[PATCH] Rewrite read-tree
[gitweb.git] / read-tree.c
index b72ee1c54c092067da0bc35492ae9dead5e990bb..0241c9c6f55c2345d5604ddbb93baee53d11ea9a 100644 (file)
  */
 #include "cache.h"
 
-static int stage = 0;
+#include "object.h"
+#include "tree.h"
+
+static int merge = 0;
 static int update = 0;
 
-static int unpack_tree(unsigned char *sha1)
-{
-       void *buffer;
-       unsigned long size;
-       int ret;
+static int head_idx = -1;
+static int merge_size = 0;
 
-       buffer = read_object_with_reference(sha1, "tree", &size, NULL);
-       if (!buffer)
-               return -1;
-       ret = read_tree(buffer, size, stage);
-       free(buffer);
-       return ret;
-}
+static struct object_list *trees = NULL;
+
+static struct cache_entry df_conflict_entry = { 
+};
+
+static struct tree_entry_list df_conflict_list = {
+       .name = NULL,
+       .next = &df_conflict_list
+};
 
-static char *lockfile_name;
+typedef int (*merge_fn_t)(struct cache_entry **src);
 
-static void remove_lock_file(void)
+static int entcmp(char *name1, int dir1, char *name2, int dir2)
 {
-       if (lockfile_name)
-               unlink(lockfile_name);
+       int len1 = strlen(name1);
+       int len2 = strlen(name2);
+       int len = len1 < len2 ? len1 : len2;
+       int ret = memcmp(name1, name2, len);
+       unsigned char c1, c2;
+       if (ret)
+               return ret;
+       c1 = name1[len];
+       c2 = name2[len];
+       if (!c1 && dir1)
+               c1 = '/';
+       if (!c2 && dir2)
+               c2 = '/';
+       ret = (c1 < c2) ? -1 : (c1 > c2) ? 1 : 0;
+       if (c1 && c2 && !ret)
+               ret = len1 - len2;
+       return ret;
 }
 
