  I specifically meant the "grafted,
" part in the message.  I know how "git log" output looks like ;-)

Re: How does git track history overwrites?

[Jens Tröger]

Hello Junio,

> Sorry, I may have been unclear.  I specifically meant the "grafted,
> " part in the message.  I know how "git log" output looks like ;-)

The “grafted” too was part of the git log output; here is the complete
cmd line output:

    /tmp/bla > git log
    commit fda77690955e9b63c6687d8806bafd56a526e45f (grafted, HEAD)
    Author: Adam Johnson <me@adamj.eu>
    Date:   Mon Sep 8 16:31:35 2025 +0100

        Version 1.20.0

On *why* the “grafted” is there in the first place, I suspect that’s got
to do with the fetch --depth=1 and that previous history isn’t available
in the shallow repo clone.

Cheers,
Jens