From: "Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: Johannes.Schindelin@gmx.de, phillip.wood@dunelm.org.uk,
liu.denton@gmail.com, gitster@pobox.com, plroskin@gmail.com,
alban.gruin@gmail.com, szeder.dev@gmail.com,
Junio C Hamano <gitster@pobox.com>,
Elijah Newren <newren@gmail.com>
Subject: [PATCH v2 08/15] git-rebase.txt: add more details about behavioral differences of backends
Date: Mon, 23 Dec 2019 18:49:52 +0000 [thread overview]
Message-ID: <b307340f7c79c87b2b5a5a2a25d98ef68be4ba4f.1577127000.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.679.v2.git.git.1577126999.gitgitgadget@gmail.com>
From: Elijah Newren <newren@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
---
Documentation/git-rebase.txt | 102 +++++++++++++++++++++---
t/t3433-rebase-options-compatibility.sh | 5 +-
2 files changed, 94 insertions(+), 13 deletions(-)
diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index ff32ca1080..f1ace07c38 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -409,13 +409,10 @@ your branch contains commits which were dropped, this option can be used
with `--keep-base` in order to drop those commits from your branch.
--ignore-whitespace::
- Behaves differently depending on which backend is selected.
-+
-'am' backend: When applying a patch, ignore changes in whitespace in
-context lines if necessary.
-+
-'interactive' backend: Treat lines with only whitespace changes as
-unchanged for the sake of a three-way merge.
+ Ignore whitespace-only changes in the commits being rebased,
+ which may avoid "unnecessary" conflicts. (Both backends
+ currently have differing edgecase bugs with this option; see
+ BEHAVIORAL DIFFERENCES.)
--whitespace=<option>::
This flag is passed to the 'git apply' program
@@ -609,9 +606,94 @@ There are some subtle differences how the backends behave.
Directory rename detection
~~~~~~~~~~~~~~~~~~~~~~~~~~
-Directory rename heuristics are enabled in the merge and interactive
-backends. Due to the lack of accurate tree information, directory
-rename detection is disabled in the am backend.
+Due to the lack of accurate tree information (arising from
+constructing fake ancestors with the limited information available in
+patches), directory rename detection is disabled in the am backend.
+Disabled directory rename detection means that if one side of history
+renames a directory and the other adds new files to the old directory,
+then the new files will be left behind in the old directory without
+any warning at the time of rebasing that you may want to move these
+files into the new directory.
+
+Directory rename detection works with the merge and interactive
+backends to provide you warnings in such cases.
+
+Context
+~~~~~~~
+
+The am backend works by creating a sequence of patches (by calling
+`format-patch` internally), and then applying the patches in sequence
+(calling `am` internally). Patches are composed of multiple hunks,
+each with line numbers, a context region, and the actual changes. The
+line numbers have to be taken with some fuzz, since the other side
+will likely have inserted or deleted lines earlier in the file. The
+context region is meant to help find how to adjust the line numbers in
+order to apply the changes to the right lines. However, if multiple
+areas of the code have the same surrounding lines of context, the
+wrong one can be picked. There are real-world cases where this has
+caused commits to be reapplied incorrectly with no conflicts reported.
+Setting diff.context to a larger value may prevent such types of
+problems, but increases the chance of spurious conflicts (since it
+will require more lines of matching context to apply).
+
+The interactive backend works with a full copy of each relevant file,
+insulating it from these types of problems.
+
+Labelling of conflicts markers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When there are content conflicts, the merge machinery tries to
+annotate each side's conflict markers with the commits where the
+content came from. Since the am backend drops the original
+information about the rebased commits and their parents (and instead
+generates new fake commits based off limited information in the
+generated patches), those commits cannot be identified; instead it has
+to fall back to a commit summary. Also, when merge.conflictStyle is
+set to diff3, the am backend will use "constructed merge base" to
+label the content from the merge base, and thus provide no information
+about the merge base commit whatsoever.
+
+The interactive backend works with the full commits on both sides of
+history and thus has no such limitations.
+
+--ignore-whitespace
+~~~~~~~~~~~~~~~~~~~
+
+The --ignore-whitespace option is supposed to ignore whitespace-only
+changes if it allows the code to merge cleanly. Unfortunately, the
+different backends implement this differently, and both have different
+edge case bugs.
