[PATCH v3 3/4] reset: add option "--merge-safe" to "git reset"

[Christian Couder <chriscool@tuxfamily.org>]

From: Stephan Beyer <s-beyer@gmx.net>

This option is nearly like "--merge" except that it is
safer.

The table below shows what happens when running
"git reset --option target" to reset the HEAD to another
commit (as a special case "target" could be the same as
HEAD) in the cases where "--merge" and "--merge-safe"
(abreviated --m-s) behave differently.

working index HEAD target         working index HEAD
  B      B     A     A   --m-s      B      A     A
                         --merge    A      A     A
  B      B     A     C   --m-s       (disallowed)
                         --merge    C      C     C

In this table, A, B and C are some different states of
a file. For example the first line of the table means
that if a file is in state B in the working tree and
the index, and in a different state A in HEAD and in
the target, then "git reset --merge-safe target" will
put the file in state B in the working tree and in
state A in the index and HEAD.

So as can be seen in the table, "--merge" discards changes
in the index, while "--merge-safe" does not.

A following patch will add some test cases for
"--merge-safe", where the differences between "--merge"
and "--merge-safe" can also be seen.

The "--merge-safe" option is implemented by doing a 2 way
merge between HEAD and the reset target, and if this
succeeds by doing a mixed reset to the target.

The code comes from the sequencer GSoC project:

git://repo.or.cz/git/sbeyer.git

(at commit 5a78908b70ceb5a4ea9fd4b82f07ceba1f019079)

But in the sequencer project the "reset" flag was set
in the "struct unpack_trees_options" passed to
"unpack_trees()". With this flag the changes in the
working tree were discarded if the file was different
between HEAD and the reset target.

Mentored-by: Daniel Barkalow <barkalow@iabervon.org>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 builtin-reset.c |   30 +++++++++++++++++++++++++-----
 1 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/builtin-reset.c b/builtin-reset.c
index ddb81f3..78d42e6 100644
--- a/builtin-reset.c
+++ b/builtin-reset.c
@@ -22,13 +22,15 @@
 #include "cache-tree.h"
 
 static const char * const git_reset_usage[] = {
-	"git reset [--mixed | --soft | --hard | --merge] [-q] [<commit>]",
+	"git reset [--mixed | --soft | --hard | --merge | --merge-safe] [-q] [<commit>]",
 	"git reset [--mixed] <commit> [--] <paths>...",
 	NULL
 };
 
-enum reset_type { MIXED, SOFT, HARD, MERGE, NONE };
-static const char *reset_type_names[] = { "mixed", "soft", "hard", "merge", NULL };
+enum reset_type { MIXED, SOFT, HARD, MERGE, MERGE_SAFE, NONE };
+static const char *reset_type_names[] = {
+	"mixed", "soft", "hard", "merge", "merge_safe", NULL
+};
 
 static char *args_to_str(const char **argv)
 {
@@ -81,6 +83,7 @@ static int reset_index_file(const unsigned char *sha1, int reset_type, int quiet
 	if (!quiet)
 		opts.verbose_update = 1;
 	switch (reset_type) {
+	case MERGE_SAFE:
 	case MERGE:
 		opts.update = 1;
 		break;
@@ -95,6 +98,16 @@ static int reset_index_file(const unsigned char *sha1, int reset_type, int quiet
 
 	read_cache_unmerged();
 
+	if (reset_type == MERGE_SAFE) {
+		unsigned char *head_sha1;
+		if (get_sha1("HEAD", head_sha1))
+			return error("You do not have a valid HEAD.");
+		if (parse_and_init_tree_desc(head_sha1, desc))
+			return error("Failed to find tree of HEAD.");
+		nr++;
+		opts.fn = twoway_merge;
+	}
+
 	if (parse_and_init_tree_desc(sha1, desc + nr - 1))
 		return error("Failed to find tree of %s.", sha1_to_hex(sha1));
 	if (unpack_trees(nr, desc, &opts))
@@ -238,6 +251,9 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 				"reset HEAD, index and working tree", HARD),
 		OPT_SET_INT(0, "merge", &reset_type,
 				"reset HEAD, index and working tree", MERGE),
+		OPT_SET_INT(0, "merge-safe", &reset_type,
+				"reset HEAD, index and working tree",
+				MERGE_SAFE),
 		OPT_BOOLEAN('q', NULL, &quiet,
 				"disable showing new HEAD in hard reset and progress message"),
 		OPT_BOOLEAN('p', "patch", &patch_mode, "select hunks interactively"),
@@ -324,9 +340,13 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (reset_type == SOFT) {
 		if (is_merge() || read_cache() < 0 || unmerged_cache())
 			die("Cannot do a soft reset in the middle of a merge.");
+	} else {
+		int err = reset_index_file(sha1, reset_type, quiet);
+		if (reset_type == MERGE_SAFE)
+			err = err || reset_index_file(sha1, MIXED, quiet);
+		if (err)
+			die("Could not reset index file to revision '%s'.", rev);
 	}
-	else if (reset_index_file(sha1, reset_type, quiet))
-		die("Could not reset index file to revision '%s'.", rev);
 
 	/* Any resets update HEAD to the head being switched to,
 	 * saving the previous head in ORIG_HEAD before. */
-- 
1.6.5.rc0.150.g38fe6