-static int path_matches(struct cache_entry *a, struct cache_entry *b)
+static int unpack_trees_rec(struct tree_entry_list **posns, int len,
+                           const char *base, merge_fn_t fn, int *indpos)
 {
-       int len = ce_namelen(a);
-       return ce_namelen(b) == len &&
-               !memcmp(a->name, b->name, len);
+       int baselen = strlen(base);
+       int src_size = len + 1;
+       do {
+               int i;
+               char *first;
+               int firstdir = 0;
+               int pathlen;
+               unsigned ce_size;
+               struct tree_entry_list **subposns;
+               struct cache_entry **src;
+               int any_files = 0;
+               int any_dirs = 0;
+               char *cache_name;
+               int ce_stage;
+
+               /* Find the first name in the input. */
+
+               first = NULL;
+               cache_name = NULL;
+
+               /* Check the cache */
+               if (merge && *indpos < active_nr) {
+                       /* This is a bit tricky: */
+                       /* If the index has a subdirectory (with
+                        * contents) as the first name, it'll get a
+                        * filename like "foo/bar". But that's after
+                        * "foo", so the entry in trees will get
+                        * handled first, at which point we'll go into
+                        * "foo", and deal with "bar" from the index,
+                        * because the base will be "foo/". The only
+                        * way we can actually have "foo/bar" first of
+                        * all the things is if the trees don't
+                        * contain "foo" at all, in which case we'll
+                        * handle "foo/bar" without going into the
+                        * directory, but that's fine (and will return
+                        * an error anyway, with the added unknown
+                        * file case.
+                        */
+
+                       cache_name = active_cache[*indpos]->name;
+                       if (strlen(cache_name) > baselen &&
+                           !memcmp(cache_name, base, baselen)) {
+                               cache_name += baselen;
+                               first = cache_name;
+                       } else {
+                               cache_name = NULL;
+                       }
+               }
+
+               if (first)
+                       printf("index %s\n", first);
+
+               for (i = 0; i < len; i++) {
+                       if (!posns[i] || posns[i] == &df_conflict_list)
+                               continue;
+                       printf("%d %s\n", i + 1, posns[i]->name);
+                       if (!first || entcmp(first, firstdir,
+                                            posns[i]->name, 
+                                            posns[i]->directory) > 0) {
+                               first = posns[i]->name;
+                               firstdir = posns[i]->directory;
+                       }
+               }
+               /* No name means we're done */
+               if (!first)
+                       return 0;
+
+               pathlen = strlen(first);
+               ce_size = cache_entry_size(baselen + pathlen);
+
+               src = xmalloc(sizeof(struct cache_entry *) * src_size);
+               memset(src, 0, sizeof(struct cache_entry *) * src_size);
+
+               subposns = xmalloc(sizeof(struct tree_list_entry *) * len);
+               memset(subposns, 0, sizeof(struct tree_list_entry *) * len);
+
+               if (cache_name && !strcmp(cache_name, first)) {
+                       any_files = 1;
+                       src[0] = active_cache[*indpos];
+                       remove_cache_entry_at(*indpos);
+               }
+
+               for (i = 0; i < len; i++) {
+                       struct cache_entry *ce;
+
+                       if (!posns[i] ||
+                           (posns[i] != &df_conflict_list &&
+                            strcmp(first, posns[i]->name))) {
+                               continue;
+                       }
+
+                       if (posns[i] == &df_conflict_list) {
+                               src[i + merge] = &df_conflict_entry;
+                               continue;
+                       }
+
+                       if (posns[i]->directory) {
+                               any_dirs = 1;
+                               parse_tree(posns[i]->item.tree);
+                               subposns[i] = posns[i]->item.tree->entries;
+                               posns[i] = posns[i]->next;
+                               src[i + merge] = &df_conflict_entry;
+                               continue;
+                       }
+
+                       if (!merge)
+                               ce_stage = 0;
+                       else if (i + 1 < head_idx)
+                               ce_stage = 1;
+                       else if (i + 1 > head_idx)
+                               ce_stage = 3;
+                       else
+                               ce_stage = 2;
+
+                       ce = xmalloc(ce_size);
+                       memset(ce, 0, ce_size);
+                       ce->ce_mode = create_ce_mode(posns[i]->mode);
+                       ce->ce_flags = create_ce_flags(baselen + pathlen,
+                                                      ce_stage);
+                       memcpy(ce->name, base, baselen);
+                       memcpy(ce->name + baselen, first, pathlen + 1);
+
+                       any_files = 1;
+
+                       memcpy(ce->sha1, posns[i]->item.any->sha1, 20);
+                       src[i + merge] = ce;
+                       subposns[i] = &df_conflict_list;
+                       posns[i] = posns[i]->next;
+               }
+               if (any_files) {
+                       if (merge) {
+                               int ret;
+
+                               printf("%s:\n", first);
+                               for (i = 0; i < src_size; i++) {
+                                       printf(" %d ", i);
+                                       if (src[i])
+                                               printf("%s\n", sha1_to_hex(src[i]->sha1));
+                                       else
+                                               printf("\n");
+                               }
+
+                               ret = fn(src);
+                               
+                               printf("Added %d entries\n", ret);
+
+                               *indpos += ret;
+                       } else {
+                               for (i = 0; i < src_size; i++) {
+                                       if (src[i]) {
+                                               add_cache_entry(src[i], ADD_CACHE_OK_TO_ADD|ADD_CACHE_SKIP_DFCHECK);
+                                       }
+                               }
+                       }
+               }
+               if (any_dirs) {
+                       char *newbase = xmalloc(baselen + 2 + pathlen);
+                       memcpy(newbase, base, baselen);
+                       memcpy(newbase + baselen, first, pathlen);
+                       newbase[baselen + pathlen] = '/';
+                       newbase[baselen + pathlen + 1] = '\0';
+                       if (unpack_trees_rec(subposns, len, newbase, fn,
+                                            indpos))
+                               return -1;
+               }
+               free(subposns);
+               free(src);
+       } while (1);
 }
 
