Merge branch 'js/difftool-builtin'

Rewrite a scripted porcelain "git difftool" in C.

* js/difftool-builtin:
  difftool: hack around -Wzero-length-format warning
  difftool: retire the scripted version
  difftool: implement the functionality in the builtin
  difftool: add a skeleton for the upcoming builtin

6 files changed, 741 insertions, 47 deletions

diff --git a/Makefile b/Makefile
index 53ecc84e28..052dce087c 100644
--- a/Makefile
+++ b/Makefile
@@ -525,7 +525,6 @@ SCRIPT_LIB += git-sh-setup
 SCRIPT_LIB += git-sh-i18n
 
 SCRIPT_PERL += git-add--interactive.perl
-SCRIPT_PERL += git-difftool.perl
 SCRIPT_PERL += git-archimport.perl
 SCRIPT_PERL += git-cvsexportcommit.perl
 SCRIPT_PERL += git-cvsimport.perl
@@ -886,6 +885,7 @@ BUILTIN_OBJS += builtin/diff-files.o
 BUILTIN_OBJS += builtin/diff-index.o
 BUILTIN_OBJS += builtin/diff-tree.o
 BUILTIN_OBJS += builtin/diff.o
+BUILTIN_OBJS += builtin/difftool.o
 BUILTIN_OBJS += builtin/fast-export.o
 BUILTIN_OBJS += builtin/fetch-pack.o
 BUILTIN_OBJS += builtin/fetch.o
diff --git a/builtin.h b/builtin.h
index b9122bc5f4..67f80519da 100644
--- a/builtin.h
+++ b/builtin.h
@@ -60,6 +60,7 @@ extern int cmd_diff_files(int argc, const char **argv, const char *prefix);
 extern int cmd_diff_index(int argc, const char **argv, const char *prefix);
 extern int cmd_diff(int argc, const char **argv, const char *prefix);
 extern int cmd_diff_tree(int argc, const char **argv, const char *prefix);
+extern int cmd_difftool(int argc, const char **argv, const char *prefix);
 extern int cmd_fast_export(int argc, const char **argv, const char *prefix);
 extern int cmd_fetch(int argc, const char **argv, const char *prefix);
 extern int cmd_fetch_pack(int argc, const char **argv, const char *prefix);
diff --git a/builtin/difftool.c b/builtin/difftool.c
new file mode 100644
index 0000000000..b5e85ab079
--- /dev/null
+++ b/builtin/difftool.c
@@ -0,0 +1,692 @@
+/*
+ * "git difftool" builtin command
+ *
+ * This is a wrapper around the GIT_EXTERNAL_DIFF-compatible
+ * git-difftool--helper script.
+ *
+ * This script exports GIT_EXTERNAL_DIFF and GIT_PAGER for use by git.
+ * The GIT_DIFF* variables are exported for use by git-difftool--helper.
+ *
+ * Any arguments that are unknown to this script are forwarded to 'git diff'.
+ *
+ * Copyright (C) 2016 Johannes Schindelin
+ */
+#include "cache.h"
+#include "builtin.h"
+#include "run-command.h"
+#include "exec_cmd.h"
+#include "parse-options.h"
+#include "argv-array.h"
+#include "strbuf.h"
+#include "lockfile.h"
+#include "dir.h"
+
+static char *diff_gui_tool;
+static int trust_exit_code;
+
+static const char *const builtin_difftool_usage[] = {
+	N_("git difftool [<options>] [<commit> [<commit>]] [--] [<path>...]"),
+	NULL
+};
+
+static int difftool_config(const char *var, const char *value, void *cb)
+{
+	if (!strcmp(var, "diff.guitool")) {
+		diff_gui_tool = xstrdup(value);
+		return 0;
+	}
+
+	if (!strcmp(var, "difftool.trustexitcode")) {
+		trust_exit_code = git_config_bool(var, value);
+		return 0;
+	}
+
+	return git_default_config(var, value, cb);
+}
+
+static int print_tool_help(void)
+{
+	const char *argv[] = { "mergetool", "--tool-help=diff", NULL };
+	return run_command_v_opt(argv, RUN_GIT_CMD);
+}
+
+static int parse_index_info(char *p, int *mode1, int *mode2,
+			    struct object_id *oid1, struct object_id *oid2,
+			    char *status)
+{
+	if (*p != ':')
+		return error("expected ':', got '%c'", *p);
+	*mode1 = (int)strtol(p + 1, &p, 8);
+	if (*p != ' ')
+		return error("expected ' ', got '%c'", *p);
+	*mode2 = (int)strtol(p + 1, &p, 8);
+	if (*p != ' ')
+		return error("expected ' ', got '%c'", *p);
+	if (get_oid_hex(++p, oid1))
+		return error("expected object ID, got '%s'", p + 1);
+	p += GIT_SHA1_HEXSZ;
+	if (*p != ' ')
+		return error("expected ' ', got '%c'", *p);
+	if (get_oid_hex(++p, oid2))
+		return error("expected object ID, got '%s'", p + 1);
+	p += GIT_SHA1_HEXSZ;
+	if (*p != ' ')
+		return error("expected ' ', got '%c'", *p);
+	*status = *++p;
+	if (!*status)
+		return error("missing status");
+	if (p[1] && !isdigit(p[1]))
+		return error("unexpected trailer: '%s'", p + 1);
+	return 0;
+}
+
+/*
+ * Remove any trailing slash from $workdir
+ * before starting to avoid double slashes in symlink targets.
+ */
+static void add_path(struct strbuf *buf, size_t base_len, const char *path)
+{
+	strbuf_setlen(buf, base_len);
+	if (buf->len && buf->buf[buf->len - 1] != '/')
+		strbuf_addch(buf, '/');
+	strbuf_addstr(buf, path);
+}
+
+/*
+ * Determine whether we can simply reuse the file in the worktree.
+ */
+static int use_wt_file(const char *workdir, const char *name,
+		       struct object_id *oid)
+{
+	struct strbuf buf = STRBUF_INIT;
+	struct stat st;
+	int use = 0;
+
+	strbuf_addstr(&buf, workdir);
+	add_path(&buf, buf.len, name);
+
+	if (!lstat(buf.buf, &st) && !S_ISLNK(st.st_mode)) {
+		struct object_id wt_oid;
+		int fd = open(buf.buf, O_RDONLY);
+
+		if (fd >= 0 &&
+		    !index_fd(wt_oid.hash, fd, &st, OBJ_BLOB, name, 0)) {
+			if (is_null_oid(oid)) {
+				oidcpy(oid, &wt_oid);
+				use = 1;
+			} else if (!oidcmp(oid, &wt_oid))
+				use = 1;
+		}
+	}
+
+	strbuf_release(&buf);
+
+	return use;
+}
+
+struct working_tree_entry {
+	struct hashmap_entry entry;
+	char path[FLEX_ARRAY];
+};
+
+static int working_tree_entry_cmp(struct working_tree_entry *a,
+				  struct working_tree_entry *b, void *keydata)
+{
+	return strcmp(a->path, b->path);
+}
+
+/*
+ * The `left` and `right` entries hold paths for the symlinks hashmap,
+ * and a SHA-1 surrounded by brief text for submodules.
+ */
+struct pair_entry {
+	struct hashmap_entry entry;
+	char left[PATH_MAX], right[PATH_MAX];
+	const char path[FLEX_ARRAY];
+};
+
