Subject: [PATCH v3 15/15] rebase: change the default backend from "am" to "merge"

From: "Elijah Newren via GitGitGadget" <[email-masked]>
Date: Tue, 24 Dec 2019 19:54:59 +0000

From: Elijah Newren <[email-masked]>

The am-backend drops information and thus limits what we can do:

  * lack of full tree information from the original commits means we
    cannot do directory rename detection and warn users that they might
    want to move some of their new files that they placed in old
    directories to prevent their becoming orphaned.[1]
  * reduction in context from only having a few lines beyond those
    changed means that when context lines are non-unique we can apply
    patches incorrectly.[2]
  * lack of access to original commits means that conflict marker
    annotation has less information available.

Also, the merge/interactive backend have far more abilities, appear to
currently have a slight performance advantage[3] and have room for more
optimizations than the am backend[4] (and work is underway to take
advantage of some of those possibilities).

[1] https://lore.kernel.org/git/[email-masked]/
[2] https://lore.kernel.org/git/[email-masked]/
[3] https://public-inbox.org/git/CABPp-BF=[email-masked]/
[4] https://lore.kernel.org/git/[email-masked]/

Signed-off-by: Elijah Newren <[email-masked]>
---
 Documentation/git-rebase.txt           |  2 +-
 builtin/rebase.c                       |  4 ++--
 t/t5520-pull.sh                        | 10 ++++++----
 t/t9106-git-svn-commit-diff-clobber.sh |  3 ++-
 4 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index cf1ac2e359..e819889a31 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -309,7 +309,7 @@ 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
diff --git a/builtin/rebase.c b/builtin/rebase.c
index d602b2da4c..938a8840b1 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -104,7 +104,7 @@ struct rebase_options {
 #define REBASE_OPTIONS_INIT {			  	\
 		.type = REBASE_UNSPECIFIED,	  	\
 		.empty = EMPTY_UNSPECIFIED,	  	\
-		.default_backend = "am",	  	\
+		.default_backend = "merge",	  	\
 		.flags = REBASE_NO_QUIET, 		\
 		.git_am_opts = ARGV_ARRAY_INIT,		\
 		.git_format_patch_opt = STRBUF_INIT	\
@@ -1945,7 +1945,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 
 	if (options.type == REBASE_UNSPECIFIED) {
 		if (!strcmp(options.default_backend, "merge"))
-			options.type = REBASE_MERGE;
+			imply_interactive(&options, "--merge");
 		else if (!strcmp(options.default_backend, "am"))
 			options.type = REBASE_AM;
 		else
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index 3fff6a06fa..4f9e7f7ff6 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -340,7 +340,7 @@ test_expect_success '--rebase with conflicts shows advice' '
 	test_tick &&
 	git commit -m "Create conflict" seq.txt &&
 	test_must_fail git pull --rebase . seq 2>err >out &&
-	test_i18ngrep "Resolve all conflicts manually" out
+	test_i18ngrep "Resolve all conflicts manually" err
 '
 
 test_expect_success 'failed --rebase shows advice' '
@@ -354,7 +354,7 @@ test_expect_success 'failed --rebase shows advice' '
 	git checkout -f -b fails-to-rebase HEAD^ &&
 	test_commit v2-without-cr file "2" file2-lf &&
 	test_must_fail git pull --rebase . diverging 2>err >out &&
-	test_i18ngrep "Resolve all conflicts manually" out
+	test_i18ngrep "Resolve all conflicts manually" err
 '
 
 test_expect_success '--rebase fails with multiple branches' '
@@ -774,8 +774,10 @@ test_expect_success 'git pull --rebase does not reapply old patches' '
 	(
 		cd dst &&
 		test_must_fail git pull --rebase &&
-		find .git/rebase-apply -name "000*" >patches &&
-		test_line_count = 1 patches
+		cat .git/rebase-merge/done .git/rebase-merge/git-rebase-todo >work &&
+		grep -v -e \# -e ^$ work >patches &&
+		test_line_count = 1 patches &&
+		rm -f work
 	)
 '
 
diff --git a/t/t9106-git-svn-commit-diff-clobber.sh b/t/t9106-git-svn-commit-diff-clobber.sh
index dbe8deac0d..aec45bca3b 100755
--- a/t/t9106-git-svn-commit-diff-clobber.sh
+++ b/t/t9106-git-svn-commit-diff-clobber.sh
@@ -92,7 +92,8 @@ test_expect_success 'multiple dcommit from git svn will not clobber svn' "

 test_expect_success 'check that rebase really failed' '
-	test -d .git/rebase-apply
+	git status >output &&
+	grep currently.rebasing output
 '
 
 test_expect_success 'resolve, continue the rebase and dcommit' "
-- 
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