-static int same(struct cache_entry *a, struct cache_entry *b)
+static void reject_merge(struct cache_entry *ce)
 {
-       return a->ce_mode == b->ce_mode && 
-               !memcmp(a->sha1, b->sha1, 20);
+       die("Entry '%s' would be overwritten by merge. Cannot merge.", 
+           ce->name);
 }
 
+static void check_updates(struct cache_entry **src, int nr)
+{
+       static struct checkout state = {
+               .base_dir = "",
+               .force = 1,
+               .quiet = 1,
+               .refresh_cache = 1,
+       };
+       unsigned short mask = htons(CE_UPDATE);
+       while (nr--) {
+               struct cache_entry *ce = *src++;
+               if (!ce->ce_mode) {
+                       if (update)
+                               unlink(ce->name);
+                       continue;
+               }
+               if (ce->ce_flags & mask) {
+                       ce->ce_flags &= ~mask;
+                       if (update)
+                               checkout_entry(ce, &state);
+               }
+       }
+}
 
-/*
- * This removes all trivial merges that don't change the tree
- * and collapses them to state 0.
- *
- * _Any_ other merge is left to user policy.  That includes "both
- * created the same file", and "both removed the same file" - which are
- * trivial, but the user might still want to _note_ it. 
- */
-static struct cache_entry *merge_entries(struct cache_entry *a,
-                                        struct cache_entry *b,
-                                        struct cache_entry *c)
+static int unpack_trees(merge_fn_t fn)
 {
-       int len = ce_namelen(a);
+       int indpos = 0;
+       unsigned len = object_list_length(trees);
+       struct tree_entry_list **posns = 
+               xmalloc(len * sizeof(struct tree_entry_list *));
+       int i;
+       struct object_list *posn = trees;
+       merge_size = len;
+       for (i = 0; i < len; i++) {
+               posns[i] = ((struct tree *) posn->item)->entries;
+               posn = posn->next;
+       }
+       if (unpack_trees_rec(posns, len, "", fn, &indpos))
+               return -1;
 
-       /*
-        * Are they all the same filename? We won't do
-        * any name merging
-        */
-       if (ce_namelen(b) != len ||
-           ce_namelen(c) != len ||
-           memcmp(a->name, b->name, len) ||
-           memcmp(a->name, c->name, len))
-               return NULL;
+       check_updates(active_cache, active_nr);
+       return 0;
+}
 
-       /*
-        * Ok, all three entries describe the same
-        * filename, but maybe the contents or file
-        * mode have changed?
-        *
-        * The trivial cases end up being the ones where two
-        * out of three files are the same:
-        *  - both destinations the same, trivially take either
-        *  - one of the destination versions hasn't changed,
-        *    take the other.
-        *
-        * The "all entries exactly the same" case falls out as
-        * a special case of any of the "two same" cases.
-        *
-        * Here "a" is "original", and "b" and "c" are the two
-        * trees we are merging.
-        */
-       if (same(b,c))
-               return c;
-       if (same(a,b))
-               return c;
-       if (same(a,c))
-               return b;
-       return NULL;
+static int list_tree(unsigned char *sha1)
+{
+       struct tree *tree = parse_tree_indirect(sha1);
+       if (!tree)
+               return -1;
+       object_list_append(&tree->object, &trees);
+       return 0;
+}
+
+static int same(struct cache_entry *a, struct cache_entry *b)
+{
+       if (!!a != !!b)
+               return 0;
+       if (!a && !b)
+               return 1;
+       return a->ce_mode == b->ce_mode && 
+               !memcmp(a->sha1, b->sha1, 20);
 }
 
+
 /*
  * When a CE gets turned into an unmerged entry, we
  * want it to be up-to-date
@@ -113,221 +309,261 @@ static void verify_uptodate(struct cache_entry *ce)
        die("Entry '%s' not uptodate. Cannot merge.", ce->name);
 }
 