+static int pair_cmp(struct pair_entry *a, struct pair_entry *b, void *keydata)
+{
+	return strcmp(a->path, b->path);
+}
+
+static void add_left_or_right(struct hashmap *map, const char *path,
+			      const char *content, int is_right)
+{
+	struct pair_entry *e, *existing;
+
+	FLEX_ALLOC_STR(e, path, path);
+	hashmap_entry_init(e, strhash(path));
+	existing = hashmap_get(map, e, NULL);
+	if (existing) {
+		free(e);
+		e = existing;
+	} else {
+		e->left[0] = e->right[0] = '\0';
+		hashmap_add(map, e);
+	}
+	strlcpy(is_right ? e->right : e->left, content, PATH_MAX);
+}
+
+struct path_entry {
+	struct hashmap_entry entry;
+	char path[FLEX_ARRAY];
+};
+
+static int path_entry_cmp(struct path_entry *a, struct path_entry *b, void *key)
+{
+	return strcmp(a->path, key ? key : b->path);
+}
+
+static void changed_files(struct hashmap *result, const char *index_path,
+			  const char *workdir)
+{
+	struct child_process update_index = CHILD_PROCESS_INIT;
+	struct child_process diff_files = CHILD_PROCESS_INIT;
+	struct strbuf index_env = STRBUF_INIT, buf = STRBUF_INIT;
+	const char *git_dir = absolute_path(get_git_dir()), *env[] = {
+		NULL, NULL
+	};
+	FILE *fp;
+
+	strbuf_addf(&index_env, "GIT_INDEX_FILE=%s", index_path);
+	env[0] = index_env.buf;
+
+	argv_array_pushl(&update_index.args,
+			 "--git-dir", git_dir, "--work-tree", workdir,
+			 "update-index", "--really-refresh", "-q",
+			 "--unmerged", NULL);
+	update_index.no_stdin = 1;
+	update_index.no_stdout = 1;
+	update_index.no_stderr = 1;
+	update_index.git_cmd = 1;
+	update_index.use_shell = 0;
+	update_index.clean_on_exit = 1;
+	update_index.dir = workdir;
+	update_index.env = env;
+	/* Ignore any errors of update-index */
+	run_command(&update_index);
+
+	argv_array_pushl(&diff_files.args,
+			 "--git-dir", git_dir, "--work-tree", workdir,
+			 "diff-files", "--name-only", "-z", NULL);
+	diff_files.no_stdin = 1;
+	diff_files.git_cmd = 1;
+	diff_files.use_shell = 0;
+	diff_files.clean_on_exit = 1;
+	diff_files.out = -1;
+	diff_files.dir = workdir;
+	diff_files.env = env;
+	if (start_command(&diff_files))
+		die("could not obtain raw diff");
+	fp = xfdopen(diff_files.out, "r");
+	while (!strbuf_getline_nul(&buf, fp)) {
+		struct path_entry *entry;
+		FLEX_ALLOC_STR(entry, path, buf.buf);
+		hashmap_entry_init(entry, strhash(buf.buf));
+		hashmap_add(result, entry);
+	}
+	if (finish_command(&diff_files))
+		die("diff-files did not exit properly");
+	strbuf_release(&index_env);
+	strbuf_release(&buf);
+}
+
+static NORETURN void exit_cleanup(const char *tmpdir, int exit_code)
+{
+	struct strbuf buf = STRBUF_INIT;
+	strbuf_addstr(&buf, tmpdir);
+	remove_dir_recursively(&buf, 0);
+	if (exit_code)
+		warning(_("failed: %d"), exit_code);
+	exit(exit_code);
+}
+
+static int ensure_leading_directories(char *path)
+{
+	switch (safe_create_leading_directories(path)) {
+		case SCLD_OK:
+		case SCLD_EXISTS:
+			return 0;
+		default:
+			return error(_("could not create leading directories "
+				       "of '%s'"), path);
+	}
+}
+
+static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
+			int argc, const char **argv)
+{
+	char tmpdir[PATH_MAX];
+	struct strbuf info = STRBUF_INIT, lpath = STRBUF_INIT;
+	struct strbuf rpath = STRBUF_INIT, buf = STRBUF_INIT;
+	struct strbuf ldir = STRBUF_INIT, rdir = STRBUF_INIT;
+	struct strbuf wtdir = STRBUF_INIT;
+	size_t ldir_len, rdir_len, wtdir_len;
+	struct cache_entry *ce = xcalloc(1, sizeof(ce) + PATH_MAX + 1);
+	const char *workdir, *tmp;
+	int ret = 0, i;
+	FILE *fp;
+	struct hashmap working_tree_dups, submodules, symlinks2;
+	struct hashmap_iter iter;
+	struct pair_entry *entry;
+	enum object_type type;
+	unsigned long size;
+	struct index_state wtindex;
+	struct checkout lstate, rstate;
+	int rc, flags = RUN_GIT_CMD, err = 0;
+	struct child_process child = CHILD_PROCESS_INIT;
+	const char *helper_argv[] = { "difftool--helper", NULL, NULL, NULL };
+	struct hashmap wt_modified, tmp_modified;
+	int indices_loaded = 0;
+
+	workdir = get_git_work_tree();
+
+	/* Setup temp directories */
+	tmp = getenv("TMPDIR");
+	xsnprintf(tmpdir, sizeof(tmpdir), "%s/git-difftool.XXXXXX", tmp ? tmp : "/tmp");
+	if (!mkdtemp(tmpdir))
+		return error("could not create '%s'", tmpdir);
+	strbuf_addf(&ldir, "%s/left/", tmpdir);
+	strbuf_addf(&rdir, "%s/right/", tmpdir);
+	strbuf_addstr(&wtdir, workdir);
+	if (!wtdir.len || !is_dir_sep(wtdir.buf[wtdir.len - 1]))
+		strbuf_addch(&wtdir, '/');
+	mkdir(ldir.buf, 0700);
+	mkdir(rdir.buf, 0700);
+
+	memset(&wtindex, 0, sizeof(wtindex));
+
+	memset(&lstate, 0, sizeof(lstate));
+	lstate.base_dir = ldir.buf;
+	lstate.base_dir_len = ldir.len;
+	lstate.force = 1;
+	memset(&rstate, 0, sizeof(rstate));
+	rstate.base_dir = rdir.buf;
+	rstate.base_dir_len = rdir.len;
+	rstate.force = 1;
+
+	ldir_len = ldir.len;
+	rdir_len = rdir.len;
+	wtdir_len = wtdir.len;
+
+	hashmap_init(&working_tree_dups,
+		     (hashmap_cmp_fn)working_tree_entry_cmp, 0);
+	hashmap_init(&submodules, (hashmap_cmp_fn)pair_cmp, 0);
+	hashmap_init(&symlinks2, (hashmap_cmp_fn)pair_cmp, 0);
+
+	child.no_stdin = 1;
+	child.git_cmd = 1;
+	child.use_shell = 0;
+	child.clean_on_exit = 1;
+	child.dir = prefix;
+	child.out = -1;
+	argv_array_pushl(&child.args, "diff", "--raw", "--no-abbrev", "-z",
+			 NULL);
+	for (i = 0; i < argc; i++)
+		argv_array_push(&child.args, argv[i]);
+	if (start_command(&child))
+		die("could not obtain raw diff");
+	fp = xfdopen(child.out, "r");
+
+	/* Build index info for left and right sides of the diff */
+	i = 0;
+	while (!strbuf_getline_nul(&info, fp)) {
+		int lmode, rmode;
+		struct object_id loid, roid;
+		char status;
+		const char *src_path, *dst_path;
+		size_t src_path_len, dst_path_len;
+
+		if (starts_with(info.buf, "::"))
