Merge branch 'maint'
[gitweb.git] / builtin-checkout.c
index cf9875c13da9c194e1f4ad4ce456ed09808b849b..b5dd9c07b42e0130384259715730f52bc8c4e6c3 100644 (file)
@@ -13,6 +13,9 @@
 #include "diff.h"
 #include "revision.h"
 #include "remote.h"
+#include "blob.h"
+#include "xdiff-interface.h"
+#include "ll-merge.h"
 
 static const char * const checkout_usage[] = {
        "git checkout [options] <branch>",
@@ -20,6 +23,18 @@ static const char * const checkout_usage[] = {
        NULL,
 };
 
+struct checkout_opts {
+       int quiet;
+       int merge;
+       int force;
+       int writeout_stage;
+       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)
 {
@@ -32,7 +47,7 @@ static int post_checkout_hook(struct commit *old, struct commit *new,
 
        memset(&proc, 0, sizeof(proc));
        argv[0] = name;
-       argv[1] = xstrdup(sha1_to_hex(old->object.sha1));
+       argv[1] = xstrdup(sha1_to_hex(old ? old->object.sha1 : null_sha1));
        argv[2] = xstrdup(sha1_to_hex(new->object.sha1));
        argv[3] = changed ? "1" : "0";
        argv[4] = NULL;
@@ -43,7 +58,7 @@ static int post_checkout_hook(struct commit *old, struct commit *new,
 }
 
 static int update_some(const unsigned char *sha1, const char *base, int baselen,
-                      const char *pathname, unsigned mode, int stage)
+               const char *pathname, unsigned mode, int stage, void *context)
 {
        int len;
        struct cache_entry *ce;
@@ -67,7 +82,7 @@ static int update_some(const unsigned char *sha1, const char *base, int baselen,
 
 static int read_tree_some(struct tree *tree, const char **pathspec)
 {
-       read_tree_recursive(tree, "", 0, 0, pathspec, update_some);
+       read_tree_recursive(tree, "", 0, 0, pathspec, update_some, NULL);
 
        /* update the index with the given tree's info
         * for all args, expanding wildcards, and exit
@@ -76,7 +91,129 @@ static int read_tree_some(struct tree *tree, const char **pathspec)
        return 0;
 }
 
