Merge branch 'dl/rebase-i-keep-base'

"git rebase --keep-base <upstream>" tries to find the original base
of the topic being rebased and rebase on top of that same base,
which is useful when running the "git rebase -i" (and its limited
variant "git rebase -x").

The command also has learned to fast-forward in more cases where it
can instead of replaying to recreate identical commits.

* dl/rebase-i-keep-base:
  rebase: teach rebase --keep-base
  rebase tests: test linear branch topology
  rebase: fast-forward --fork-point in more cases
  rebase: fast-forward --onto in more cases
  rebase: refactor can_fast_forward into goto tower
  t3432: test for --no-ff's interaction with fast-forward
  t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests
  t3432: test rebase fast-forward behavior
  t3431: add rebase --fork-point tests

9 files changed, 360 insertions, 28 deletions

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 7ecf766077..639a4179d1 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -8,8 +8,8 @@ git-rebase - Reapply commits on top of another base tip
 SYNOPSIS
 --------
 [verse]
-'git rebase' [-i | --interactive] [<options>] [--exec <cmd>] [--onto <newbase>]
-	[<upstream> [<branch>]]
+'git rebase' [-i | --interactive] [<options>] [--exec <cmd>]
+	[--onto <newbase> | --keep-base] [<upstream> [<branch>]]
 'git rebase' [-i | --interactive] [<options>] [--exec <cmd>] [--onto <newbase>]
 	--root [<branch>]
 'git rebase' (--continue | --skip | --abort | --quit | --edit-todo | --show-current-patch)
@@ -217,6 +217,24 @@ As a special case, you may use "A\...B" as a shortcut for the
 merge base of A and B if there is exactly one merge base. You can
 leave out at most one of A and B, in which case it defaults to HEAD.
 
+--keep-base::
+	Set the starting point at which to create the new commits to the
+	merge base of <upstream> <branch>. Running
+	'git rebase --keep-base <upstream> <branch>' is equivalent to
+	running 'git rebase --onto <upstream>... <upstream>'.
++
+This option is useful in the case where one is developing a feature on
+top of an upstream branch. While the feature is being worked on, the
+upstream branch may advance and it may not be the best idea to keep
+rebasing on top of the upstream but to keep the base commit as-is.
++
+Although both this option and --fork-point find the merge base between
+<upstream> and <branch>, this option uses the merge base as the _starting
+point_ on which new commits will be created, whereas --fork-point uses
+the merge base to determine the _set of commits_ which will be rebased.
++
+See also INCOMPATIBLE OPTIONS below.
+
 <upstream>::
 	Upstream branch to compare against.  May be any valid commit,
 	not just an existing branch name. Defaults to the configured
@@ -369,6 +387,10 @@ ends up being empty, the <upstream> will be used as a fallback.
 +
 If either <upstream> or --root is given on the command line, then the
 default is `--no-fork-point`, otherwise the default is `--fork-point`.
++
+If your branch was based on <upstream> but <upstream> was rewound and
+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::
 --whitespace=<option>::
@@ -543,6 +565,8 @@ In addition, the following pairs of options are incompatible:
  * --preserve-merges and --interactive
  * --preserve-merges and --signoff
  * --preserve-merges and --rebase-merges
+ * --keep-base and --onto
+ * --keep-base and --root
 
 BEHAVIORAL DIFFERENCES
 -----------------------
@@ -869,7 +893,7 @@ NOTE: While an "easy case recovery" sometimes appears to be successful
       --interactive` will be **resurrected**!
 
 The idea is to manually tell 'git rebase' "where the old 'subsystem'
-ended and your 'topic' began", that is, what the old merge-base
+ended and your 'topic' began", that is, what the old merge base
 between them was.  You will have to find a way to name the last commit
 of the old 'subsystem', for example:
 
diff --git a/builtin/rebase.c b/builtin/rebase.c
index e8319d5946..f730b15a78 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -29,8 +29,8 @@
 #include "rebase-interactive.h"
 
 static char const * const builtin_rebase_usage[] = {
-	N_("git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] "
-		"[<upstream>] [<branch>]"),
+	N_("git rebase [-i] [options] [--exec <cmd>] "
+		"[--onto <newbase> | --keep-base] [<upstream> [<branch>]]"),
 	N_("git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] "
 		"--root [<branch>]"),
 	N_("git rebase --continue | --abort | --skip | --edit-todo"),
@@ -1261,24 +1261,44 @@ static int is_linear_history(struct commit *from, struct commit *to)
 	return 1;
 }
 