+			die(N_("combined diff formats('-c' and '--cc') are "
+			       "not supported in\n"
+			       "directory diff mode('-d' and '--dir-diff')."));
+
+		if (parse_index_info(info.buf, &lmode, &rmode, &loid, &roid,
+				     &status))
+			break;
+		if (strbuf_getline_nul(&lpath, fp))
+			break;
+		src_path = lpath.buf;
+		src_path_len = lpath.len;
+
+		i++;
+		if (status != 'C' && status != 'R') {
+			dst_path = src_path;
+			dst_path_len = src_path_len;
+		} else {
+			if (strbuf_getline_nul(&rpath, fp))
+				break;
+			dst_path = rpath.buf;
+			dst_path_len = rpath.len;
+		}
+
+		if (S_ISGITLINK(lmode) || S_ISGITLINK(rmode)) {
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "Subproject commit %s",
+				    oid_to_hex(&loid));
+			add_left_or_right(&submodules, src_path, buf.buf, 0);
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "Subproject commit %s",
+				    oid_to_hex(&roid));
+			if (!oidcmp(&loid, &roid))
+				strbuf_addstr(&buf, "-dirty");
+			add_left_or_right(&submodules, dst_path, buf.buf, 1);
+			continue;
+		}
+
+		if (S_ISLNK(lmode)) {
+			char *content = read_sha1_file(loid.hash, &type, &size);
+			add_left_or_right(&symlinks2, src_path, content, 0);
+			free(content);
+		}
+
+		if (S_ISLNK(rmode)) {
+			char *content = read_sha1_file(roid.hash, &type, &size);
+			add_left_or_right(&symlinks2, dst_path, content, 1);
+			free(content);
+		}
+
+		if (lmode && status != 'C') {
+			ce->ce_mode = lmode;
+			oidcpy(&ce->oid, &loid);
+			strcpy(ce->name, src_path);
+			ce->ce_namelen = src_path_len;
+			if (checkout_entry(ce, &lstate, NULL))
+				return error("could not write '%s'", src_path);
+		}
+
+		if (rmode) {
+			struct working_tree_entry *entry;
+
+			/* Avoid duplicate working_tree entries */
+			FLEX_ALLOC_STR(entry, path, dst_path);
+			hashmap_entry_init(entry, strhash(dst_path));
+			if (hashmap_get(&working_tree_dups, entry, NULL)) {
+				free(entry);
+				continue;
+			}
+			hashmap_add(&working_tree_dups, entry);
+
+			if (!use_wt_file(workdir, dst_path, &roid)) {
+				ce->ce_mode = rmode;
+				oidcpy(&ce->oid, &roid);
+				strcpy(ce->name, dst_path);
+				ce->ce_namelen = dst_path_len;
+				if (checkout_entry(ce, &rstate, NULL))
+					return error("could not write '%s'",
+						     dst_path);
+			} else if (!is_null_oid(&roid)) {
+				/*
+				 * Changes in the working tree need special
+				 * treatment since they are not part of the
+				 * index.
+				 */
+				struct cache_entry *ce2 =
+					make_cache_entry(rmode, roid.hash,
+							 dst_path, 0, 0);
+
+				add_index_entry(&wtindex, ce2,
+						ADD_CACHE_JUST_APPEND);
+
+				add_path(&rdir, rdir_len, dst_path);
+				if (ensure_leading_directories(rdir.buf))
+					return error("could not create "
+						     "directory for '%s'",
+						     dst_path);
+				add_path(&wtdir, wtdir_len, dst_path);
+				if (symlinks) {
+					if (symlink(wtdir.buf, rdir.buf)) {
+						ret = error_errno("could not symlink '%s' to '%s'", wtdir.buf, rdir.buf);
+						goto finish;
+					}
+				} else {
+					struct stat st;
+					if (stat(wtdir.buf, &st))
+						st.st_mode = 0644;
+					if (copy_file(rdir.buf, wtdir.buf,
+						      st.st_mode)) {
+						ret = error("could not copy '%s' to '%s'", wtdir.buf, rdir.buf);
+						goto finish;
+					}
+				}
+			}
+		}
+	}
+
+	if (finish_command(&child)) {
+		ret = error("error occurred running diff --raw");
+		goto finish;
+	}
+
+	if (!i)
+		return 0;
+
+	/*
+	 * Changes to submodules require special treatment.This loop writes a
+	 * temporary file to both the left and right directories to show the
+	 * change in the recorded SHA1 for the submodule.
+	 */
+	hashmap_iter_init(&submodules, &iter);
+	while ((entry = hashmap_iter_next(&iter))) {
+		if (*entry->left) {
+			add_path(&ldir, ldir_len, entry->path);
+			ensure_leading_directories(ldir.buf);
+			write_file(ldir.buf, "%s", entry->left);
+		}
+		if (*entry->right) {
+			add_path(&rdir, rdir_len, entry->path);
+			ensure_leading_directories(rdir.buf);
+			write_file(rdir.buf, "%s", entry->right);
+		}
+	}
+
+	/*
+	 * Symbolic links require special treatment.The standard "git diff"
+	 * shows only the link itself, not the contents of the link target.
+	 * This loop replicates that behavior.
+	 */
+	hashmap_iter_init(&symlinks2, &iter);
+	while ((entry = hashmap_iter_next(&iter))) {
+		if (*entry->left) {
+			add_path(&ldir, ldir_len, entry->path);
+			ensure_leading_directories(ldir.buf);
+			write_file(ldir.buf, "%s", entry->left);
+		}
+		if (*entry->right) {
+			add_path(&rdir, rdir_len, entry->path);
+			ensure_leading_directories(rdir.buf);
+			write_file(rdir.buf, "%s", entry->right);
+		}
+	}
+
+	strbuf_release(&buf);
+
+	strbuf_setlen(&ldir, ldir_len);
+	helper_argv[1] = ldir.buf;
+	strbuf_setlen(&rdir, rdir_len);
+	helper_argv[2] = rdir.buf;
+
+	if (extcmd) {
+		helper_argv[0] = extcmd;
+		flags = 0;
+	} else
+		setenv("GIT_DIFFTOOL_DIRDIFF", "true", 1);
+	rc = run_command_v_opt(helper_argv, flags);
+
+	/*
+	 * If the diff includes working copy files and those
+	 * files were modified during the diff, then the changes
+	 * should be copied back to the working tree.
+	 * Do not copy back files when symlinks are used and the
+	 * external tool did not replace the original link with a file.
+	 *
+	 * These hashes are loaded lazily since they aren't needed
+	 * in the common case of --symlinks and the difftool updating
+	 * files through the symlink.
+	 */
+	hashmap_init(&wt_modified, (hashmap_cmp_fn)path_entry_cmp,
+		     wtindex.cache_nr);
+	hashmap_init(&tmp_modified, (hashmap_cmp_fn)path_entry_cmp,
+		     wtindex.cache_nr);
+
+	for (i = 0; i < wtindex.cache_nr; i++) {
+		struct hashmap_entry dummy;
+		const char *name = wtindex.cache[i]->name;
