[PATCH 1/2] Introduce CHERRY_PICK_HEAD

[Jay Soffian <jaysoffian@gmail.com>]

When a cherry-pick conflicts git advises to use:

 $ git commit -c <original commit id>

to preserve the original commit message and authorship. Instead, let's
record the original commit id in CHERRY_PICK_HEAD and advise to use:

  $ git commit -c CHERRY_PICK_HEAD

In the next commit, we teach git to handle the '-c CHERRY_PICK_HEAD'
part. Note that wWe record CHERRY_PICK_HEAD even in the case where there
are no conflicts so that we may use it to communicate authorship to
commit; this will then allow us to remove set_author_ident_env from
revert.c.

Contributions-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
---
 Documentation/git-cherry-pick.txt |   19 +++++++++++++++++++
 Documentation/revisions.txt       |    5 ++++-
 branch.c                          |    1 +
 builtin/commit.c                  |    1 +
 builtin/merge.c                   |    7 +++++++
 builtin/revert.c                  |   20 ++++++++++++++++++--
 t/t3507-cherry-pick-conflict.sh   |   22 +++++++++++++++++++++-
 7 files changed, 71 insertions(+), 4 deletions(-)

diff --git a/Documentation/git-cherry-pick.txt b/Documentation/git-cherry-pick.txt
index 749d68a..5d85daa 100644
--- a/Documentation/git-cherry-pick.txt
+++ b/Documentation/git-cherry-pick.txt
@@ -16,6 +16,25 @@ Given one or more existing commits, apply the change each one
 introduces, recording a new commit for each.  This requires your
 working tree to be clean (no modifications from the HEAD commit).
 
+When it is not obvious how to apply a change, the following
+happens:
+
+1. The current branch and `HEAD` pointer stay at the last commit
+   successfully made.
+2. The `CHERRY_PICK_HEAD` ref is set to point at the commit that
+   introduced the change that is difficult to apply.
+3. Paths in which the change applied cleanly are updated both
+   in the index file and in your working tree.
+4. For conflicting paths, the index file records up to three
+   versions, as described in the "TRUE MERGE" section of
+   linkgit:git-merge[1].  The working tree files will include
+   a description of the conflict bracketed by the usual
+   conflict markers `<<<<<<<` and `>>>>>>>`.
+5. No other modifications are made.
+
+See linkgit:git-merge[1] for some hints on resolving such
+conflicts.
+
 OPTIONS
 -------
 <commit>...::
diff --git a/Documentation/revisions.txt b/Documentation/revisions.txt
index 9e92734..04fceee 100644
--- a/Documentation/revisions.txt
+++ b/Documentation/revisions.txt
@@ -25,7 +25,8 @@ blobs contained in a commit.
   first match in the following rules:
 
   . if `$GIT_DIR/<name>` exists, that is what you mean (this is usually
-    useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD` and `MERGE_HEAD`);
+    useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD`, `MERGE_HEAD`
+    and `CHERRY_PICK_HEAD`);
 
   . otherwise, `refs/<name>` if exists;
 
@@ -46,6 +47,8 @@ you can change the tip of the branch back to the state before you ran
 them easily.
 MERGE_HEAD records the commit(s) you are merging into your branch
 when you run 'git merge'.
+CHERRY_PICK_HEAD records the commit you are cherry-picking
+when you run 'git cherry-pick'.
 +
 Note that any of the `refs/*` cases above may come either from
 the `$GIT_DIR/refs` directory or from the `$GIT_DIR/packed-refs` file.
diff --git a/branch.c b/branch.c
index 93dc866..dc23e95 100644
--- a/branch.c
+++ b/branch.c
@@ -217,6 +217,7 @@ void create_branch(const char *head,
 
 void remove_branch_state(void)
 {
+	unlink(git_path("CHERRY_PICK_HEAD"));
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_RR"));
 	unlink(git_path("MERGE_MSG"));
diff --git a/builtin/commit.c b/builtin/commit.c
index 03cff5a..0def540 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1424,6 +1424,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		die("cannot update HEAD ref");
 	}
 