-static int checkout_paths(struct tree *source_tree, const char **pathspec)
+static int skip_same_name(struct cache_entry *ce, int pos)
+{
+       while (++pos < active_nr &&
+              !strcmp(active_cache[pos]->name, ce->name))
+               ; /* skip */
+       return pos;
+}
+
+static int check_stage(int stage, struct cache_entry *ce, int pos)
+{
+       while (pos < active_nr &&
+              !strcmp(active_cache[pos]->name, ce->name)) {
+               if (ce_stage(active_cache[pos]) == stage)
+                       return 0;
+               pos++;
+       }
+       return error("path '%s' does not have %s version",
+                    ce->name,
+                    (stage == 2) ? "our" : "their");
+}
+
+static int check_all_stages(struct cache_entry *ce, int pos)
+{
+       if (ce_stage(ce) != 1 ||
+           active_nr <= pos + 2 ||
+           strcmp(active_cache[pos+1]->name, ce->name) ||
+           ce_stage(active_cache[pos+1]) != 2 ||
+           strcmp(active_cache[pos+2]->name, ce->name) ||
+           ce_stage(active_cache[pos+2]) != 3)
+               return error("path '%s' does not have all three versions",
+                            ce->name);
+       return 0;
+}
+
+static int checkout_stage(int stage, struct cache_entry *ce, int pos,
+                         struct checkout *state)
+{
+       while (pos < active_nr &&
+              !strcmp(active_cache[pos]->name, ce->name)) {
+               if (ce_stage(active_cache[pos]) == stage)
+                       return checkout_entry(active_cache[pos], state, NULL);
+               pos++;
+       }
+       return error("path '%s' does not have %s version",
+                    ce->name,
+                    (stage == 2) ? "our" : "their");
+}
+
+/* NEEDSWORK: share with merge-recursive */
+static void fill_mm(const unsigned char *sha1, mmfile_t *mm)
+{
+       unsigned long size;
+       enum object_type type;
+
+       if (!hashcmp(sha1, null_sha1)) {
+               mm->ptr = xstrdup("");
+               mm->size = 0;
+               return;
+       }
+
+       mm->ptr = read_sha1_file(sha1, &type, &size);
+       if (!mm->ptr || type != OBJ_BLOB)
+               die("unable to read blob object %s", sha1_to_hex(sha1));
+       mm->size = size;
+}
+
+static int checkout_merged(int pos, struct checkout *state)
+{
+       struct cache_entry *ce = active_cache[pos];
+       const char *path = ce->name;
+       mmfile_t ancestor, ours, theirs;
+       int status;
+       unsigned char sha1[20];
+       mmbuffer_t result_buf;
+
+       if (ce_stage(ce) != 1 ||
+           active_nr <= pos + 2 ||
+           strcmp(active_cache[pos+1]->name, path) ||
+           ce_stage(active_cache[pos+1]) != 2 ||
+           strcmp(active_cache[pos+2]->name, path) ||
+           ce_stage(active_cache[pos+2]) != 3)
+               return error("path '%s' does not have all 3 versions", path);
+
+       fill_mm(active_cache[pos]->sha1, &ancestor);
+       fill_mm(active_cache[pos+1]->sha1, &ours);
+       fill_mm(active_cache[pos+2]->sha1, &theirs);
+
+       status = ll_merge(&result_buf, path, &ancestor,
+                         &ours, "ours", &theirs, "theirs", 1);
+       free(ancestor.ptr);
+       free(ours.ptr);
+       free(theirs.ptr);
+       if (status < 0 || !result_buf.ptr) {
+               free(result_buf.ptr);
+               return error("path '%s': cannot merge", path);
+       }
+
+       /*
+        * NEEDSWORK:
+        * There is absolutely no reason to write this as a blob object
+        * and create a phoney cache entry just to leak.  This hack is
+        * primarily to get to the write_entry() machinery that massages
+        * the contents to work-tree format and writes out which only
+        * allows it for a cache entry.  The code in write_entry() needs
+        * to be refactored to allow us to feed a <buffer, size, mode>
+        * instead of a cache entry.  Such a refactoring would help
+        * merge_recursive as well (it also writes the merge result to the
+        * object database even when it may contain conflicts).
+        */
+       if (write_sha1_file(result_buf.ptr, result_buf.size,
+                           blob_type, sha1))
+               die("Unable to add merge result for '%s'", path);
+       ce = make_cache_entry(create_ce_mode(active_cache[pos+1]->ce_mode),
+                             sha1,
+                             path, 2, 0);
+       if (!ce)
+               die("make_cache_entry failed for path '%s'", path);
+       status = checkout_entry(ce, state, NULL);
+       return status;
+}
+
+static int checkout_paths(struct tree *source_tree, const char **pathspec,
+                         struct checkout_opts *opts)
 {
        int pos;
        struct checkout state;
@@ -84,12 +221,15 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec)
        unsigned char rev[20];
        int flag;
        struct commit *head;
-
+       int errs = 0;
+       int stage = opts->writeout_stage;
+       int merge = opts->merge;
        int newfd;
        struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 
        newfd = hold_locked_index(lock_file, 1);
-       read_cache();
+       if (read_cache() < 0)
+               return error("corrupt index file");
 
        if (source_tree)
                read_tree_some(source_tree, pathspec);
@@ -106,13 +246,44 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec)
        if (report_path_error(ps_matched, pathspec, 0))
                return 1;
 