+		struct stat st;
+
+		add_path(&rdir, rdir_len, name);
+		if (lstat(rdir.buf, &st))
+			continue;
+
+		if ((symlinks && S_ISLNK(st.st_mode)) || !S_ISREG(st.st_mode))
+			continue;
+
+		if (!indices_loaded) {
+			static struct lock_file lock;
+			strbuf_reset(&buf);
+			strbuf_addf(&buf, "%s/wtindex", tmpdir);
+			if (hold_lock_file_for_update(&lock, buf.buf, 0) < 0 ||
+			    write_locked_index(&wtindex, &lock, COMMIT_LOCK)) {
+				ret = error("could not write %s", buf.buf);
+				rollback_lock_file(&lock);
+				goto finish;
+			}
+			changed_files(&wt_modified, buf.buf, workdir);
+			strbuf_setlen(&rdir, rdir_len);
+			changed_files(&tmp_modified, buf.buf, rdir.buf);
+			add_path(&rdir, rdir_len, name);
+			indices_loaded = 1;
+		}
+
+		hashmap_entry_init(&dummy, strhash(name));
+		if (hashmap_get(&tmp_modified, &dummy, name)) {
+			add_path(&wtdir, wtdir_len, name);
+			if (hashmap_get(&wt_modified, &dummy, name)) {
+				warning(_("both files modified: '%s' and '%s'."),
+					wtdir.buf, rdir.buf);
+				warning(_("working tree file has been left."));
+				warning("%s", "");
+				err = 1;
+			} else if (unlink(wtdir.buf) ||
+				   copy_file(wtdir.buf, rdir.buf, st.st_mode))
+				warning_errno(_("could not copy '%s' to '%s'"),
+					      rdir.buf, wtdir.buf);
+		}
+	}
+
+	if (err) {
+		warning(_("temporary files exist in '%s'."), tmpdir);
+		warning(_("you may want to cleanup or recover these."));
+		exit(1);
+	} else
+		exit_cleanup(tmpdir, rc);
+
+finish:
+	free(ce);
+	strbuf_release(&ldir);
+	strbuf_release(&rdir);
+	strbuf_release(&wtdir);
+	strbuf_release(&buf);
+
+	return ret;
+}
+
+static int run_file_diff(int prompt, const char *prefix,
+			 int argc, const char **argv)
+{
+	struct argv_array args = ARGV_ARRAY_INIT;
+	const char *env[] = {
+		"GIT_PAGER=", "GIT_EXTERNAL_DIFF=git-difftool--helper", NULL,
+		NULL
+	};
+	int ret = 0, i;
+
+	if (prompt > 0)
+		env[2] = "GIT_DIFFTOOL_PROMPT=true";
+	else if (!prompt)
+		env[2] = "GIT_DIFFTOOL_NO_PROMPT=true";
+
+
+	argv_array_push(&args, "diff");
+	for (i = 0; i < argc; i++)
+		argv_array_push(&args, argv[i]);
+	ret = run_command_v_opt_cd_env(args.argv, RUN_GIT_CMD, prefix, env);
+	exit(ret);
+}
+
+int cmd_difftool(int argc, const char **argv, const char *prefix)
+{
+	int use_gui_tool = 0, dir_diff = 0, prompt = -1, symlinks = 0,
+	    tool_help = 0;
+	static char *difftool_cmd = NULL, *extcmd = NULL;
+	struct option builtin_difftool_options[] = {
+		OPT_BOOL('g', "gui", &use_gui_tool,
+			 N_("use `diff.guitool` instead of `diff.tool`")),
+		OPT_BOOL('d', "dir-diff", &dir_diff,
+			 N_("perform a full-directory diff")),
+		{ OPTION_SET_INT, 'y', "no-prompt", &prompt, NULL,
+			N_("do not prompt before launching a diff tool"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG, NULL, 0},
+		{ OPTION_SET_INT, 0, "prompt", &prompt, NULL, NULL,
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG | PARSE_OPT_HIDDEN,
+			NULL, 1 },
+		OPT_BOOL(0, "symlinks", &symlinks,
+			 N_("use symlinks in dir-diff mode")),
+		OPT_STRING('t', "tool", &difftool_cmd, N_("<tool>"),
+			   N_("use the specified diff tool")),
+		OPT_BOOL(0, "tool-help", &tool_help,
+			 N_("print a list of diff tools that may be used with "
+			    "`--tool`")),
+		OPT_BOOL(0, "trust-exit-code", &trust_exit_code,
+			 N_("make 'git-difftool' exit when an invoked diff "
+			    "tool returns a non - zero exit code")),
+		OPT_STRING('x', "extcmd", &extcmd, N_("<command>"),
+			   N_("specify a custom command for viewing diffs")),
+		OPT_END()
+	};
+
+	/* NEEDSWORK: once we no longer spawn anything, remove this */
+	setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
+	setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
+
+	git_config(difftool_config, NULL);
+	symlinks = has_symlinks;
+
+	argc = parse_options(argc, argv, prefix, builtin_difftool_options,
+			     builtin_difftool_usage, PARSE_OPT_KEEP_UNKNOWN |
+			     PARSE_OPT_KEEP_DASHDASH);
+
+	if (tool_help)
+		return print_tool_help();
+
+	if (use_gui_tool && diff_gui_tool && *diff_gui_tool)
+		setenv("GIT_DIFF_TOOL", diff_gui_tool, 1);
+	else if (difftool_cmd) {
+		if (*difftool_cmd)
+			setenv("GIT_DIFF_TOOL", difftool_cmd, 1);
+		else
+			die(_("no <tool> given for --tool=<tool>"));
+	}
+
+	if (extcmd) {
+		if (*extcmd)
+			setenv("GIT_DIFFTOOL_EXTCMD", extcmd, 1);
+		else
+			die(_("no <cmd> given for --extcmd=<cmd>"));
+	}
+
+	setenv("GIT_DIFFTOOL_TRUST_EXIT_CODE",
+	       trust_exit_code ? "true" : "false", 1);
+
+	/*
+	 * In directory diff mode, 'git-difftool--helper' is called once
+	 * to compare the a / b directories. In file diff mode, 'git diff'
+	 * will invoke a separate instance of 'git-difftool--helper' for
+	 * each file that changed.
+	 */
+	if (dir_diff)
+		return run_dir_diff(extcmd, symlinks, prefix, argc, argv);
+	return run_file_diff(prompt, prefix, argc, argv);
+}
diff --git a/git-difftool.perl b/contrib/examples/git-difftool.perl
index df59bdfe97..df59bdfe97 100755
--- a/git-difftool.perl
+++ b/contrib/examples/git-difftool.perl
diff --git a/git.c b/git.c
index b367cf6686..1cf125cd28 100644
--- a/git.c
+++ b/git.c
@@ -424,6 +424,7 @@ static struct cmd_struct commands[] = {
 	{ "diff-files", cmd_diff_files, RUN_SETUP | NEED_WORK_TREE },
 	{ "diff-index", cmd_diff_index, RUN_SETUP },
 	{ "diff-tree", cmd_diff_tree, RUN_SETUP },
+	{ "difftool", cmd_difftool, RUN_SETUP | NEED_WORK_TREE },
 	{ "fast-export", cmd_fast_export, RUN_SETUP },
 	{ "fetch", cmd_fetch, RUN_SETUP },
 	{ "fetch-pack", cmd_fetch_pack, RUN_SETUP },
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 99d4123461..aa0ef02597 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -24,7 +24,7 @@ prompt_given ()
 }
 