-static int can_fast_forward(struct commit *onto, struct object_id *head_oid,
-			    struct object_id *merge_base)
+static int can_fast_forward(struct commit *onto, struct commit *upstream,
+			    struct commit *restrict_revision,
+			    struct object_id *head_oid, struct object_id *merge_base)
 {
 	struct commit *head = lookup_commit(the_repository, head_oid);
-	struct commit_list *merge_bases;
-	int res;
+	struct commit_list *merge_bases = NULL;
+	int res = 0;
 
 	if (!head)
-		return 0;
+		goto done;
 
 	merge_bases = get_merge_bases(onto, head);
-	if (merge_bases && !merge_bases->next) {
-		oidcpy(merge_base, &merge_bases->item->object.oid);
-		res = oideq(merge_base, &onto->object.oid);
-	} else {
+	if (!merge_bases || merge_bases->next) {
 		oidcpy(merge_base, &null_oid);
-		res = 0;
+		goto done;
 	}
+
+	oidcpy(merge_base, &merge_bases->item->object.oid);
+	if (!oideq(merge_base, &onto->object.oid))
+		goto done;
+
+	if (restrict_revision && !oideq(&restrict_revision->object.oid, merge_base))
+		goto done;
+
+	if (!upstream)
+		goto done;
+
+	free_commit_list(merge_bases);
+	merge_bases = get_merge_bases(upstream, head);
+	if (!merge_bases || merge_bases->next)
+		goto done;
+
+	if (!oideq(&onto->object.oid, &merge_bases->item->object.oid))
+		goto done;
+
+	res = 1;
+
+done:
 	free_commit_list(merge_bases);
 	return res && is_linear_history(onto, head);
 }
@@ -1377,6 +1397,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	struct rebase_options options = REBASE_OPTIONS_INIT;
 	const char *branch_name;
 	int ret, flags, total_argc, in_progress = 0;
+	int keep_base = 0;
 	int ok_to_skip_pre_rebase = 0;
 	struct strbuf msg = STRBUF_INIT;
 	struct strbuf revisions = STRBUF_INIT;
@@ -1395,6 +1416,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		OPT_STRING(0, "onto", &options.onto_name,
 			   N_("revision"),
 			   N_("rebase onto given branch instead of upstream")),
+		OPT_BOOL(0, "keep-base", &keep_base,
+			 N_("use the merge-base of upstream and branch as the current base")),
 		OPT_BOOL(0, "no-verify", &ok_to_skip_pre_rebase,
 			 N_("allow pre-rebase hook to run")),
 		OPT_NEGBIT('q', "quiet", &options.flags,
@@ -1548,6 +1571,13 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		warning(_("git rebase --preserve-merges is deprecated. "
 			  "Use --rebase-merges instead."));
 
+	if (keep_base) {
+		if (options.onto_name)
+			die(_("cannot combine '--keep-base' with '--onto'"));
+		if (options.root)
+			die(_("cannot combine '--keep-base' with '--root'"));
+	}
+
 	if (action != ACTION_NONE && !in_progress)
 		die(_("No rebase in progress?"));
 	setenv(GIT_REFLOG_ACTION_ENVIRONMENT, "rebase", 0);
@@ -1876,12 +1906,22 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	}
 
 	/* Make sure the branch to rebase onto is valid. */
-	if (!options.onto_name)
+	if (keep_base) {
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, options.upstream_name);
+		strbuf_addstr(&buf, "...");
+		options.onto_name = xstrdup(buf.buf);
+	} else if (!options.onto_name)
 		options.onto_name = options.upstream_name;
 	if (strstr(options.onto_name, "...")) {
-		if (get_oid_mb(options.onto_name, &merge_base) < 0)
-			die(_("'%s': need exactly one merge base"),
-			    options.onto_name);
+		if (get_oid_mb(options.onto_name, &merge_base) < 0) {
+			if (keep_base)
+				die(_("'%s': need exactly one merge base with branch"),
+				    options.upstream_name);
+			else
+				die(_("'%s': need exactly one merge base"),
+				    options.onto_name);
+		}
 		options.onto = lookup_commit_or_die(&merge_base,
 						    options.onto_name);
 	} else {
@@ -2016,13 +2056,13 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 
 	/*
 	 * Check if we are already based on onto with linear history,
-	 * but this should be done only when upstream and onto are the same
-	 * and if this is not an interactive rebase.
+	 * in which case we could fast-forward without replacing the commits
+	 * with new commits recreated by replaying their changes. This
+	 * optimization must not be done if this is an interactive rebase.
 	 */
-	if (can_fast_forward(options.onto, &options.orig_head, &merge_base) &&
-	    !is_interactive(&options) && !options.restrict_revision &&
-	    options.upstream &&
-	    !oidcmp(&options.upstream->object.oid, &options.onto->object.oid)) {
+	if (can_fast_forward(options.onto, options.upstream, options.restrict_revision,
+		    &options.orig_head, &merge_base) &&
+	    !is_interactive(&options)) {
 		int flag;
 