+       /* Any unmerged paths? */
+       for (pos = 0; pos < active_nr; pos++) {
+               struct cache_entry *ce = active_cache[pos];
+               if (pathspec_match(pathspec, NULL, ce->name, 0)) {
+                       if (!ce_stage(ce))
+                               continue;
+                       if (opts->force) {
+                               warning("path '%s' is unmerged", ce->name);
+                       } else if (stage) {
+                               errs |= check_stage(stage, ce, pos);
+                       } else if (opts->merge) {
+                               errs |= check_all_stages(ce, pos);
+                       } else {
+                               errs = 1;
+                               error("path '%s' is unmerged", ce->name);
+                       }
+                       pos = skip_same_name(ce, pos) - 1;
+               }
+       }
+       if (errs)
+               return 1;
+
+       /* Now we are committed to check them out */
        memset(&state, 0, sizeof(state));
        state.force = 1;
        state.refresh_cache = 1;
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
                if (pathspec_match(pathspec, NULL, ce->name, 0)) {
-                       checkout_entry(ce, &state, NULL);
+                       if (!ce_stage(ce)) {
+                               errs |= checkout_entry(ce, &state, NULL);
+                               continue;
+                       }
+                       if (stage)
+                               errs |= checkout_stage(stage, ce, pos, &state);
+                       else if (merge)
+                               errs |= checkout_merged(pos, &state);
+                       pos = skip_same_name(ce, pos) - 1;
                }
        }
 
@@ -123,7 +294,8 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec)
        resolve_ref("HEAD", rev, 0, &flag);
        head = lookup_commit_reference_gently(rev, 1);
 
-       return post_checkout_hook(head, head, 0);
+       errs |= post_checkout_hook(head, head, 0);
+       return errs;
 }
 
 static void show_local_changes(struct object *head)
@@ -139,8 +311,7 @@ static void show_local_changes(struct object *head)
 
 static void describe_detached_head(char *msg, struct commit *commit)
 {
-       struct strbuf sb;
-       strbuf_init(&sb, 0);
+       struct strbuf sb = STRBUF_INIT;
        parse_commit(commit);
        pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, 0, NULL, NULL, 0, 0);
        fprintf(stderr, "%s %s... %s\n", msg,
@@ -148,57 +319,39 @@ static void describe_detached_head(char *msg, struct commit *commit)
        strbuf_release(&sb);
 }
 
-static int reset_to_new(struct tree *tree, int quiet)
+static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
 {
        struct unpack_trees_options opts;
        struct tree_desc tree_desc;
 
        memset(&opts, 0, sizeof(opts));
        opts.head_idx = -1;
-       opts.update = 1;
+       opts.update = worktree;
+       opts.skip_unmerged = !worktree;
        opts.reset = 1;
        opts.merge = 1;
        opts.fn = oneway_merge;
-       opts.verbose_update = !quiet;
+       opts.verbose_update = !o->quiet;
        opts.src_index = &the_index;
        opts.dst_index = &the_index;
        parse_tree(tree);
        init_tree_desc(&tree_desc, tree->buffer, tree->size);
-       if (unpack_trees(1, &tree_desc, &opts))
+       switch (unpack_trees(1, &tree_desc, &opts)) {
+       case -2:
+               o->writeout_error = 1;
+               /*
+                * We return 0 nevertheless, as the index is all right
+                * and more importantly we have made best efforts to
+                * update paths in the work tree, and we cannot revert
+                * them.
+                */
+       case 0:
+               return 0;
+       default:
                return 128;
-       return 0;
-}
-
-static void reset_clean_to_new(struct tree *tree, int quiet)
-{
-       struct unpack_trees_options opts;
-       struct tree_desc tree_desc;
-
-       memset(&opts, 0, sizeof(opts));
-       opts.head_idx = -1;
-       opts.skip_unmerged = 1;
-       opts.reset = 1;
-       opts.merge = 1;
-       opts.fn = oneway_merge;
-       opts.verbose_update = !quiet;
-       opts.src_index = &the_index;
-       opts.dst_index = &the_index;
-       parse_tree(tree);
-       init_tree_desc(&tree_desc, tree->buffer, tree->size);
-       if (unpack_trees(1, &tree_desc, &opts))
-               exit(128);
+       }
 }
 