 # Create a file on master and change it on branch
-test_expect_success PERL 'setup' '
+test_expect_success 'setup' '
 	echo master >file &&
 	git add file &&
 	git commit -m "added file" &&
@@ -36,7 +36,7 @@ test_expect_success PERL 'setup' '
 '
 
 # Configure a custom difftool.<tool>.cmd and use it
-test_expect_success PERL 'custom commands' '
+test_expect_success 'custom commands' '
 	difftool_test_setup &&
 	test_config difftool.test-tool.cmd "cat \"\$REMOTE\"" &&
 	echo master >expect &&
@@ -49,21 +49,21 @@ test_expect_success PERL 'custom commands' '
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'custom tool commands override built-ins' '
+test_expect_success 'custom tool commands override built-ins' '
 	test_config difftool.vimdiff.cmd "cat \"\$REMOTE\"" &&
 	echo master >expect &&
 	git difftool --tool vimdiff --no-prompt branch >actual &&
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool ignores bad --tool values' '
+test_expect_success 'difftool ignores bad --tool values' '
 	: >expect &&
 	test_must_fail \
 		git difftool --no-prompt --tool=bad-tool branch >actual &&
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool forwards arguments to diff' '
+test_expect_success 'difftool forwards arguments to diff' '
 	difftool_test_setup &&
 	>for-diff &&
 	git add for-diff &&
@@ -76,40 +76,40 @@ test_expect_success PERL 'difftool forwards arguments to diff' '
 	rm for-diff
 '
 