 		if (!(options.flags & REBASE_FORCE)) {
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 6f7b4f9607..59cd3e8e90 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -2043,7 +2043,7 @@ _git_rebase ()
 			--autosquash --no-autosquash
 			--fork-point --no-fork-point
 			--autostash --no-autostash
-			--verify --no-verify
+			--verify --no-verify --keep-base
 			--keep-empty --root --force-rebase --no-ff
 			--rerere-autoupdate
 			--exec
diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index 23469cc789..ab18ac5f28 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -295,7 +295,7 @@ test_expect_success 'rebase --am and --show-current-patch' '
 		echo two >>init.t &&
 		git commit -a -m two &&
 		git tag two &&
-		test_must_fail git rebase --onto init HEAD^ &&
+		test_must_fail git rebase -f --onto init HEAD^ &&
 		GIT_TRACE=1 git rebase --show-current-patch >/dev/null 2>stderr &&
 		grep "show.*$(git rev-parse two)" stderr
 	)
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 04affcc38a..29a35840ed 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -1056,7 +1056,7 @@ test_expect_success C_LOCALE_OUTPUT 'rebase --edit-todo does not work on non-int
 	git reset --hard &&
 	git checkout conflict-branch &&
 	set_fake_editor &&
-	test_must_fail git rebase --onto HEAD~2 HEAD~ &&
+	test_must_fail git rebase -f --onto HEAD~2 HEAD~ &&
 	test_must_fail git rebase --edit-todo &&
 	git rebase --abort
 '
diff --git a/t/t3416-rebase-onto-threedots.sh b/t/t3416-rebase-onto-threedots.sh
index ddf2f64853..9c2548423b 100755
--- a/t/t3416-rebase-onto-threedots.sh
+++ b/t/t3416-rebase-onto-threedots.sh
@@ -99,7 +99,64 @@ test_expect_success 'rebase -i --onto master...side' '
 	git checkout side &&
 	git reset --hard K &&
 
+	set_fake_editor &&
 	test_must_fail git rebase -i --onto master...side J
 '
 
+test_expect_success 'rebase --keep-base --onto incompatible' '
+	test_must_fail git rebase --keep-base --onto master...
+'
+
+test_expect_success 'rebase --keep-base --root incompatible' '
+	test_must_fail git rebase --keep-base --root
+'
+
+test_expect_success 'rebase --keep-base master from topic' '
+	git reset --hard &&
+	git checkout topic &&
+	git reset --hard G &&
+
+	git rebase --keep-base master &&
+	git rev-parse C >base.expect &&
+	git merge-base master HEAD >base.actual &&
+	test_cmp base.expect base.actual &&
+
+	git rev-parse HEAD~2 >actual &&
+	git rev-parse C^0 >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase --keep-base master from side' '
+	git reset --hard &&
+	git checkout side &&
+	git reset --hard K &&
+
+	test_must_fail git rebase --keep-base master
+'
+
+test_expect_success 'rebase -i --keep-base master from topic' '
+	git reset --hard &&
+	git checkout topic &&
+	git reset --hard G &&
+
+	set_fake_editor &&
+	EXPECT_COUNT=2 git rebase -i --keep-base master &&
+	git rev-parse C >base.expect &&
+	git merge-base master HEAD >base.actual &&
+	test_cmp base.expect base.actual &&
+
+	git rev-parse HEAD~2 >actual &&
+	git rev-parse C^0 >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase -i --keep-base master from side' '
+	git reset --hard &&
+	git checkout side &&
+	git reset --hard K &&
+
+	set_fake_editor &&
+	test_must_fail git rebase -i --keep-base master
+'
+
 test_done
diff --git a/t/t3421-rebase-topology-linear.sh b/t/t3421-rebase-topology-linear.sh
index 7274dca40b..b847064f91 100755
--- a/t/t3421-rebase-topology-linear.sh
+++ b/t/t3421-rebase-topology-linear.sh
@@ -31,6 +31,16 @@ test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase success -p
 