+	unlink(git_path("CHERRY_PICK_HEAD"));
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_MSG"));
 	unlink(git_path("MERGE_MODE"));
diff --git a/builtin/merge.c b/builtin/merge.c
index 9403747..454dad2 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -971,6 +971,13 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		else
 			die("You have not concluded your merge (MERGE_HEAD exists).");
 	}
+	if (file_exists(git_path("CHERRY_PICK_HEAD"))) {
+		if (advice_resolve_conflict)
+			die("You have not concluded your cherry-pick (CHERRY_PICK_HEAD exists).\n"
+			    "Please, commit your changes before you can merge.");
+		else
+			die("You have not concluded your cherry-pick (CHERRY_PICK_HEAD exists).");
+	}
 	resolve_undo_clear();
 
 	if (verbosity < 0)
diff --git a/builtin/revert.c b/builtin/revert.c
index dc1b702..cced2e4 100644
--- a/builtin/revert.c
+++ b/builtin/revert.c
@@ -248,6 +248,22 @@ static void set_author_ident_env(const char *message)
 			sha1_to_hex(commit->object.sha1));
 }
 
+static void write_cherry_pick_head(void)
+{
+	int fd;
+	struct strbuf buf = STRBUF_INIT;
+
+	strbuf_addf(&buf, "%s\n", sha1_to_hex(commit->object.sha1));
+
+	fd = open(git_path("CHERRY_PICK_HEAD"), O_WRONLY | O_CREAT, 0666);
+	if (fd < 0)
+		die_errno("Could not open '%s' for writing",
+			  git_path("CHERRY_PICK_HEAD"));
+	if (write_in_full(fd, buf.buf, buf.len) != buf.len || close(fd))
+		die_errno("Could not write to '%s'", git_path("CHERRY_PICK_HEAD"));
+	strbuf_release(&buf);
+}
+
 static void advise(const char *advice, ...)
 {
 	va_list params;
@@ -270,8 +286,7 @@ static void print_advice(void)
 	advise("with 'git add <paths>' or 'git rm <paths>'");
 
 	if (action == CHERRY_PICK)
-		advise("and commit the result with 'git commit -c %s'",
-		       find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
+		advise("and commit the result with 'git commit -c CHERRY_PICK_HEAD'");
 }
 
 static void write_message(struct strbuf *msgbuf, const char *filename)
@@ -504,6 +519,7 @@ static int do_pick_commit(void)
 			strbuf_addstr(&msgbuf, sha1_to_hex(commit->object.sha1));
 			strbuf_addstr(&msgbuf, ")\n");
 		}
+		write_cherry_pick_head();
 	}
 
 	if (!strategy || !strcmp(strategy, "recursive") || action == REVERT) {
diff --git a/t/t3507-cherry-pick-conflict.sh b/t/t3507-cherry-pick-conflict.sh
index 607bf25..fd569c8 100755
--- a/t/t3507-cherry-pick-conflict.sh
+++ b/t/t3507-cherry-pick-conflict.sh
@@ -11,6 +11,12 @@ test_description='test cherry-pick and revert with conflicts
 
 . ./test-lib.sh
 
+test_cmp_rev () {
+	git rev-parse --verify "$1" >expect.rev &&
+	git rev-parse --verify "$2" >actual.rev &&
+	test_cmp expect.rev actual.rev
+}
+
 test_expect_success setup '
 
 	echo unrelated >unrelated &&
@@ -51,13 +57,27 @@ test_expect_success 'advice from failed cherry-pick' "
 	error: could not apply \$picked... picked
 	hint: after resolving the conflicts, mark the corrected paths
 	hint: with 'git add <paths>' or 'git rm <paths>'
-	hint: and commit the result with 'git commit -c \$picked'
+	hint: and commit the result with 'git commit -c CHERRY_PICK_HEAD'
 	EOF
 	test_must_fail git cherry-pick picked 2>actual &&
 
 	test_cmp expected actual
 "
 
+test_expect_success 'failed cherry-pick sets CHERRY_PICK_HEAD' '
+
+	git checkout -f initial^0 &&
+	git read-tree -u --reset HEAD &&
+	git clean -d -f -f -q -x &&
+
+	git update-index --refresh &&
+	git diff-index --exit-code HEAD &&
+
+	test_must_fail git cherry-pick picked &&
+
+	test_cmp_rev picked CHERRY_PICK_HEAD
+'
+
 test_expect_success 'failed cherry-pick produces dirty index' '
 
 	git checkout -f initial^0 &&
-- 
1.7.4.1.28.gd46b3