-/*
- * If the old tree contained a CE that isn't even in the
- * result, that's always a problem, regardless of whether
- * it's up-to-date or not (ie it can be a file that we
- * have updated but not committed yet).
- */
-static void reject_merge(struct cache_entry *ce)
+static int merged_entry(struct cache_entry *merge, struct cache_entry *old)
 {
-       die("Entry '%s' would be overwritten by merge. Cannot merge.", ce->name);
+       merge->ce_flags |= htons(CE_UPDATE);
+       if (old) {
+               /*
+                * See if we can re-use the old CE directly?
+                * That way we get the uptodate stat info.
+                *
+                * This also removes the UPDATE flag on
+                * a match.
+                */
+               if (same(old, merge)) {
+                       *merge = *old;
+               } else {
+                       verify_uptodate(old);
+               }
+       }
+       merge->ce_flags &= ~htons(CE_STAGEMASK);
+       add_cache_entry(merge, ADD_CACHE_OK_TO_ADD);
+       return 1;
 }
 
-#define CHECK_OLD(ce) if (old && same(old, ce)) { verify_uptodate(old); old = NULL; }
+static int deleted_entry(struct cache_entry *ce, struct cache_entry *old)
+{
+       if (old)
+               verify_uptodate(old);
+       ce->ce_mode = 0;
+       add_cache_entry(ce, ADD_CACHE_OK_TO_ADD);
+       return 1;
+}
 
-static void trivially_merge_cache(struct cache_entry **src, int nr)
+static int keep_entry(struct cache_entry *ce)
 {
-       struct cache_entry **dst = src;
-       struct cache_entry *old = NULL;
+       add_cache_entry(ce, ADD_CACHE_OK_TO_ADD);
+       return 1;
+}
 
-       while (nr--) {
-               struct cache_entry *ce, *result;
+static int threeway_merge(struct cache_entry **stages)
+{
+       struct cache_entry *index;
+       struct cache_entry *head; 
+       struct cache_entry *remote = stages[head_idx + 1];
+       int count;
+       int head_match = 0;
+       int remote_match = 0;
+
+       int df_conflict_head = 0;
+       int df_conflict_remote = 0;
+
+       int any_anc_missing = 0;
+       int i;
+
+       for (i = 1; i < head_idx; i++) {
+               if (!stages[i])
+                       any_anc_missing = 1;
+       }
 
-               ce = *src++;
+       index = stages[0];
+       head = stages[head_idx];
 
-               /* We throw away original cache entries except for the stat information */
-               if (!ce_stage(ce)) {
-                       if (old)
-                               reject_merge(old);
-                       old = ce;
-                       active_nr--;
-                       continue;
-               }
-               if (old && !path_matches(old, ce))
-                       reject_merge(old);
-               if (nr > 2 && (result = merge_entries(ce, src[0], src[1])) != NULL) {
-                       result->ce_flags |= htons(CE_UPDATE);
-                       /*
-                        * See if we can re-use the old CE directly?
-                        * That way we get the uptodate stat info.
-                        *
-                        * This also removes the UPDATE flag on
-                        * a match.
-                        */
-                       if (old && same(old, result)) {
-                               *result = *old;
-                               old = NULL;
+       if (head == &df_conflict_entry) {
+               df_conflict_head = 1;
+               head = NULL;
+       }
+
+       if (remote == &df_conflict_entry) {
+               df_conflict_remote = 1;
+               remote = NULL;
+       }
+
+       /* First, if there's a #16 situation, note that to prevent #13
+        * and #14. 
+        */
+       if (!same(remote, head)) {
+               for (i = 1; i < head_idx; i++) {
+                       if (same(stages[i], head)) {
+                               head_match = 1;
+                       }
+                       if (same(stages[i], remote)) {
+                               remote_match = 1;
                        }
-                       CHECK_OLD(ce);
-                       CHECK_OLD(src[0]);
-                       CHECK_OLD(src[1]);
-                       ce = result;
-                       ce->ce_flags &= ~htons(CE_STAGEMASK);
-                       src += 2;
-                       nr -= 2;
-                       active_nr -= 2;
                }
+       }
 