+test_expect_success 'setup branches and remote tracking' '
+	git tag -l >tags &&
+	for tag in $(cat tags)
+	do
+		git branch branch-$tag $tag || return 1
+	done &&
+	git remote add origin "file://$PWD" &&
+	git fetch origin
+'
+
 test_run_rebase () {
 	result=$1
 	shift
@@ -57,6 +67,7 @@ test_run_rebase () {
 	"
 }
 test_run_rebase success ''
+test_run_rebase success --fork-point
 test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase failure -p
@@ -64,6 +75,23 @@ test_have_prereq !REBASE_P || test_run_rebase failure -p
 test_run_rebase () {
 	result=$1
 	shift
+	test_expect_$result "rebase $* -f rewrites even if remote upstream is an ancestor" "
+		reset_rebase &&
+		git rebase $* -f branch-b branch-e &&
+		! test_cmp_rev branch-e origin/branch-e &&
+		test_cmp_rev branch-b HEAD~2 &&
+		test_linear_range 'd e' branch-b..
+	"
+}
+test_run_rebase success ''
+test_run_rebase success --fork-point
+test_run_rebase success -m
+test_run_rebase success -i
+test_have_prereq !REBASE_P || test_run_rebase success -p
+
+test_run_rebase () {
+	result=$1
+	shift
 	test_expect_$result "rebase $* fast-forwards from ancestor of upstream" "
 		reset_rebase &&
 		git rebase $* e b &&
@@ -71,6 +99,7 @@ test_run_rebase () {
 	"
 }
 test_run_rebase success ''
+test_run_rebase success --fork-point
 test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase success -p
diff --git a/t/t3431-rebase-fork-point.sh b/t/t3431-rebase-fork-point.sh
new file mode 100755
index 0000000000..78851b9a2a
--- /dev/null
+++ b/t/t3431-rebase-fork-point.sh
@@ -0,0 +1,57 @@
+#!/bin/sh
+#
+# Copyright (c) 2019 Denton Liu
+#
+
+test_description='git rebase --fork-point test'
+
+. ./test-lib.sh
+
+# A---B---D---E    (master)
+#      \
+#       C*---F---G (side)
+#
+# C was formerly part of master but master was rewound to remove C
+#
+test_expect_success setup '
+	test_commit A &&
+	test_commit B &&
+	test_commit C &&
+	git branch -t side &&
+	git reset --hard HEAD^ &&
+	test_commit D &&
+	test_commit E &&
+	git checkout side &&
+	test_commit F &&
+	test_commit G
+'
+
+test_rebase () {
+	expected="$1" &&
+	shift &&
+	test_expect_success "git rebase $*" "
+		git checkout master &&
+		git reset --hard E &&
+		git checkout side &&
+		git reset --hard G &&
+		git rebase $* &&
+		test_write_lines $expected >expect &&
+		git log --pretty=%s >actual &&
+		test_cmp expect actual
+	"
+}
+
+test_rebase 'G F E D B A'
+test_rebase 'G F D B A' --onto D
+test_rebase 'G F B A' --keep-base
+test_rebase 'G F C E D B A' --no-fork-point
+test_rebase 'G F C D B A' --no-fork-point --onto D
+test_rebase 'G F C B A' --no-fork-point --keep-base
+test_rebase 'G F E D B A' --fork-point refs/heads/master
+test_rebase 'G F D B A' --fork-point --onto D refs/heads/master
+test_rebase 'G F B A' --fork-point --keep-base refs/heads/master
+test_rebase 'G F C E D B A' refs/heads/master
+test_rebase 'G F C D B A' --onto D refs/heads/master
+test_rebase 'G F C B A' --keep-base refs/heads/master
+
+test_done
diff --git a/t/t3432-rebase-fast-forward.sh b/t/t3432-rebase-fast-forward.sh
new file mode 100755
index 0000000000..034ffc7e76
--- /dev/null
+++ b/t/t3432-rebase-fast-forward.sh
@@ -0,0 +1,125 @@
+#!/bin/sh
+#
+# Copyright (c) 2019 Denton Liu
+#
+
+test_description='ensure rebase fast-forwards commits when possible'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	test_commit A &&
+	test_commit B &&
+	test_commit C &&
+	test_commit D &&
+	git checkout -t -b side
+'
+
+test_rebase_same_head () {
+	status_n="$1" &&
+	shift &&
+	what_n="$1" &&
+	shift &&
+	cmp_n="$1" &&
+	shift &&
+	status_f="$1" &&
+	shift &&
+	what_f="$1" &&
+	shift &&
+	cmp_f="$1" &&
+	shift &&
+	test_rebase_same_head_ $status_n $what_n $cmp_n "" "$*" &&
+	test_rebase_same_head_ $status_f $what_f $cmp_f " --no-ff" "$*"
+}
+
+test_rebase_same_head_ () {
+	status="$1" &&
+	shift &&
+	what="$1" &&
+	shift &&
+	cmp="$1" &&
+	shift &&
+	flag="$1"
+	shift &&
+	test_expect_$status "git rebase$flag $* with $changes is $what with $cmp HEAD" "
