In this documentation it is mentioned
A commit object may have any number of parents.
But from my understanding, the only case where a commit will have more than 1 parent is when a merge has happened, and in that case there will only be two parents. So my question is, can a commit have more than 2 parents? If so, when?
You can use git merge to merge more than one commit into your current branch. From man git-merge (or git help merge):
git-merge - Join two or more development histories together
The result will be a commit with more than two parents when you do that.
Source: worked at GitHub for five years.
– gjtorikian Nov 13 '17 at 19:50git merge says [fat emphasis mine] "Specifying more than one commit will create a merge with more than two parents (affectionately called an Octopus merge)." The command line option to select a merge strategy for more than two parents is called octopus. The executable that implements said merge strategy is called git-merge-octopus.
– Jörg W Mittag
Nov 13 '17 at 20:21
git revert takes an optional -m <parent-number> argument. Quoting from the git revert man page, emphasis mine, "As a result, later merges will only bring in tree changes introduced by commits that are not ancestors of the previously reverted merge. This may or may not be what you want."
– David Hammen
Mar 24 '20 at 07:03
Yes, how about 100k parents?
Here is a live GitHub example with a merge of 100k commits: https://github.com/cirosantilli/test-octopus-100k Generated with this script.
Trivia
Linus does not like commits with more than 60 parents: https://www.destroyallsoftware.com/blog/2017/the-biggest-and-weirdest-commits-in-linux-kernel-git-history
It's pulled, and it's fine, but there's clearly a balance between "octopus merges are fine" and "Christ, that's not an octopus, that's a Cthulhu merge".
Have a look at the format for the Git commit object
From that analysis, we can see that the list of parents list is an arbitrary newline separated list of type:
parent {parent_1_sha}
parent {parent_2_sha}
...
parent {parent_N_sha}
and so an arbitrary number of parents is allowed.
Minimal example
Script:
#!/usr/bin/env bash
set -eu
mkdir tmp
cd tmp
git init
touch root
git add .
git commit -m root
sha_root="$(git log -1 --format="%H")"
touch 1
git add .
git commit -m 1
sha1="$(git log -1 --format="%H")"
git reset --hard "$sha_root"
touch 2
git add .
git commit -m 2
sha2="$(git log -1 --format="%H")"
git reset --hard "$sha_root"
touch 3
git add .
git commit -m 3
sha3="$(git log -1 --format="%H")"
git merge -m merge "$sha1" "$sha2"
View it:
git log --abbrev-commit --decorate --graph --pretty=oneline --all
Output:
*-. 70b4d8c (HEAD -> master) merge
|\ \
| | * e54f235 2
| * | c7ab8ef 1
| |/
* / ae5f932 3
|/
* ef2f89e root
How it shows on git log:
commit 70b4d8c93b8ba47d3a2b2d4d45619ad5e0f34289 (HEAD -> master)
Merge: ae5f932 c7ab8ef e54f235
Author: Ciro Santilli <[email protected]>
Date: Thu Dec 3 23:45:07 2020 +0000
merge
commit ae5f932c1de34255400806f85f7f58ff52b7ae86
Author: Ciro Santilli <[email protected]>
Date: Thu Dec 3 23:45:07 2020 +0000
3
commit c7ab8efcb25e49ffc653ce5308cd5151d6a3fb94
Author: Ciro Santilli <[email protected]>
Date: Thu Dec 3 23:45:07 2020 +0000
1
commit e54f235a54da0900cec99301099673e3bc1cd01e
Author: Ciro Santilli <[email protected]>
Date: Thu Dec 3 23:45:07 2020 +0000
2
commit ef2f89eeb8fa83779b58b604f9814ec6fdfcc20c
Author: Ciro Santilli <[email protected]>
Date: Thu Dec 3 23:45:07 2020 +0000
root
so note how a commit with multiple parents gets the:
Merge: ae5f932 c7ab8ef e54f235
line with the parents. The parent of commits with a single parent is not as clearly shown in that format however, but we could get it with:
git log --pretty=raw
which gives:
commit 4210da20504fc08136421a351983c179baf0dccb
tree 2d509da01229d1c0f2f0efa0951fd623f6e26f62
parent 5b1eba2de78f303d5a3a8b0ca18fcf1219d5431a
parent 802c349017b0f911ccb8e59b1a730b51113d198d
parent 228b5883376dce7874c1c2269666077a9db285b6
author Ciro Santilli <[email protected]> 1642065317 +0000
committer Ciro Santilli <[email protected]> 1642065317 +0000
merge
commit 5b1eba2de78f303d5a3a8b0ca18fcf1219d5431a
tree 235125b9eb17ab8889306536ca28a192cbb5499b
parent 1a257992f676efa966ffaf826e1bd1b6ece9325b
author Ciro Santilli <[email protected]> 1642065317 +0000
committer Ciro Santilli <[email protected]> 1642065317 +0000
3
commit 802c349017b0f911ccb8e59b1a730b51113d198d
tree 64b425efe9e0a742c6a085eb4c0771641be57582
parent 1a257992f676efa966ffaf826e1bd1b6ece9325b
author Ciro Santilli <[email protected]> 1642065317 +0000
committer Ciro Santilli <[email protected]> 1642065317 +0000
1
commit 228b5883376dce7874c1c2269666077a9db285b6
tree 40693988bf998a8acbc7cdec9982cdfd358915c9
parent 1a257992f676efa966ffaf826e1bd1b6ece9325b
author Ciro Santilli <[email protected]> 1642065317 +0000
committer Ciro Santilli <[email protected]> 1642065317 +0000
2
commit 1a257992f676efa966ffaf826e1bd1b6ece9325b
tree f5ba07a8f892d043d1b8f3429d3bec69a2331d4b
author Ciro Santilli <[email protected]> 1642065317 +0000
committer Ciro Santilli <[email protected]> 1642065317 +0000
root
related: https://stackoverflow.com/questions/9059335/how-can-i-get-the-parents-of-a-merge-commit-in-git
Another related git log option is: --min-parents which filters commits by the number of parents.
Tested on git version 2.25.1.
You can specify more than one branch when merging.
For example:
git merge branch_A branch_B branch_C [...]
Then commit has more parents.