-               /*
-                * If we had an old entry that we now effectively
-                * overwrite, make sure it wasn't dirty.
-                */
-               CHECK_OLD(ce);
-               *dst++ = ce;
+       /* We start with cases where the index is allowed to match
+        * something other than the head: #14(ALT) and #2ALT, where it
+        * is permitted to match the result instead.
+        */
+       /* #14, #14ALT, #2ALT */
+       if (remote && !df_conflict_head && head_match && !remote_match) {
+               if (index && !same(index, remote) && !same(index, head))
+                       reject_merge(index);
+               return merged_entry(remote, index);
        }
-       if (old)
-               reject_merge(old);
+       /*
+        * If we have an entry in the index cache, then we want to
+        * make sure that it matches head.
+        */
+       if (index && !same(index, head)) {
+               reject_merge(index);
+       }
+
+       if (head) {
+               /* #5ALT, #15 */
+               if (same(head, remote))
+                       return merged_entry(head, index);
+               /* #13, #3ALT */
+               if (!df_conflict_remote && remote_match && !head_match)
+                       return merged_entry(head, index);
+       }
+
+       /* #1 */
+       if (!head && !remote && any_anc_missing)
+               return 0;
+
+       /* Below are "no merge" cases, which require that the index be
+        * up-to-date to avoid the files getting overwritten with
+        * conflict resolution files. 
+        */
+       if (index) {
+               verify_uptodate(index);
+       }
+
+       /* #2, #3, #4, #6, #7, #9, #11. */
+       count = 0;
+       if (!head_match || !remote_match) {
+               for (i = 1; i < head_idx; i++) {
+                       if (stages[i]) {
+                               keep_entry(stages[i]);
+                               count++;
+                               break;
+                       }
+               }
+       }
+       if (head) { count += keep_entry(head); }
+       if (remote) { count += keep_entry(remote); }
+       return count;
 }
 
 /*
- * When we find a "stage2" entry in the two-way merge, that's
- * the one that will remain. If we have an exact old match,
- * we don't care whether the file is up-to-date or not, we just
- * re-use the thing directly.
+ * Two-way merge.
+ *
+ * The rule is to "carry forward" what is in the index without losing
+ * information across a "fast forward", favoring a successful merge
+ * over a merge failure when it makes sense.  For details of the
+ * "carry forward" rule, please see <Documentation/git-read-tree.txt>.
  *
- * If we didn't have an exact match, then we want to make sure
- * that we've seen a stage1 that matched the old, and that the
- * old file was up-to-date. Because it will be gone after this
- * merge..
  */
-static void twoway_check(struct cache_entry *old, int seen_stage1, struct cache_entry *ce)
+static int twoway_merge(struct cache_entry **src)
 {
-       if (path_matches(old, ce)) {
-               /*
-                * This also removes the UPDATE flag on
-                * a match
-                */
-               if (same(old, ce)) {
-                       *ce = *old;
-                       return;
+       struct cache_entry *current = src[0];
+       struct cache_entry *oldtree = src[1], *newtree = src[2];
+
+       if (merge_size != 2)
+               return error("Cannot do a twoway merge of %d trees\n",
+                            merge_size);
+
+       if (current) {
+               if ((!oldtree && !newtree) || /* 4 and 5 */
+                   (!oldtree && newtree &&
+                    same(current, newtree)) || /* 6 and 7 */
+                   (oldtree && newtree &&
+                    same(oldtree, newtree)) || /* 14 and 15 */
+                   (oldtree && newtree &&
+                    !same(oldtree, newtree) && /* 18 and 19*/
+                    same(current, newtree))) {
+                       return keep_entry(current);
+               }
+               else if (oldtree && !newtree && same(current, oldtree)) {
+                       /* 10 or 11 */
+                       return deleted_entry(oldtree, current);
+               }
+               else if (oldtree && newtree &&
+                        same(current, oldtree) && !same(current, newtree)) {
+                       /* 20 or 21 */
+                       return merged_entry(newtree, current);
+               }
+               else {
+                       /* all other failures */
+                       if (oldtree)
+                               reject_merge(oldtree);
+                       if (current)
+                               reject_merge(current);
+                       if (newtree)
+                               reject_merge(newtree);
+                       return -1;
                }
-               if (!seen_stage1)
-                       reject_merge(old);
        }
-       verify_uptodate(old);
+       else if (newtree)
+               return merged_entry(newtree, current);
+       else
+               return deleted_entry(oldtree, current);
 }
 
