absorb: preserve changesets which were already empty
Most commands in Mercurial (commit, rebase, absorb itself) don’t create empty
changesets or drop them if they become empty. If there’s a changeset that’s
empty, it must be a deliberate choice of the user. At least it shouldn’t be
absorb’s responsibility to prune them. The fact that changesets that became
empty during absorb are pruned, is unaffected by this.
This case was found while writing patches which make it possible to configure
absorb and rebase to not drop empty changesets. Even without having such config
set, I think it’s valuable to preserve changesets which were already empty.
$ cat <<EOF >> $HGRCPATH
> [extensions]
> hooklib =
>
> [phases]
> publish = False
> EOF
$ hg init a
$ hg --cwd a debugbuilddag '.:parent.*parent'
$ hg --cwd a log -G
o changeset: 2:fa942426a6fd
| tag: tip
| parent: 0:1ea73414a91b
| user: debugbuilddag
| date: Thu Jan 01 00:00:02 1970 +0000
| summary: r2
|
| o changeset: 1:66f7d451a68b
|/ user: debugbuilddag
| date: Thu Jan 01 00:00:01 1970 +0000
| summary: r1
|
o changeset: 0:1ea73414a91b
tag: parent
user: debugbuilddag
date: Thu Jan 01 00:00:00 1970 +0000
summary: r0
$ hg init b
$ cat <<EOF >> b/.hg/hgrc
> [hooks]
> pretxnclose.reject_new_heads = \
> python:hgext.hooklib.reject_new_heads.hook
> EOF
$ hg --cwd b pull ../a
pulling from ../a
requesting all changes
adding changesets
adding manifests
adding file changes
error: pretxnclose.reject_new_heads hook failed: Changes on branch 'default' resulted in multiple heads
transaction abort!
rollback completed
abort: Changes on branch 'default' resulted in multiple heads
[255]
$ hg --cwd b pull ../a -r 1ea73414a91b
pulling from ../a
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files
new changesets 1ea73414a91b (1 drafts)
(run 'hg update' to get a working copy)