+		oldhead=\$(git rev-parse HEAD) &&
+		test_when_finished 'git reset --hard \$oldhead' &&
+		git rebase$flag $* >stdout &&
+		if test $what = work
+		then
+			# Must check this case first, for 'is up to
+			# date, rebase forced[...]rewinding head' cases
+			test_i18ngrep 'rewinding head' stdout
+		elif test $what = noop
+		then
+			test_i18ngrep 'is up to date' stdout &&
+			test_i18ngrep ! 'rebase forced' stdout
+		elif test $what = noop-force
+		then
+			test_i18ngrep 'is up to date, rebase forced' stdout
+		fi &&
+		newhead=\$(git rev-parse HEAD) &&
+		if test $cmp = same
+		then
+			test_cmp_rev \$oldhead \$newhead
+		elif test $cmp = diff
+		then
+			! test_cmp_rev \$oldhead \$newhead
+		fi
+	"
+}
+
+changes='no changes'
+test_rebase_same_head success noop same success work same
+test_rebase_same_head success noop same success noop-force same master
+test_rebase_same_head success noop same success noop-force diff --onto B B
+test_rebase_same_head success noop same success noop-force diff --onto B... B
+test_rebase_same_head success noop same success noop-force same --onto master... master
+test_rebase_same_head success noop same success noop-force same --keep-base master
+test_rebase_same_head success noop same success noop-force same --keep-base
+test_rebase_same_head success noop same success noop-force same --no-fork-point
+test_rebase_same_head success noop same success noop-force same --keep-base --no-fork-point
+test_rebase_same_head success noop same success work same --fork-point master
+test_rebase_same_head success noop same success work diff --fork-point --onto B B
+test_rebase_same_head success noop same success work diff --fork-point --onto B... B
+test_rebase_same_head success noop same success work same --fork-point --onto master... master
+test_rebase_same_head success noop same success work same --keep-base --keep-base master
+
+test_expect_success 'add work same to side' '
+	test_commit E
+'
+
+changes='our changes'
+test_rebase_same_head success noop same success work same
+test_rebase_same_head success noop same success noop-force same master
+test_rebase_same_head success noop same success noop-force diff --onto B B
+test_rebase_same_head success noop same success noop-force diff --onto B... B
+test_rebase_same_head success noop same success noop-force same --onto master... master
+test_rebase_same_head success noop same success noop-force same --keep-base master
+test_rebase_same_head success noop same success noop-force same --keep-base
+test_rebase_same_head success noop same success noop-force same --no-fork-point
+test_rebase_same_head success noop same success noop-force same --keep-base --no-fork-point
+test_rebase_same_head success noop same success work same --fork-point master
+test_rebase_same_head success noop same success work diff --fork-point --onto B B
+test_rebase_same_head success noop same success work diff --fork-point --onto B... B
+test_rebase_same_head success noop same success work same --fork-point --onto master... master
+test_rebase_same_head success noop same success work same --fork-point --keep-base master
+
+test_expect_success 'add work same to upstream' '
+	git checkout master &&
+	test_commit F &&
+	git checkout side
+'
+
+changes='our and their changes'
+test_rebase_same_head success noop same success noop-force diff --onto B B
+test_rebase_same_head success noop same success noop-force diff --onto B... B
+test_rebase_same_head success noop same success work diff --onto master... master
+test_rebase_same_head success noop same success work diff --keep-base master
+test_rebase_same_head success noop same success work diff --keep-base
+test_rebase_same_head failure work same success work diff --fork-point --onto B B
+test_rebase_same_head failure work same success work diff --fork-point --onto B... B
+test_rebase_same_head success noop same success work diff --fork-point --onto master... master
+test_rebase_same_head success noop same success work diff --fork-point --keep-base master
+
+test_done