 /*
- * Two-way merge.
+ * One-way merge.
  *
- * The rule is: 
- *  - every current entry has to match the old tree
- *  - if the current entry matches the new tree, we leave it
- *    as-is. Otherwise we require that it be up-to-date.
+ * The rule is:
+ * - take the stat information from stage0, take the data from stage1
  */
-static void twoway_merge(struct cache_entry **src, int nr)
+static int oneway_merge(struct cache_entry **src)
 {
-       int seen_stage1 = 0;
-       struct cache_entry *old = NULL;
-       struct cache_entry **dst = src;
-
-       while (nr--) {
-               struct cache_entry *ce = *src++;
-               int stage = ce_stage(ce);
-
-               switch (stage) {
-               case 0:
-                       if (old)
-                               reject_merge(old);
-                       old = ce;
-                       seen_stage1 = 0;
-                       active_nr--;
-                       continue;
+       struct cache_entry *old = src[0];
+       struct cache_entry *a = src[1];
 
-               case 1:
-                       active_nr--;
-                       if (!old)
-                               continue;
-                       if (!path_matches(old, ce) || !same(old, ce))
-                               reject_merge(old);
-                       seen_stage1 = 1;
-                       continue;
+       if (merge_size != 1)
+               return error("Cannot do a oneway merge of %d trees\n",
+                            merge_size);
 
-               case 2:
-                       ce->ce_flags |= htons(CE_UPDATE);
-                       if (old) {
-                               twoway_check(old, seen_stage1, ce);
-                               old = NULL;
-                       }
-                       ce->ce_flags &= ~htons(CE_STAGEMASK);
-                       *dst++ = ce;
-                       continue;
-               }
-               die("impossible two-way stage");
-       }
-
-       /*
-        * Unmatched with a new entry? Make sure it was
-        * at least uptodate in the working directory _and_
-        * the original tree..
-        */
-       if (old) {
-               if (!seen_stage1)
-                       reject_merge(old);
-               verify_uptodate(old);
+       if (!a)
+               return 0;
+       if (old && same(old, a)) {
+               return keep_entry(old);
        }
+       return merged_entry(a, NULL);
 }
 
-static void merge_stat_info(struct cache_entry **src, int nr)
+static int read_cache_unmerged(void)
 {
-       static struct cache_entry null_entry;
-       struct cache_entry **dst = src;
-       struct cache_entry *stat = &null_entry;
-
-       while (nr--) {
-               struct cache_entry *ce = *src++;
-
-               /* We throw away original cache entries except for the stat information */
-               if (!ce_stage(ce)) {
-                       stat = ce;
-                       active_nr--;
+       int i, deleted;
+       struct cache_entry **dst;
+
+       read_cache();
+       dst = active_cache;
+       deleted = 0;
+       for (i = 0; i < active_nr; i++) {
+               struct cache_entry *ce = active_cache[i];
+               if (ce_stage(ce)) {
+                       deleted++;
                        continue;
                }
-               if (path_matches(ce, stat) && same(ce, stat))
-                       *ce = *stat;
-               ce->ce_flags &= ~htons(CE_STAGEMASK);
-               *dst++ = ce;
+               if (deleted)
+                       *dst = ce;
+               dst++;
        }
+       active_nr -= deleted;
+       return deleted;
 }
 
-static void check_updates(struct cache_entry **src, int nr)
-{
-       static struct checkout state = {
-               .base_dir = "",
-               .force = 1,
-               .quiet = 1,
-               .refresh_cache = 1,
-       };
-       unsigned short mask = htons(CE_UPDATE);
-       while (nr--) {
-               struct cache_entry *ce = *src++;
-               if (ce->ce_flags & mask) {
-                       ce->ce_flags &= ~mask;
-                       if (update)
-                               checkout_entry(ce, &state);
-               }
-       }
-}
+static const char read_tree_usage[] = "git-read-tree (<sha> | -m [-u] <sha1> [<sha2> [<sha3>]])";
 