++
+'am' backend: When applying a patch, ignore changes in whitespace in
+context lines if necessary. (Which implies that if the whitespace
+change was not in the context lines but on a line with a real change,
+then the rebase will still fail with "unnecessary" content conflicts.)
++
+'interactive' backend: Treat lines with only whitespace changes as
+unchanged for the sake of a three-way merge. This means that if one
+side made no changes and the commits being rebased had whitespace-only
+changes, those whitespaces fixups will be discarded despite the fact
+that they present no content conflict.
+
+Miscellaneous differences
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are a few more behavioral differences that most folks would
+probably consider inconsequential but which are mentioned for
+completeness:
+
+* Reflog: The two backends will use different wording when describing
+ the changes made in the reflog, though both will make use of the
+ word "rebase".
+
+* Progress, informational, and error messages: The two backends
+ provide slightly different progress and informational messages.
+ Also, the am backend writes error messages (such as "Your files
+ would be overwritten...") to stdout, while the interactive backend
+ writes them to stderr.
+
+* State directories: The two backends keep their state in different
+ directories under .git/
include::merge-strategies.txt[]
diff --git a/t/t3433-rebase-options-compatibility.sh b/t/t3433-rebase-options-compatibility.sh
index 5166f158dd..bd4d2d2f63 100755
--- a/t/t3433-rebase-options-compatibility.sh
+++ b/t/t3433-rebase-options-compatibility.sh
@@ -10,9 +10,8 @@ test_description='tests to ensure compatibility between am and interactive backe
GIT_AUTHOR_DATE="1999-04-02T08:03:20+05:30"
export GIT_AUTHOR_DATE
-# This is a special case in which both am and interactive backends
-# provide the same output. It was done intentionally because
-# both the backends fall short of optimal behaviour.
+# This is a common case in which both am and interactive backends
+# provide the same output with --ignore-whitespace.
test_expect_success 'setup' '
git checkout -b topic &&
q_to_tab >file <<-\EOF &&
--
gitgitgadget
next prev parent reply other threads:[~2019-12-23 18:50 UTC|newest]
Thread overview: 161+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-20 17:09 [PATCH 00/15] rebase: make the default backend configurable Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 01/15] rebase: extend the options for handling of empty commits Elijah Newren via GitGitGadget
2019-12-20 21:29 ` Junio C Hamano
2019-12-21 0:32 ` Elijah Newren
2019-12-21 18:52 ` Elijah Newren
2019-12-21 23:49 ` Junio C Hamano
2019-12-20 17:09 ` [PATCH 02/15] t3406: simplify an already simple test Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 03/15] rebase, sequencer: remove the broken GIT_QUIET handling Elijah Newren via GitGitGadget
2019-12-20 21:34 ` Junio C Hamano
2019-12-20 17:09 ` [PATCH 04/15] rebase: make sure to pass along the quiet flag to the sequencer Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 05/15] rebase: fix handling of restrict_revision Elijah Newren via GitGitGadget
2019-12-20 21:37 ` Junio C Hamano
2019-12-20 17:09 ` [PATCH 06/15] t3432: make these tests work with either am or merge backends Elijah Newren via GitGitGadget
2019-12-22 5:11 ` Denton Liu
2019-12-23 17:17 ` Elijah Newren
2019-12-20 17:09 ` [PATCH 07/15] rebase: allow more types of rebases to fast-forward Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 08/15] git-rebase.txt: add more details about behavioral differences of backends Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 09/15] rebase: move incompatibility checks between backend options a bit earlier Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 10/15] rebase: add an --am option Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 11/15] contrib: change the prompt for am-based rebases Elijah Newren via GitGitGadget
2019-12-20 23:07 ` SZEDER Gábor
2019-12-21 0:17 ` Elijah Newren
2019-12-20 17:09 ` [PATCH 12/15] rebase tests: mark tests specific to the am-backend with --am Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 13/15] rebase tests: repeat some tests using the merge backend instead of am Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 14/15] rebase: make the backend configurable via config setting Elijah Newren via GitGitGadget
2019-12-20 17:09 ` [PATCH 15/15] rebase: change the default backend from "am" to "merge" Elijah Newren via GitGitGadget
2019-12-20 18:51 ` [PATCH 00/15] rebase: make the default backend configurable Alban Gruin
2019-12-20 18:55 ` Elijah Newren
2019-12-23 18:49 ` [PATCH v2 " Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 01/15] rebase: extend the options for handling of empty commits Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 02/15] t3406: simplify an already simple test Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 03/15] rebase, sequencer: remove the broken GIT_QUIET handling Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 04/15] rebase: make sure to pass along the quiet flag to the sequencer Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 05/15] rebase: fix handling of restrict_revision Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 06/15] t3432: make these tests work with either am or merge backends Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 07/15] rebase: allow more types of rebases to fast-forward Elijah Newren via GitGitGadget