-test_expect_success PERL 'difftool ignores exit code' '
+test_expect_success 'difftool ignores exit code' '
 	test_config difftool.error.cmd false &&
 	git difftool -y -t error branch
 '
 
-test_expect_success PERL 'difftool forwards exit code with --trust-exit-code' '
+test_expect_success 'difftool forwards exit code with --trust-exit-code' '
 	test_config difftool.error.cmd false &&
 	test_must_fail git difftool -y --trust-exit-code -t error branch
 '
 
-test_expect_success PERL 'difftool forwards exit code with --trust-exit-code for built-ins' '
+test_expect_success 'difftool forwards exit code with --trust-exit-code for built-ins' '
 	test_config difftool.vimdiff.path false &&
 	test_must_fail git difftool -y --trust-exit-code -t vimdiff branch
 '
 
-test_expect_success PERL 'difftool honors difftool.trustExitCode = true' '
+test_expect_success 'difftool honors difftool.trustExitCode = true' '
 	test_config difftool.error.cmd false &&
 	test_config difftool.trustExitCode true &&
 	test_must_fail git difftool -y -t error branch
 '
 
-test_expect_success PERL 'difftool honors difftool.trustExitCode = false' '
+test_expect_success 'difftool honors difftool.trustExitCode = false' '
 	test_config difftool.error.cmd false &&
 	test_config difftool.trustExitCode false &&
 	git difftool -y -t error branch
 '
 
-test_expect_success PERL 'difftool ignores exit code with --no-trust-exit-code' '
+test_expect_success 'difftool ignores exit code with --no-trust-exit-code' '
 	test_config difftool.error.cmd false &&
 	test_config difftool.trustExitCode true &&
 	git difftool -y --no-trust-exit-code -t error branch
 '
 