-static char *read_tree_usage = "git-read-tree (<sha> | -m <sha1> [<sha2> [<sha3>]])";
+static struct cache_file cache_file;
 
 int main(int argc, char **argv)
 {
-       int i, newfd, merge;
+       int i, newfd, reset, stage = 0;
        unsigned char sha1[20];
-       static char lockfile[MAXPATHLEN+1];
-       const char *indexfile = get_index_file();
-
-       snprintf(lockfile, sizeof(lockfile), "%s.lock", indexfile);
+       merge_fn_t fn = NULL;
 
-       newfd = open(lockfile, O_RDWR | O_CREAT | O_EXCL, 0600);
+       newfd = hold_index_file_for_update(&cache_file, get_index_file());
        if (newfd < 0)
                die("unable to create new cachefile");
-       atexit(remove_lock_file);
-       lockfile_name = lockfile;
 
        merge = 0;
+       reset = 0;
        for (i = 1; i < argc; i++) {
                const char *arg = argv[i];
 
@@ -337,47 +573,70 @@ int main(int argc, char **argv)
                        continue;
                }
 
+               /* This differs from "-m" in that we'll silently ignore unmerged entries */
+               if (!strcmp(arg, "--reset")) {
+                       if (stage || merge)
+                               usage(read_tree_usage);
+                       reset = 1;
+                       merge = 1;
+                       stage = 1;
+                       read_cache_unmerged();
+                       continue;
+               }
+
+               if (!strcmp(arg, "--head")) {
+                       head_idx = stage - 1;
+                       fn = threeway_merge;
+               }
+
                /* "-m" stands for "merge", meaning we start in stage 1 */
                if (!strcmp(arg, "-m")) {
-                       int i;
-                       if (stage)
-                               die("-m needs to come first");
-                       read_cache();
-                       for (i = 0; i < active_nr; i++) {
-                               if (ce_stage(active_cache[i]))
-                                       die("you need to resolve your current index first");
-                       }
+                       if (stage || merge)
+                               usage(read_tree_usage);
+                       if (read_cache_unmerged())
+                               die("you need to resolve your current index first");
                        stage = 1;
                        merge = 1;
                        continue;
                }
+
                if (get_sha1(arg, sha1) < 0)
                        usage(read_tree_usage);
-               if (stage > 3)
-                       usage(read_tree_usage);
-               if (unpack_tree(sha1) < 0)
+               if (list_tree(sha1) < 0)
                        die("failed to unpack tree object %s", arg);
                stage++;
        }
-       if (merge) {
-               switch (stage) {
-               case 4: /* Three-way merge */
-                       trivially_merge_cache(active_cache, active_nr);
-                       check_updates(active_cache, active_nr);
+       if (update && !merge)
+               usage(read_tree_usage);
+       if (merge && !fn) {
+               if (stage < 2)
+                       die("just how do you expect me to merge %d trees?", stage-1);
+               switch (stage - 1) {
+               case 1:
+                       fn = oneway_merge;
                        break;
-               case 3: /* Update from one tree to another */
-                       twoway_merge(active_cache, active_nr);
-                       check_updates(active_cache, active_nr);
+               case 2:
+                       fn = twoway_merge;
                        break;
-               case 2: /* Just read a tree, merge with old cache contents */
-                       merge_stat_info(active_cache, active_nr);
+               case 3:
+                       fn = threeway_merge;
                        break;
                default:
-                       die("just how do you expect me to merge %d trees?", stage-1);
+                       fn = threeway_merge;
+                       break;
                }
        }
-       if (write_cache(newfd, active_cache, active_nr) || rename(lockfile, indexfile))
+
+       if (head_idx < 0) {
+               if (stage - 1 >= 3)
+                       head_idx = stage - 2;
+               else
+                       head_idx = 1;
+       }
+
+       unpack_trees(fn);
+       if (write_cache(newfd, active_cache, active_nr) ||
+           commit_index_file(&cache_file))
                die("unable to write new index file");
-       lockfile_name = NULL;
        return 0;
 }