checkout -f: allow ignoring unmerged paths when checking out of the index

Earlier we made "git checkout $pathspec" to atomically refuse
the operation of $pathspec matched any path with unmerged
stages.  This patch allows:

    $ git checkout -f a b c

to ignore, instead of error out on, such unmerged paths.  The
fix to prevent checkout of an unmerged path from random stages
is still there.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 59 insertions, 24 deletions

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 5aa69c0e1..15fdb08ce 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt
@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git checkout' [-q] [-f] [[--track | --no-track] -b <new_branch> [-l]] [-m] [<branch>]
-'git checkout' [<tree-ish>] [--] <paths>...
+'git checkout' [-f] [<tree-ish>] [--] <paths>...
 
 DESCRIPTION
 -----------
@@ -23,14 +23,17 @@ options, which will be passed to `git branch`.
 
 When <paths> are given, this command does *not* switch
 branches.  It updates the named paths in the working tree from
-the index file (i.e. it runs `git checkout-index -f -u`), or
-from a named commit.  In
-this case, the `-f` and `-b` options are meaningless and giving
+the index file, or from a named commit.  In
+this case, the `-b` options is meaningless and giving
 either of them results in an error.  <tree-ish> argument can be
 used to specify a specific tree-ish (i.e. commit, tag or tree)
 to update the index for the given paths before updating the
 working tree.
 
+The index may contain unmerged entries after a failed merge.  By
+default, if you try to check out such an entry from the index, the
+checkout operation will fail and nothing will be checked out.
+Using -f will ignore these unmerged entries.
 
 OPTIONS
 -------
@@ -38,8 +41,12 @@ OPTIONS
 	Quiet, suppress feedback messages.
 
 -f::
-	Proceed even if the index or the working tree differs
-	from HEAD.  This is used to throw away local changes.
+	When switching branches, proceed even if the index or the
+	working tree differs from HEAD.  This is used to throw away
+	local changes.
++
+When checking out paths from the index, do not fail upon unmerged
+entries; instead, unmerged entries are ignored.
 
 -b::
 	Create a new branch named <new_branch> and start it at
diff --git a/builtin-checkout.c b/builtin-checkout.c
index 854401099..1303f3b5b 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -20,6 +20,17 @@ static const char * const checkout_usage[] = {
 	NULL,
 };
 
+struct checkout_opts {
+	int quiet;
+	int merge;
+	int force;
+	int writeout_error;
+
+	const char *new_branch;
+	int new_branch_log;
+	enum branch_track track;
+};
+
 static int post_checkout_hook(struct commit *old, struct commit *new,
 			      int changed)
 {
@@ -85,7 +96,8 @@ static int skip_same_name(struct cache_entry *ce, int pos)
 }
 
 
-static int checkout_paths(struct tree *source_tree, const char **pathspec)
+static int checkout_paths(struct tree *source_tree, const char **pathspec,
+			  struct checkout_opts *opts)
 {
 	int pos;
 	struct checkout state;
@@ -122,8 +134,12 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec)
 		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
 			if (!ce_stage(ce))
 				continue;
-			errs = 1;
-			error("path '%s' is unmerged", ce->name);
+			if (opts->force) {
+				warning("path '%s' is unmerged", ce->name);
+			} else {
+				errs = 1;
+				error("path '%s' is unmerged", ce->name);
+			}
 			pos = skip_same_name(ce, pos) - 1;
 		}
 	}
@@ -178,17 +194,6 @@ static void describe_detached_head(char *msg, struct commit *commit)
 	strbuf_release(&sb);
 }
 
-struct checkout_opts {
-	int quiet;
-	int merge;
-	int force;
-	int writeout_error;
-
-	char *new_branch;
-	int new_branch_log;
-	enum branch_track track;
-};
-
 static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
 {
 	struct unpack_trees_options opts;
@@ -554,15 +559,15 @@ no_reference:
 			die("invalid path specification");
 
 		/* Checkout paths */
-		if (opts.new_branch || opts.force || opts.merge) {
+		if (opts.new_branch || opts.merge) {
 			if (argc == 1) {
-				die("git checkout: updating paths is incompatible with switching branches/forcing\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
+				die("git checkout: updating paths is incompatible with switching branches.\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
 			} else {
-				die("git checkout: updating paths is incompatible with switching branches/forcing");
+				die("git checkout: updating paths is incompatible with switching branches.");
 			}
 		}
 
-		return checkout_paths(source_tree, pathspec);
+		return checkout_paths(source_tree, pathspec, &opts);
 	}
 
 	if (new.name && !new.commit) {
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index 83a366f1e..88692f987 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -359,4 +359,27 @@ test_expect_success 'checkout an unmerged path should fail' '
 	test_cmp sample file
 '
 
+test_expect_success 'checkout with an unmerged path can be ignored' '
+	rm -f .git/index &&
+	O=$(echo original | git hash-object -w --stdin) &&
+	A=$(echo ourside | git hash-object -w --stdin) &&
+	B=$(echo theirside | git hash-object -w --stdin) &&
+	(
+		echo "100644 $A 0	fild" &&
+		echo "100644 $O 1	file" &&
+		echo "100644 $A 2	file" &&
+		echo "100644 $B 3	file" &&
+		echo "100644 $A 0	filf"
+	) | git update-index --index-info &&
+	echo "none of the above" >sample &&
+	echo ourside >expect &&
+	cat sample >fild &&
+	cat sample >file &&
+	cat sample >filf &&
+	git checkout -f fild file filf &&
+	test_cmp expect fild &&
+	test_cmp expect filf &&
+	test_cmp sample file
+'
+
 test_done