Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
supported resuming from a failed patch application by skipping the
current patch. Re-implement this feature by introducing am_skip().

Signed-off-by: Paul Tan <pyoka...@gmail.com>
---
 builtin/am.c | 121 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 119 insertions(+), 2 deletions(-)

diff --git a/builtin/am.c b/builtin/am.c
index 4381164..0d7af19 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -16,6 +16,8 @@
 #include "lockfile.h"
 #include "diff.h"
 #include "diffcore.h"
+#include "unpack-trees.h"
+#include "branch.h"
 
 /**
  * Returns 1 if the file is empty or does not exist, 0 otherwise.
@@ -735,6 +737,114 @@ static void am_resolve(struct am_state *state)
 }
 
 /**
+ * Performs a checkout fast-forward from `head` to `remote`. If `reset` is
+ * true, any unmerged entries will be discarded. Returns 0 on success, -1 on
+ * failure.
+ */
+static int fast_forward_to(struct tree *head, struct tree *remote, int reset)
+{
+       struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+       struct unpack_trees_options opts;
+       struct tree_desc t[2];
+
+       if (parse_tree(head) || parse_tree(remote))
+               return -1;
+
+       hold_locked_index(lock_file, 1);
+
+       refresh_cache(REFRESH_QUIET);
+
+       memset(&opts, 0, sizeof(opts));
+       opts.head_idx = 1;
+       opts.src_index = &the_index;
+       opts.dst_index = &the_index;
+       opts.update = 1;
+       opts.merge = 1;
+       opts.reset = reset;
+       opts.fn = twoway_merge;
+       init_tree_desc(&t[0], head->buffer, head->size);
+       init_tree_desc(&t[1], remote->buffer, remote->size);
+
+       if (unpack_trees(2, t, &opts)) {
+               rollback_lock_file(lock_file);
+               return -1;
+       }
+
+       if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+               die(_("unable to write new index file"));
+
+       return 0;
+}
+
+/**
+ * Clean the index without touching entries that are not modified between
+ * `head` and `remote`.
+ */
+static int clean_index(const unsigned char *head, const unsigned char *remote)
+{
+       struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+       struct tree *head_tree, *remote_tree, *index_tree;
+       unsigned char index[GIT_SHA1_RAWSZ];
+       struct pathspec pathspec;
+
+       head_tree = parse_tree_indirect(head);
+       if (!head_tree)
+               return error(_("Could not parse object '%s'."), 
sha1_to_hex(head));
+
+       remote_tree = parse_tree_indirect(remote);
+       if (!remote_tree)
+               return error(_("Could not parse object '%s'."), 
sha1_to_hex(remote));
+
+       read_cache_unmerged();
+
+       if (fast_forward_to(head_tree, head_tree, 1))
+               return -1;
+
+       if (write_cache_as_tree(index, 0, NULL))
+               return -1;
+
+       index_tree = parse_tree_indirect(index);
+       if (!index_tree)
+               return error(_("Could not parse object '%s'."), 
sha1_to_hex(index));
+
+       if (fast_forward_to(index_tree, remote_tree, 0))
+               return -1;
+
+       memset(&pathspec, 0, sizeof(pathspec));
+
+       hold_locked_index(lock_file, 1);
+
+       if (read_tree(remote_tree, 0, &pathspec)) {
+               rollback_lock_file(lock_file);
+               return -1;
+       }
+
+       if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+               die(_("unable to write new index file"));
+
+       remove_branch_state();
+
+       return 0;
+}
+
+/**
+ * Resume the current am session by skipping the current patch.
+ */
+static void am_skip(struct am_state *state)
+{
+       unsigned char head[GIT_SHA1_RAWSZ];
+
+       if (get_sha1("HEAD", head))
+               hashcpy(head, EMPTY_TREE_SHA1_BIN);
+
+       if (clean_index(head, head))
+               die(_("failed to clean index"));
+
+       am_next(state);
+       am_run(state);
+}
+
+/**
  * parse_options() callback that validates and sets opt->value to the
  * PATCH_FORMAT_* enum value corresponding to `arg`.
  */
@@ -751,7 +861,8 @@ static int parse_opt_patchformat(const struct option *opt, 
const char *arg, int
 
 enum resume_mode {
        RESUME_FALSE = 0,
-       RESUME_RESOLVED
+       RESUME_RESOLVED,
+       RESUME_SKIP
 };
 
 static struct am_state state;
@@ -760,7 +871,7 @@ static enum resume_mode opt_resume;
 
 static const char * const am_usage[] = {
        N_("git am [options] [(<mbox>|<Maildir>)...]"),
-       N_("git am [options] --continue"),
+       N_("git am [options] (--continue | --skip)"),
        NULL
 };
 
@@ -773,6 +884,9 @@ static struct option am_options[] = {
        OPT_CMDMODE('r', "resolved", &opt_resume,
                N_("synonyms for --continue"),
                RESUME_RESOLVED),
+       OPT_CMDMODE(0, "skip", &opt_resume,
+               N_("skip the current patch"),
+               RESUME_SKIP),
        OPT_END()
 };
 
@@ -831,6 +945,9 @@ int cmd_am(int argc, const char **argv, const char *prefix)
        case RESUME_RESOLVED:
                am_resolve(&state);
                break;
+       case RESUME_SKIP:
+               am_skip(&state);
+               break;
        default:
                die("BUG: invalid resume value");
        }
-- 
2.1.4

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