2019-12-23 18:49 ` Elijah Newren via GitGitGadget [this message]
2019-12-23 18:49 ` [PATCH v2 09/15] rebase: move incompatibility checks between backend options a bit earlier Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 10/15] rebase: add an --am option Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 11/15] contrib: change the prompt for interactive-based rebases Elijah Newren via GitGitGadget
2019-12-23 22:00 ` Denton Liu
2019-12-23 18:49 ` [PATCH v2 12/15] rebase tests: mark tests specific to the am-backend with --am Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 13/15] rebase tests: repeat some tests using the merge backend instead of am Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 14/15] rebase: make the backend configurable via config setting Elijah Newren via GitGitGadget
2019-12-23 18:49 ` [PATCH v2 15/15] rebase: change the default backend from "am" to "merge" Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 00/15] rebase: make the default backend configurable Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 01/15] rebase: extend the options for handling of empty commits Elijah Newren via GitGitGadget
2020-01-07 14:37 ` Phillip Wood
2020-01-07 19:15 ` Elijah Newren
2020-01-08 14:27 ` Phillip Wood
2020-01-09 21:32 ` Johannes Schindelin
2019-12-24 19:54 ` [PATCH v3 02/15] t3406: simplify an already simple test Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 03/15] rebase, sequencer: remove the broken GIT_QUIET handling Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 04/15] rebase: make sure to pass along the quiet flag to the sequencer Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 05/15] rebase: fix handling of restrict_revision Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 06/15] t3432: make these tests work with either am or merge backends Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 07/15] rebase: allow more types of rebases to fast-forward Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 08/15] git-rebase.txt: add more details about behavioral differences of backends Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 09/15] rebase: move incompatibility checks between backend options a bit earlier Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 10/15] rebase: add an --am option Elijah Newren via GitGitGadget
2020-01-07 14:43 ` Phillip Wood
2020-01-07 19:26 ` Elijah Newren
2020-01-07 20:11 ` Junio C Hamano
2020-01-08 14:32 ` Phillip Wood
2020-01-08 17:18 ` Junio C Hamano
2020-01-08 18:55 ` Phillip Wood
2019-12-24 19:54 ` [PATCH v3 11/15] git-prompt: change the prompt for interactive-based rebases Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 12/15] rebase tests: mark tests specific to the am-backend with --am Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 13/15] rebase tests: repeat some tests using the merge backend instead of am Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 14/15] rebase: make the backend configurable via config setting Elijah Newren via GitGitGadget
2019-12-24 19:54 ` [PATCH v3 15/15] rebase: change the default backend from "am" to "merge" Elijah Newren via GitGitGadget
2020-01-10 23:14 ` Jonathan Nieder
2020-01-11 1:16 ` Elijah Newren
2020-01-11 14:41 ` Phillip Wood
2020-01-12 17:59 ` Johannes Schindelin
2020-01-16 6:32 ` Elijah Newren
2020-01-16 7:58 ` Jonathan Nieder
2020-01-16 8:06 ` Jonathan Nieder
2020-01-16 16:18 ` Elijah Newren
2020-01-16 20:35 ` Jonathan Nieder
2020-01-16 21:30 ` Elijah Newren
2020-01-16 22:39 ` Jonathan Nieder
2020-01-16 23:19 ` Elijah Newren
2020-01-16 23:25 ` Junio C Hamano
2020-01-17 0:51 ` Elijah Newren
2020-01-16 15:35 ` Elijah Newren
2020-01-16 20:05 ` Junio C Hamano
2020-01-16 10:48 ` Johannes Schindelin
2020-01-12 21:23 ` Junio C Hamano
2020-01-15 19:50 ` Jonathan Nieder
2020-01-15 21:59 ` Emily Shaffer
2020-01-16 6:14 ` [PATCH v4 00/19] rebase: make the default backend configurable Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 01/19] git-rebase.txt: update description of --allow-empty-message Elijah Newren via GitGitGadget
2020-02-09 15:59 ` Phillip Wood
2020-02-13 18:35 ` Elijah Newren
2020-01-16 6:14 ` [PATCH v4 02/19] t3404: directly test the behavior of interest Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 03/19] rebase (interactive-backend): make --keep-empty the default Elijah Newren via GitGitGadget
2020-02-09 15:59 ` Phillip Wood
2020-02-13 18:52 ` Elijah Newren
2020-01-16 6:14 ` [PATCH v4 04/19] rebase (interactive-backend): fix handling of commits that become empty Elijah Newren via GitGitGadget
2020-02-10 14:27 ` Phillip Wood
2020-02-13 18:54 ` Elijah Newren
2020-02-16 14:46 ` Phillip Wood
