git-rebase.txt: document behavioral differences between modes
There are a variety of aspects that are common to all rebases regardless
of which backend is in use; however, the behavior for these different
aspects varies in ways that could surprise users. (In fact, it's not
clear -- to me at least -- that these differences were even desirable or
intentional.) Document these differences.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 2f47495..a67df4c 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -546,6 +546,38 @@
* --rebase-merges and --strategy
* --rebase-merges and --strategy-option
+BEHAVIORAL DIFFERENCES
+-----------------------
+
+ * empty commits:
+
+ am-based rebase will drop any "empty" commits, 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).
+
+ merge-based rebase does the same.
+
+ interactive-based rebase will by default drop commits that
+ started empty and halt if it hits a commit that ended up empty.
+ The `--keep-empty` option exists for interactive rebases to allow
+ it to keep commits that started empty.
+
+ * empty commit messages:
+
+ am-based rebase will silently apply commits with empty commit
+ messages.
+
+ merge-based and interactive-based rebases will by default halt
+ on any such commits. The `--allow-empty-message` option exists to
+ allow interactive-based rebases to apply such commits without
+ halting.
+
+ * directory rename detection:
+
+ merge-based and interactive-based rebases work fine with
+ directory rename detection. am-based rebases sometimes do not.
+
include::merge-strategies.txt[]
NOTES
diff --git a/Documentation/technical/directory-rename-detection.txt b/Documentation/technical/directory-rename-detection.txt
index 6e22920..1c0086e 100644
--- a/Documentation/technical/directory-rename-detection.txt
+++ b/Documentation/technical/directory-rename-detection.txt
@@ -90,3 +90,26 @@
simply not implemented. Further, to implement this, directory rename
detection logic would need to move from merge-recursive to
diffcore-rename.
+
+ * am
+
+ git-am tries to avoid a full three way merge, instead calling
+ git-apply. That prevents us from detecting renames at all, which may
+ defeat the directory rename detection. There is a fallback, though; if
+ the initial git-apply fails and the user has specified the -3 option,
+ git-am will fall back to a three way merge. However, git-am lacks the
+ necessary information to do a "real" three way merge. Instead, it has
+ to use build_fake_ancestor() to get a merge base that is missing files
+ whose rename may have been important to detect for directory rename
+ detection to function.
+
+ * rebase
+
+ Since am-based rebases work by first generating a bunch of patches
+ (which no longer record what the original commits were and thus don't
+ have the necessary info from which we can find a real merge-base), and
+ then calling git-am, this implies that am-based rebases will not always
+ successfully detect directory renames either (see the 'am' section
+ above). merged-based rebases (rebase -m) and cherry-pick-based rebases
+ (rebase -i) are not affected by this shortcoming, and fully support
+ directory rename detection.