-test_expect_success PERL 'difftool stops on error with --trust-exit-code' '
+test_expect_success 'difftool stops on error with --trust-exit-code' '
 	test_when_finished "rm -f for-diff .git/fail-right-file" &&
 	test_when_finished "git reset -- for-diff" &&
 	write_script .git/fail-right-file <<-\EOF &&
@@ -124,13 +124,13 @@ test_expect_success PERL 'difftool stops on error with --trust-exit-code' '
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool honors exit status if command not found' '
+test_expect_success 'difftool honors exit status if command not found' '
 	test_config difftool.nonexistent.cmd i-dont-exist &&
 	test_config difftool.trustExitCode false &&
 	test_must_fail git difftool -y -t nonexistent branch
 '
 
-test_expect_success PERL 'difftool honors --gui' '
+test_expect_success 'difftool honors --gui' '
 	difftool_test_setup &&
 	test_config merge.tool bogus-tool &&
 	test_config diff.tool bogus-tool &&
@@ -141,7 +141,7 @@ test_expect_success PERL 'difftool honors --gui' '
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool --gui last setting wins' '
+test_expect_success 'difftool --gui last setting wins' '
 	difftool_test_setup &&
 	: >expect &&
 	git difftool --no-prompt --gui --no-gui >actual &&
@@ -155,7 +155,7 @@ test_expect_success PERL 'difftool --gui last setting wins' '
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool --gui works without configured diff.guitool' '
+test_expect_success 'difftool --gui works without configured diff.guitool' '
 	difftool_test_setup &&
 	echo branch >expect &&
 	git difftool --no-prompt --gui branch >actual &&
@@ -163,7 +163,7 @@ test_expect_success PERL 'difftool --gui works without configured diff.guitool'
 '
 
 # Specify the diff tool using $GIT_DIFF_TOOL
-test_expect_success PERL 'GIT_DIFF_TOOL variable' '
+test_expect_success 'GIT_DIFF_TOOL variable' '
 	difftool_test_setup &&
 	git config --unset diff.tool &&
 	echo branch >expect &&
@@ -173,7 +173,7 @@ test_expect_success PERL 'GIT_DIFF_TOOL variable' '
 
 # Test the $GIT_*_TOOL variables and ensure
 # that $GIT_DIFF_TOOL always wins unless --tool is specified
-test_expect_success PERL 'GIT_DIFF_TOOL overrides' '
+test_expect_success 'GIT_DIFF_TOOL overrides' '
 	difftool_test_setup &&
 	test_config diff.tool bogus-tool &&
 	test_config merge.tool bogus-tool &&
@@ -191,7 +191,7 @@ test_expect_success PERL 'GIT_DIFF_TOOL overrides' '
 
 # Test that we don't have to pass --no-prompt to difftool
 # when $GIT_DIFFTOOL_NO_PROMPT is true
-test_expect_success PERL 'GIT_DIFFTOOL_NO_PROMPT variable' '
+test_expect_success 'GIT_DIFFTOOL_NO_PROMPT variable' '
 	difftool_test_setup &&
 	echo branch >expect &&
 	GIT_DIFFTOOL_NO_PROMPT=true git difftool branch >actual &&
@@ -200,7 +200,7 @@ test_expect_success PERL 'GIT_DIFFTOOL_NO_PROMPT variable' '
 
 # git-difftool supports the difftool.prompt variable.
 # Test that GIT_DIFFTOOL_PROMPT can override difftool.prompt = false
-test_expect_success PERL 'GIT_DIFFTOOL_PROMPT variable' '
+test_expect_success 'GIT_DIFFTOOL_PROMPT variable' '
 	difftool_test_setup &&
 	test_config difftool.prompt false &&
 	echo >input &&
@@ -210,7 +210,7 @@ test_expect_success PERL 'GIT_DIFFTOOL_PROMPT variable' '
 '
 
 # Test that we don't have to pass --no-prompt when difftool.prompt is false
-test_expect_success PERL 'difftool.prompt config variable is false' '
+test_expect_success 'difftool.prompt config variable is false' '
 	difftool_test_setup &&
 	test_config difftool.prompt false &&
 	echo branch >expect &&
@@ -219,7 +219,7 @@ test_expect_success PERL 'difftool.prompt config variable is false' '
 '
 
 # Test that we don't have to pass --no-prompt when mergetool.prompt is false
-test_expect_success PERL 'difftool merge.prompt = false' '
+test_expect_success 'difftool merge.prompt = false' '
 	difftool_test_setup &&
 	test_might_fail git config --unset difftool.prompt &&
 	test_config mergetool.prompt false &&
@@ -229,7 +229,7 @@ test_expect_success PERL 'difftool merge.prompt = false' '
 '
 
 # Test that the -y flag can override difftool.prompt = true
-test_expect_success PERL 'difftool.prompt can overridden with -y' '
+test_expect_success 'difftool.prompt can overridden with -y' '
 	difftool_test_setup &&
 	test_config difftool.prompt true &&
 	echo branch >expect &&
@@ -238,7 +238,7 @@ test_expect_success PERL 'difftool.prompt can overridden with -y' '
 '
 
 # Test that the --prompt flag can override difftool.prompt = false
-test_expect_success PERL 'difftool.prompt can overridden with --prompt' '
+test_expect_success 'difftool.prompt can overridden with --prompt' '
 	difftool_test_setup &&
 	test_config difftool.prompt false &&
 	echo >input &&
@@ -248,7 +248,7 @@ test_expect_success PERL 'difftool.prompt can overridden with --prompt' '
 '
 