2020-01-16 6:14 ` [PATCH v4 05/19] t3406: simplify an already simple test Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 06/19] rebase, sequencer: remove the broken GIT_QUIET handling Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 07/19] rebase: make sure to pass along the quiet flag to the sequencer Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 08/19] rebase: fix handling of restrict_revision Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 09/19] t3432: make these tests work with either am or merge backends Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 10/19] rebase: allow more types of rebases to fast-forward Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 11/19] git-rebase.txt: add more details about behavioral differences of backends Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 12/19] rebase: move incompatibility checks between backend options a bit earlier Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 13/19] rebase: add an --am option Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 14/19] git-prompt: change the prompt for interactive-based rebases Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 15/19] rebase: drop '-i' from the reflog " Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 16/19] rebase tests: mark tests specific to the am-backend with --am Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 17/19] rebase tests: repeat some tests using the merge backend instead of am Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 18/19] rebase: make the backend configurable via config setting Elijah Newren via GitGitGadget
2020-01-16 6:14 ` [PATCH v4 19/19] rebase: change the default backend from "am" to "merge" Elijah Newren via GitGitGadget
2020-01-17 16:58 ` [PATCH v4 00/19] rebase: make the default backend configurable Phillip Wood
2020-02-05 21:06 ` Junio C Hamano
2020-02-05 22:38 ` Elijah Newren
2020-02-15 21:36 ` [PATCH v5 00/20] rebase: make the default backend configurable and change the default Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 01/20] git-rebase.txt: update description of --allow-empty-message Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 02/20] t3404: directly test the behavior of interest Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 03/20] rebase (interactive-backend): make --keep-empty the default Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 04/20] rebase (interactive-backend): fix handling of commits that become empty Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 05/20] t3406: simplify an already simple test Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 06/20] rebase, sequencer: remove the broken GIT_QUIET handling Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 07/20] rebase: make sure to pass along the quiet flag to the sequencer Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 08/20] rebase: fix handling of restrict_revision Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 09/20] t3432: make these tests work with either am or merge backends Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 10/20] rebase: allow more types of rebases to fast-forward Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 11/20] git-rebase.txt: add more details about behavioral differences of backends Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 12/20] rebase: move incompatibility checks between backend options a bit earlier Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 13/20] rebase: add an --am option Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 14/20] git-prompt: change the prompt for interactive-based rebases Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 15/20] rebase: drop '-i' from the reflog " Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 16/20] rebase tests: mark tests specific to the am-backend with --am Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 17/20] rebase tests: repeat some tests using the merge backend instead of am Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 18/20] rebase: make the backend configurable via config setting Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 19/20] rebase: change the default backend from "am" to "merge" Elijah Newren via GitGitGadget
2020-02-15 21:36 ` [PATCH v5 20/20] rebase: rename the two primary rebase backends Elijah Newren via GitGitGadget
2020-03-12 15:13 ` Emily Shaffer
2020-03-12 16:33 ` Elijah Newren
2020-03-12 17:55 ` Jonathan Nieder
2020-03-12 18:39 ` Elijah Newren
2020-03-12 18:46 ` Jonathan Nieder
2020-03-12 19:31 ` Elijah Newren
2020-03-17 2:58 ` Jonathan Nieder
2020-03-17 4:45 ` Elijah Newren
2020-03-12 19:54 ` Junio C Hamano
2020-03-12 19:07 ` Junio C Hamano
2020-03-12 19:12 ` Jonathan Nieder
2020-03-12 19:12 ` Junio C Hamano
2020-03-12 19:29 ` Elijah Newren
2020-03-12 20:37 ` Jeff King
2020-03-12 21:27 ` Junio C Hamano
2020-03-12 22:06 ` Elijah Newren
2020-03-13 0:04 ` Junio C Hamano
2020-03-12 23:30 ` Jonathan Nieder
2020-02-16 15:01 ` [PATCH v5 00/20] rebase: make the default backend configurable and change the default Phillip Wood
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