git@vger.kernel.org list mirror (unofficial, one of many)
 help / color / mirror / Atom feed
From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: <git@vger.kernel.org>
Cc: "Martin Ågren" <martin.agren@gmail.com>
Subject: [PATCH 1/3] docs: explain why squash merges are broken with long-running branches
Date: Sat, 12 Sep 2020 20:48:22 +0000
Message-ID: <20200912204824.2824106-2-sandals@crustytoothpaste.net> (raw)
In-Reply-To: <20200912204824.2824106-1-sandals@crustytoothpaste.net>

In many projects, squash merges are commonly used, primarily to keep a
tidy history in the face of developers who do not use logically
independent, bisectable commits.  As common as this is, this tends to
cause significant problems when squash merges are used to merge
long-running branches due to the lack of any new merge bases.  Even very
experienced developers may make this mistakes, so let's add a FAQ entry
explaining why this is problematic and explaining that regular merge
commits should be used to merge two long-running branches.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
---
 Documentation/gitfaq.txt | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/Documentation/gitfaq.txt b/Documentation/gitfaq.txt
index 9cd7a592ac..550f4e30d6 100644
--- a/Documentation/gitfaq.txt
+++ b/Documentation/gitfaq.txt
@@ -241,6 +241,39 @@ How do I know if I want to do a fetch or a pull?::
 	ignore the upstream changes.  A pull consists of a fetch followed
 	immediately by either a merge or rebase.  See linkgit:git-pull[1].
 
+Merging and Rebasing
+--------------------
+
+[[long-running-squash-merge]]
+What kinds of problems can occur when merging long-running branches with squash merges?::
+	In general, there are a variety of problems that can occur when using squash
+	merges with long-running branches.  These can include seeing extra commits in
+	`git log` output, with a GUI, or when using the `...` notation to express a
+	range, as well as the possibility of needing to re-resolve conflicts again and
+	again.
++
+When Git does a normal merge between two branches, it considers exactly three
+points: the two branches and a third commit, called the _merge base_, which is
+usually the common ancestor of the commits.  The result of the merge is the sum
+of the changes between the merge base and each head.  When you merge two
+long-running branches with a regular merge commit, this results in a new commit
+which will end up as a merge base when they're merged again, because there is
+now a new common ancestor.  Git doesn't have to consider changes that occurred
+before the merge base, so you don't have to re-resolve any conflicts you
+resolved before.
++
+When you perform a squash merge, a merge commit isn't created; instead, the
+changes from one side are applied as a regular commit to the other side.  This
+means that the merge base for these branches won't have changed, and so when Git
+goes to perform its next merge, it considers all of the changes that it
+considered the last time plus the new changes.  That means any conflicts may
+need to be re-resolved.  Similarly, anything using the `...` notation in `git
+diff`, `git log`, or a GUI will result in showing all of the changes since the
+original merge base.
++
+As a consequence, if you want to merge two long-running branches, it's best to
+always use a regular merge commit.
+
 Hooks
 -----
 

  reply	other threads:[~2020-09-12 20:52 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-12 20:48 [PATCH 0/3] FAQ entries for merges and modified files brian m. carlson
2020-09-12 20:48 ` brian m. carlson [this message]
2020-09-13 15:05   ` [PATCH 1/3] docs: explain why squash merges are broken with long-running branches Martin Ågren
2020-09-13 17:12     ` brian m. carlson
2020-09-12 20:48 ` [PATCH 2/3] docs: explain why reverts are not always applied on merge brian m. carlson
2020-09-13 15:12   ` Martin Ågren
2020-09-12 20:48 ` [PATCH 3/3] docs: explain how to deal with files that are always modified brian m. carlson
2020-09-13 15:13   ` Martin Ågren
2020-09-12 21:48 ` [PATCH 0/3] FAQ entries for merges and modified files Junio C Hamano
2020-09-20 23:22 ` [PATCH v2 " brian m. carlson
2020-09-20 23:22   ` [PATCH v2 1/3] docs: explain why squash merges are broken with long-running branches brian m. carlson
2020-09-20 23:22   ` [PATCH v2 2/3] docs: explain why reverts are not always applied on merge brian m. carlson
2020-09-20 23:22   ` [PATCH v2 3/3] docs: explain how to deal with files that are always modified brian m. carlson

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200912204824.2824106-2-sandals@crustytoothpaste.net \
    --to=sandals@crustytoothpaste.net \
    --cc=git@vger.kernel.org \
    --cc=martin.agren@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

git@vger.kernel.org list mirror (unofficial, one of many)

This inbox may be cloned and mirrored by anyone:

	git clone --mirror https://public-inbox.org/git
	git clone --mirror http://ou63pmih66umazou.onion/git
	git clone --mirror http://czquwvybam4bgbro.onion/git
	git clone --mirror http://hjrcffqmbrq6wope.onion/git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V1 git git/ https://public-inbox.org/git \
		git@vger.kernel.org
	public-inbox-index git

Example config snippet for mirrors.
Newsgroups are available over NNTP:
	nntp://news.public-inbox.org/inbox.comp.version-control.git
	nntp://ou63pmih66umazou.onion/inbox.comp.version-control.git
	nntp://czquwvybam4bgbro.onion/inbox.comp.version-control.git
	nntp://hjrcffqmbrq6wope.onion/inbox.comp.version-control.git
	nntp://news.gmane.io/gmane.comp.version-control.git
 note: .onion URLs require Tor: https://www.torproject.org/

code repositories for the project(s) associated with this inbox:

	https://80x24.org/mirrors/git.git

AGPL code for this site: git clone https://public-inbox.org/public-inbox.git