-struct checkout_opts {
-       int quiet;
-       int merge;
-       int force;
-
-       char *new_branch;
-       int new_branch_log;
-       enum branch_track track;
-};
-
 struct branch_info {
        const char *name; /* The short name used */
        const char *path; /* The full name of a real branch */
@@ -207,8 +360,7 @@ struct branch_info {
 
 static void setup_branch_path(struct branch_info *branch)
 {
-       struct strbuf buf;
-       strbuf_init(&buf, 0);
+       struct strbuf buf = STRBUF_INIT;
        strbuf_addstr(&buf, "refs/heads/");
        strbuf_addstr(&buf, branch->name);
        branch->path = strbuf_detach(&buf, NULL);
@@ -220,10 +372,12 @@ static int merge_working_tree(struct checkout_opts *opts,
        int ret;
        struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
        int newfd = hold_locked_index(lock_file, 1);
-       read_cache();
+
+       if (read_cache() < 0)
+               return error("corrupt index file");
 
        if (opts->force) {
-               ret = reset_to_new(new->commit->tree, opts->quiet);
+               ret = reset_tree(new->commit->tree, opts, 1);
                if (ret)
                        return ret;
        } else {
@@ -236,6 +390,8 @@ static int merge_working_tree(struct checkout_opts *opts,
                topts.src_index = &the_index;
                topts.dst_index = &the_index;
 
+               topts.msgs.not_uptodate_file = "You have local changes to '%s'; cannot switch branches.";
+
                refresh_cache(REFRESH_QUIET);
 
                if (unmerged_cache()) {
@@ -244,6 +400,7 @@ static int merge_working_tree(struct checkout_opts *opts,
                }
 
                /* 2-way merge to the new branch */
+               topts.initial_checkout = is_cache_unborn();
                topts.update = 1;
                topts.merge = 1;
                topts.gently = opts->merge;
@@ -257,7 +414,8 @@ static int merge_working_tree(struct checkout_opts *opts,
                tree = parse_tree_indirect(new->commit->object.sha1);
                init_tree_desc(&trees[1], tree->buffer, tree->size);
 
-               if (unpack_trees(2, trees, &topts)) {
+               ret = unpack_trees(2, trees, &topts);
+               if (ret == -1) {
                        /*
                         * Unpack couldn't do a trivial merge; either
                         * give up or do a real merge, depending on
@@ -265,6 +423,7 @@ static int merge_working_tree(struct checkout_opts *opts,
                         */
                        struct tree *result;
                        struct tree *work;
+                       struct merge_options o;
                        if (!opts->merge)
                                return 1;
                        parse_commit(old->commit);
@@ -282,15 +441,21 @@ static int merge_working_tree(struct checkout_opts *opts,
                         * entries in the index.
                         */
 
-                       add_files_to_cache(0, NULL, NULL);
-                       work = write_tree_from_memory();
+                       add_files_to_cache(NULL, NULL, 0);
+                       init_merge_options(&o);
+                       o.verbosity = 0;
+                       work = write_tree_from_memory(&o);
 
-                       ret = reset_to_new(new->commit->tree, opts->quiet);
+                       ret = reset_tree(new->commit->tree, opts, 1);
+                       if (ret)
+                               return ret;
+                       o.branch1 = new->name;
+                       o.branch2 = "local";
+                       merge_trees(&o, new->commit->tree, work,
+                               old->commit->tree, &result);
+                       ret = reset_tree(new->commit->tree, opts, 0);
                        if (ret)
                                return ret;
-                       merge_trees(new->commit->tree, work, old->commit->tree,
-                                   new->name, "local", &result);
-                       reset_clean_to_new(new->commit->tree, opts->quiet);
                }
        }
 
@@ -298,110 +463,28 @@ static int merge_working_tree(struct checkout_opts *opts,
            commit_locked_index(lock_file))
                die("unable to write new index file");
 
