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-rw-r--r--Documentation/git-rebase.txt170
1 files changed, 148 insertions, 22 deletions
diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 0c4f038..8c1f4b8 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -258,16 +258,45 @@ See also INCOMPATIBLE OPTIONS below.
original branch. The index and working tree are also left
unchanged as a result.
---keep-empty::
- Keep the commits that do not change anything from its
- parents in the result.
+--apply:
+ Use applying strategies to rebase (calling `git-am`
+ internally). This option may become a no-op in the future
+ once the merge backend handles everything the apply one does.
++
+See also INCOMPATIBLE OPTIONS below.
+
+--empty={drop,keep,ask}::
+ How to handle commits that are not empty to start and are not
+ clean cherry-picks of any upstream commit, but which become
+ empty after rebasing (because they contain a subset of already
+ upstream changes). With drop (the default), commits that
+ become empty are dropped. With keep, such commits are kept.
+ With ask (implied by --interactive), the rebase will halt when
+ an empty commit is applied allowing you to choose whether to
+ drop it, edit files more, or just commit the empty changes.
+ Other options, like --exec, will use the default of drop unless
+ -i/--interactive is explicitly specified.
++
+Note that commits which start empty are kept, and commits which are
+clean cherry-picks (as determined by `git log --cherry-mark ...`) are
+always dropped.
+
See also INCOMPATIBLE OPTIONS below.
+--keep-empty::
+ No-op. Rebasing commits that started empty (had no change
+ relative to their parent) used to fail and this option would
+ override that behavior, allowing commits with empty changes to
+ be rebased. Now commits with no changes do not cause rebasing
+ to halt.
++
+See also BEHAVIORAL DIFFERENCES and INCOMPATIBLE OPTIONS below.
+
--allow-empty-message::
- By default, rebasing commits with an empty message will fail.
- This option overrides that behavior, allowing commits with empty
- messages to be rebased.
+ No-op. Rebasing commits with an empty message used to fail
+ and this option would override that behavior, allowing commits
+ with empty messages to be rebased. Now commits with an empty
+ message do not cause rebasing to halt.
+
See also INCOMPATIBLE OPTIONS below.
@@ -286,7 +315,7 @@ See also INCOMPATIBLE OPTIONS below.
--merge::
Use merging strategies to rebase. When the recursive (default) merge
strategy is used, this allows rebase to be aware of renames on the
- upstream side.
+ upstream side. This is the default.
+
Note that a rebase merge works by replaying each commit from the working
branch on top of the <upstream> branch. Because of this, when a merge
@@ -356,7 +385,7 @@ See also INCOMPATIBLE OPTIONS below.
Ensure at least <n> lines of surrounding context match before
and after each change. When fewer lines of surrounding
context exist they all must match. By default no context is
- ever ignored.
+ ever ignored. Implies --apply.
+
See also INCOMPATIBLE OPTIONS below.
@@ -394,8 +423,9 @@ with `--keep-base` in order to drop those commits from your branch.
--ignore-whitespace::
--whitespace=<option>::
- These flag are passed to the 'git apply' program
+ These flags are passed to the 'git apply' program
(see linkgit:git-apply[1]) that applies the patch.
+ Implies --apply.
+
See also INCOMPATIBLE OPTIONS below.
@@ -539,10 +569,11 @@ INCOMPATIBLE OPTIONS
The following options:
+ * --apply
* --committer-date-is-author-date
* --ignore-date
- * --whitespace
* --ignore-whitespace
+ * --whitespace
* -C
are incompatible with the following options:
@@ -557,6 +588,7 @@ are incompatible with the following options:
* --interactive
* --exec
* --keep-empty
+ * --empty=
* --edit-todo
* --root when used in combination with --onto
@@ -565,33 +597,127 @@ In addition, the following pairs of options are incompatible:
* --preserve-merges and --interactive
* --preserve-merges and --signoff
* --preserve-merges and --rebase-merges
+ * --preserve-merges and --empty=
* --keep-base and --onto
* --keep-base and --root
BEHAVIORAL DIFFERENCES
-----------------------
-There are some subtle differences how the backends behave.
+git rebase has two primary backends: apply and merge. (The apply
+backend used to known as the 'am' backend, but the name led to
+confusion as it looks like a verb instead of a noun. Also, the merge
+backend used to be known as the interactive backend, but it is now
+used for non-interactive cases as well. Both were renamed based on
+lower-level functionality that underpinned each.) There are some
+subtle differences in how these two backends behave:
Empty commits
~~~~~~~~~~~~~
-The am backend drops any "empty" commits, regardless of whether the
-commit started empty (had no changes relative to its parent to
-start with) or ended empty (all changes were already applied
-upstream in other commits).
+The apply backend unfortunately drops intentionally empty commits, i.e.
+commits that started empty, though these are rare in practice. It
+also drops commits that become empty and has no option for controlling
+this behavior.
-The interactive backend drops commits by default that
-started empty and halts if it hits a commit that ended up empty.
-The `--keep-empty` option exists for the interactive backend to allow
-it to keep commits that started empty.
+The merge backend keeps intentionally empty commits. Similar to the
+apply backend, by default the merge backend drops commits that become
+empty unless -i/--interactive is specified (in which case it stops and
+asks the user what to do). The merge backend also has an
+--empty={drop,keep,ask} option for changing the behavior of handling
+commits that become empty.
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 apply 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 backend to provide you
+warnings in such cases.
+
+Context
+~~~~~~~
+
+The apply 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 merge 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 apply 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 apply 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 merge backend works with the full commits on both sides of history
+and thus has no such limitations.
+
+Hooks
+~~~~~
+
+The apply backend has not traditionally called the post-commit hook,
+while the merge backend has. However, this was by accident of
+implementation rather than by design. Both backends should have the
+same behavior, though it is not clear which one is correct.
+
+Interruptability
+~~~~~~~~~~~~~~~~
+
+The apply backend has safety problems with an ill-timed interrupt; if
+the user presses Ctrl-C at the wrong time to try to abort the rebase,
+the rebase can enter a state where it cannot be aborted with a
+subsequent `git rebase --abort`. The merge backend does not appear to
+suffer from the same shortcoming. (See
+https://lore.kernel.org/git/20200207132152.GC2868@szeder.dev/ for
+details.)
+
+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 apply backend writes error messages (such as "Your files
+ would be overwritten...") to stdout, while the merge backend writes
+ them to stderr.
+
+* State directories: The two backends keep their state in different
+ directories under .git/
include::merge-strategies.txt[]