Introduce CHERRY_PICK_HEAD

When a cherry-pick conflicts git advises:

 $ 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:

  $ git commit -c CHERRY_PICK_HEAD

A later patch teaches git to handle the '-c CHERRY_PICK_HEAD' part.
Note that we 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. However, we do not record CHERRY_PICK_HEAD when --no-commit
is used, as presumably the user intends to further edit the commit
and possibly even cherry-pick additional commits on top.

Tests and documentation contributed by Jonathan Nieder.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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>...::

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.

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"));

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"));

builtin/merge.c

@@ -969,6 +969,13 @@ 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)

builtin/revert.c

@@ -231,6 +231,22 @@ 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;
@@ -246,15 +262,20 @@ static void print_advice(void)
 
 	if (msg) {
 		fprintf(stderr, "%s\n", msg);
+		/*
+		 * A conflict has occured but the porcelain
+		 * (typically rebase --interactive) wants to take care
+		 * of the commit itself so remove CHERRY_PICK_HEAD
+		 */
+		unlink(git_path("CHERRY_PICK_HEAD"));
 		return;
 	}
 
 	advise("after resolving the conflicts, mark the corrected paths");
 	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)
@@ -489,6 +510,8 @@ static int do_pick_commit(void)
 			strbuf_addstr(&msgbuf, sha1_to_hex(commit->object.sha1));
 			strbuf_addstr(&msgbuf, ")\n");
 		}
+		if (!no_commit)
+			write_cherry_pick_head();
 	}
 
 	if (!strategy || !strcmp(strategy, "recursive") || action == REVERT) {

t/t3507-cherry-pick-conflict.sh

@@ -52,13 +52,87 @@ 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' '
+	pristine_detach initial &&
+	test_must_fail git cherry-pick picked &&
+	test_cmp_rev picked CHERRY_PICK_HEAD
+'
+
+test_expect_success 'successful cherry-pick does not set CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+	git cherry-pick base &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD
+'
+
+test_expect_success 'cherry-pick --no-commit does not set CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+	git cherry-pick --no-commit base &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD
+'
+
+test_expect_success 'GIT_CHERRY_PICK_HELP suppresses CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+	(
+		GIT_CHERRY_PICK_HELP="and then do something else" &&
+		export GIT_CHERRY_PICK_HELP &&
+		test_must_fail git cherry-pick picked
+	) &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD
+'
+
+test_expect_success 'git reset clears CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+
+	test_must_fail git cherry-pick picked &&
+	git reset &&
+
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD
+'
+
+test_expect_success 'failed commit does not clear CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+
+	test_must_fail git cherry-pick picked &&
+	test_must_fail git commit &&
+
+	test_cmp_rev picked CHERRY_PICK_HEAD
+'
+
+test_expect_success 'cancelled commit does not clear CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+
+	test_must_fail git cherry-pick picked &&
+	echo resolved >foo &&
+	git add foo &&
+	git update-index --refresh -q &&
+	test_must_fail git diff-index --exit-code HEAD &&
+	(
+		GIT_EDITOR=false &&
+		export GIT_EDITOR &&
+		test_must_fail git commit
+	) &&
+
+	test_cmp_rev picked CHERRY_PICK_HEAD
+'
+
+test_expect_success 'successful commit clears CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+
+	test_must_fail git cherry-pick picked &&
+	echo resolved >foo &&
+	git add foo &&
+	git commit &&
+
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD
+'
+
 test_expect_success 'failed cherry-pick produces dirty index' '
 	pristine_detach initial &&