-       if (!opts->force)
+       if (!opts->force && !opts->quiet)
                show_local_changes(&new->commit->object);
 
        return 0;
 }
 
-static void report_tracking(struct branch_info *new, struct checkout_opts *opts)
+static void report_tracking(struct branch_info *new)
 {
-       /*
-        * We have switched to a new branch; is it building on
-        * top of another branch, and if so does that other branch
-        * have changes we do not have yet?
-        */
-       char *base;
-       unsigned char sha1[20];
-       struct commit *ours, *theirs;
-       char symmetric[84];
-       struct rev_info revs;
-       const char *rev_argv[10];
-       int rev_argc;
-       int num_ours, num_theirs;
-       const char *remote_msg;
+       struct strbuf sb = STRBUF_INIT;
        struct branch *branch = branch_get(new->name);
 
-       /*
-        * Nothing to report unless we are marked to build on top of
-        * somebody else.
-        */
-       if (!branch || !branch->merge || !branch->merge[0] || !branch->merge[0]->dst)
+       if (!format_tracking_info(branch, &sb))
                return;
-
-       /*
-        * If what we used to build on no longer exists, there is
-        * nothing to report.
-        */
-       base = branch->merge[0]->dst;
-       if (!resolve_ref(base, sha1, 1, NULL))
-               return;
-
-       theirs = lookup_commit(sha1);
-       ours = new->commit;
-       if (!hashcmp(sha1, ours->object.sha1))
-               return; /* we are the same */
-
-       /* Run "rev-list --left-right ours...theirs" internally... */
-       rev_argc = 0;
-       rev_argv[rev_argc++] = NULL;
-       rev_argv[rev_argc++] = "--left-right";
-       rev_argv[rev_argc++] = symmetric;
-       rev_argv[rev_argc++] = "--";
-       rev_argv[rev_argc] = NULL;
-
-       strcpy(symmetric, sha1_to_hex(ours->object.sha1));
-       strcpy(symmetric + 40, "...");
-       strcpy(symmetric + 43, sha1_to_hex(theirs->object.sha1));
-
-       init_revisions(&revs, NULL);
-       setup_revisions(rev_argc, rev_argv, &revs, NULL);
-       prepare_revision_walk(&revs);
-
-       /* ... and count the commits on each side. */
-       num_ours = 0;
-       num_theirs = 0;
-       while (1) {
-               struct commit *c = get_revision(&revs);
-               if (!c)
-                       break;
-               if (c->object.flags & SYMMETRIC_LEFT)
-                       num_ours++;
-               else
-                       num_theirs++;
-       }
-
-       if (!prefixcmp(base, "refs/remotes/")) {
-               remote_msg = " remote";
-               base += strlen("refs/remotes/");
-       } else {
-               remote_msg = "";
-       }
-
-       if (!num_theirs)
-               printf("Your branch is ahead of the tracked%s branch '%s' "
-                      "by %d commit%s.\n",
-                      remote_msg, base,
-                      num_ours, (num_ours == 1) ? "" : "s");
-       else if (!num_ours)
-               printf("Your branch is behind the tracked%s branch '%s' "
-                      "by %d commit%s,\n"
-                      "and can be fast-forwarded.\n",
-                      remote_msg, base,
-                      num_theirs, (num_theirs == 1) ? "" : "s");
-       else
-               printf("Your branch and the tracked%s branch '%s' "
-                      "have diverged,\nand respectively "
-                      "have %d and %d different commit(s) each.\n",
-                      remote_msg, base,
-                      num_ours, num_theirs);
+       fputs(sb.buf, stdout);
+       strbuf_release(&sb);
 }
 
 static void update_refs_for_switch(struct checkout_opts *opts,
                                   struct branch_info *old,
                                   struct branch_info *new)
 {
-       struct strbuf msg;
+       struct strbuf msg = STRBUF_INIT;
        const char *old_desc;
        if (opts->new_branch) {
                create_branch(old->name, opts->new_branch, new->name, 0,
@@ -410,12 +493,11 @@ static void update_refs_for_switch(struct checkout_opts *opts,
                setup_branch_path(new);
        }
 
-       strbuf_init(&msg, 0);
        old_desc = old->name;
-       if (!old_desc)
+       if (!old_desc && old->commit)
                old_desc = sha1_to_hex(old->commit->object.sha1);
        strbuf_addf(&msg, "checkout: moving from %s to %s",
-                   old_desc, new->name);
+                   old_desc ? old_desc : "(invalid)", new->name);
 
        if (new->path) {
                create_symref("HEAD", new->path, msg.buf);
@@ -440,7 +522,7 @@ static void update_refs_for_switch(struct checkout_opts *opts,
        remove_branch_state();
        strbuf_release(&msg);
        if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
-               report_tracking(new, opts);
+               report_tracking(new);
 }
 
 static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
@@ -467,16 +549,14 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
        }
 
        /*
-        * If the new thing isn't a branch and isn't HEAD and we're
-        * not starting a new branch, and we want messages, and we
-        * weren't on a branch, and we're moving to a new commit,
-        * describe the old commit.
+        * If we were on a detached HEAD, but we are now moving to
+        * a new commit, we want to mention the old commit once more
+        * to remind the user that it might be lost.
         */
-       if (!new->path && strcmp(new->name, "HEAD") && !opts->new_branch &&
-           !opts->quiet && !old.path && new->commit != old.commit)
+       if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
                describe_detached_head("Previous HEAD position was", old.commit);
 
-       if (!old.commit) {
+       if (!old.commit && !opts->force) {
                if (!opts->quiet) {
                        fprintf(stderr, "warning: You appear to be on a branch yet to be born.\n");
                        fprintf(stderr, "warning: Forcing checkout of %s.\n", new->name);
@@ -490,7 +570,13 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 
        update_refs_for_switch(opts, &old, new);
 
-       return post_checkout_hook(old.commit, new->commit, 1);
+       ret = post_checkout_hook(old.commit, new->commit, 1);
+       return ret || opts->writeout_error;
+}
+
+static int git_checkout_config(const char *var, const char *value, void *cb)
+{
+       return git_xmerge_config(var, value, cb);
 }
 
 int cmd_checkout(int argc, const char **argv, const char *prefix)
@@ -500,31 +586,103 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
        const char *arg;
        struct branch_info new;
        struct tree *source_tree = NULL;
+       char *conflict_style = NULL;
        struct option options[] = {
                OPT__QUIET(&opts.quiet),
                OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
                OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
-               OPT_SET_INT( 0 , "track",  &opts.track, "track",
+               OPT_SET_INT('t', "track",  &opts.track, "track",
                        BRANCH_TRACK_EXPLICIT),
+               OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
+                           2),
+               OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
+                           3),
                OPT_BOOLEAN('f', NULL, &opts.force, "force"),
-               OPT_BOOLEAN('m', NULL, &opts.merge, "merge"),
+               OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
+               OPT_STRING(0, "conflict", &conflict_style, "style",
+                          "conflict style (merge or diff3)"),
                OPT_END(),
        };
+       int has_dash_dash;
 
        memset(&opts, 0, sizeof(opts));
        memset(&new, 0, sizeof(new));
 
-       git_config(git_default_config);
+       git_config(git_checkout_config, NULL);
+
+       opts.track = BRANCH_TRACK_UNSPECIFIED;
+
+       argc = parse_options(argc, argv, options, checkout_usage,
+                            PARSE_OPT_KEEP_DASHDASH);
+
+       /* --track without -b should DWIM */
+       if (0 < opts.track && !opts.new_branch) {
+               const char *argv0 = argv[0];
+               if (!argc || !strcmp(argv0, "--"))
+                       die ("--track needs a branch name");
+               if (!prefixcmp(argv0, "refs/"))
+                       argv0 += 5;
+               if (!prefixcmp(argv0, "remotes/"))
+                       argv0 += 8;
+               argv0 = strchr(argv0, '/');
+               if (!argv0 || !argv0[1])
+                       die ("Missing branch name; try -b");
+               opts.new_branch = argv0 + 1;
+       }
 
-       opts.track = git_branch_track;
+       if (opts.track == BRANCH_TRACK_UNSPECIFIED)
+               opts.track = git_branch_track;
+       if (conflict_style) {
+               opts.merge = 1; /* implied */
+               git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
+       }
 
-       argc = parse_options(argc, argv, options, checkout_usage, 0);
+       if (opts.force && opts.merge)
+               die("git checkout: -f and -m are incompatible");
+
+       /*
+        * case 1: git checkout <ref> -- [<paths>]
+        *
+        *   <ref> must be a valid tree, everything after the '--' must be
+        *   a path.
+        *
+        * case 2: git checkout -- [<paths>]
+        *
+        *   everything after the '--' must be paths.
+        *
+        * case 3: git checkout <something> [<paths>]
+        *
+        *   With no paths, if <something> is a commit, that is to
+        *   switch to the branch or detach HEAD at it.
+        *
+        *   Otherwise <something> shall not be ambiguous.
+        *   - If it's *only* a reference, treat it like case (1).
+        *   - If it's only a path, treat it like case (2).
+        *   - else: fail.
+        *
+        */
        if (argc) {
+               if (!strcmp(argv[0], "--")) {       /* case (2) */
+                       argv++;
+                       argc--;
+                       goto no_reference;
+               }
+
                arg = argv[0];
-               if (get_sha1(arg, rev))
-                       ;
-               else if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
-                       new.name = arg;
+               has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");
+
+               if (get_sha1(arg, rev)) {
+                       if (has_dash_dash)          /* case (1) */
+                               die("invalid reference: %s", arg);
+                       goto no_reference;          /* case (3 -> 2) */
+               }
+
+               /* we can't end up being in (2) anymore, eat the argument */
+               argv++;
+               argc--;
+
+               new.name = arg;
+               if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
                        setup_branch_path(&new);
                        if (resolve_ref(new.path, rev, 1, NULL))
                                new.commit = lookup_commit_reference(rev);
@@ -532,25 +690,28 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
                                new.path = NULL;
                        parse_commit(new.commit);
                        source_tree = new.commit->tree;
-                       argv++;
-                       argc--;
-               } else if ((source_tree = parse_tree_indirect(rev))) {
+               } else
+                       source_tree = parse_tree_indirect(rev);
+
+               if (!source_tree)                   /* case (1): want a tree */
+                       die("reference is not a tree: %s", arg);
+               if (!has_dash_dash) {/* case (3 -> 1) */
+                       /*
+                        * Do not complain the most common case
+                        *      git checkout branch
+                        * even if there happen to be a file called 'branch';
+                        * it would be extremely annoying.
+                        */
+                       if (argc)
+                               verify_non_filename(NULL, arg);
+               }
+               else {
                        argv++;
                        argc--;
                }
        }
 
-       if (argc && !strcmp(argv[0], "--")) {
-               argv++;
-               argc--;
-       }
-
-       if (!opts.new_branch && (opts.track != git_branch_track))
-               die("git checkout: --track and --no-track require -b");
-
-       if (opts.force && opts.merge)
-               die("git checkout: -f and -m are incompatible");
-
+no_reference:
        if (argc) {
                const char **pathspec = get_pathspec(prefix, argv);
 
@@ -558,20 +719,36 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
                        die("invalid path specification");
 
                /* Checkout paths */
-               if (opts.new_branch || opts.force || opts.merge) {
+               if (opts.new_branch) {
                        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);
+               if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
+                       die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");
+
+               return checkout_paths(source_tree, pathspec, &opts);
+       }
+
+       if (opts.new_branch) {
+               struct strbuf buf = STRBUF_INIT;
+               strbuf_addstr(&buf, "refs/heads/");
+               strbuf_addstr(&buf, opts.new_branch);
+               if (!get_sha1(buf.buf, rev))
+                       die("git checkout: branch %s already exists", opts.new_branch);
+               if (check_ref_format(buf.buf))
+                       die("git checkout: we do not like '%s' as a branch name.", opts.new_branch);
+               strbuf_release(&buf);
        }
 
        if (new.name && !new.commit) {
                die("Cannot switch branch to a non-commit.");
        }
+       if (opts.writeout_stage)
+               die("--ours/--theirs is incompatible with switching branches.");
 
        return switch_branches(&opts, &new);
 }