 # Test that the last flag passed on the command-line wins
-test_expect_success PERL 'difftool last flag wins' '
+test_expect_success 'difftool last flag wins' '
 	difftool_test_setup &&
 	echo branch >expect &&
 	git difftool --prompt --no-prompt branch >actual &&
@@ -261,7 +261,7 @@ test_expect_success PERL 'difftool last flag wins' '
 
 # git-difftool falls back to git-mergetool config variables
 # so test that behavior here
-test_expect_success PERL 'difftool + mergetool config variables' '
+test_expect_success 'difftool + mergetool config variables' '
 	test_config merge.tool test-tool &&
 	test_config mergetool.test-tool.cmd "cat \$LOCAL" &&
 	echo branch >expect &&
@@ -275,49 +275,49 @@ test_expect_success PERL 'difftool + mergetool config variables' '
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool.<tool>.path' '
+test_expect_success 'difftool.<tool>.path' '
 	test_config difftool.tkdiff.path echo &&
 	git difftool --tool=tkdiff --no-prompt branch >output &&
 	lines=$(grep file output | wc -l) &&
 	test "$lines" -eq 1
 '
 
-test_expect_success PERL 'difftool --extcmd=cat' '
+test_expect_success 'difftool --extcmd=cat' '
 	echo branch >expect &&
 	echo master >>expect &&
 	git difftool --no-prompt --extcmd=cat branch >actual &&
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool --extcmd cat' '
+test_expect_success 'difftool --extcmd cat' '
 	echo branch >expect &&
 	echo master >>expect &&
 	git difftool --no-prompt --extcmd=cat branch >actual &&
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool -x cat' '
+test_expect_success 'difftool -x cat' '
 	echo branch >expect &&
 	echo master >>expect &&
 	git difftool --no-prompt -x cat branch >actual &&
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool --extcmd echo arg1' '
+test_expect_success 'difftool --extcmd echo arg1' '
 	echo file >expect &&
 	git difftool --no-prompt \
 		--extcmd sh\ -c\ \"echo\ \$1\" branch >actual &&
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool --extcmd cat arg1' '
+test_expect_success 'difftool --extcmd cat arg1' '
 	echo master >expect &&
 	git difftool --no-prompt \
 		--extcmd sh\ -c\ \"cat\ \$1\" branch >actual &&
 	test_cmp expect actual
 '
 
-test_expect_success PERL 'difftool --extcmd cat arg2' '
+test_expect_success 'difftool --extcmd cat arg2' '
 	echo branch >expect &&
 	git difftool --no-prompt \
 		--extcmd sh\ -c\ \"cat\ \$2\" branch >actual &&
@@ -325,7 +325,7 @@ test_expect_success PERL 'difftool --extcmd cat arg2' '
 '
 
 # Create a second file on master and a different version on branch
-test_expect_success PERL 'setup with 2 files different' '
+test_expect_success 'setup with 2 files different' '
 	echo m2 >file2 &&
 	git add file2 &&
 	git commit -m "added file2" &&
@@ -337,7 +337,7 @@ test_expect_success PERL 'setup with 2 files different' '
 	git checkout master
 '
 
-test_expect_success PERL 'say no to the first file' '
+test_expect_success 'say no to the first file' '
 	(echo n && echo) >input &&
 	git difftool -x cat branch <input >output &&
 	grep m2 output &&
@@ -346,7 +346,7 @@ test_expect_success PERL 'say no to the first file' '
 	! grep branch output
 '
 
-test_expect_success PERL 'say no to the second file' '
+test_expect_success 'say no to the second file' '
 	(echo && echo n) >input &&
 	git difftool -x cat branch <input >output &&
 	grep master output &&
@@ -355,7 +355,7 @@ test_expect_success PERL 'say no to the second file' '
 	! grep br2 output
 '
 
-test_expect_success PERL 'ending prompt input with EOF' '
+test_expect_success 'ending prompt input with EOF' '
 	git difftool -x cat branch </dev/null >output &&
 	! grep master output &&
 	! grep branch output &&
@@ -363,12 +363,12 @@ test_expect_success PERL 'ending prompt input with EOF' '
 	! grep br2 output
 '
 
-test_expect_success PERL 'difftool --tool-help' '
+test_expect_success 'difftool --tool-help' '
 	git difftool --tool-help >output &&
 	grep tool output
 '
 
-test_expect_success PERL 'setup change in subdirectory' '
+test_expect_success 'setup change in subdirectory' '
 	git checkout master &&
 	mkdir sub &&
 	echo master >sub/sub &&
@@ -382,11 +382,11 @@ test_expect_success PERL 'setup change in subdirectory' '
 '
 
 run_dir_diff_test () {
-	test_expect_success PERL "$1 --no-symlinks" "
+	test_expect_success "$1 --no-symlinks" "
 		symlinks=--no-symlinks &&
 		$2
 	"
-	test_expect_success PERL,SYMLINKS "$1 --symlinks" "
+	test_expect_success SYMLINKS "$1 --symlinks" "
 		symlinks=--symlinks &&
 		$2
 	"
@@ -508,7 +508,7 @@ do
 done >actual
 EOF
 
-test_expect_success PERL,SYMLINKS 'difftool --dir-diff --symlink without unstaged changes' '
+test_expect_success SYMLINKS 'difftool --dir-diff --symlink without unstaged changes' '
 	cat >expect <<-EOF &&
 	file
 	$PWD/file
@@ -545,7 +545,7 @@ write_script modify-file <<\EOF
 echo "new content" >file
 EOF
 
-test_expect_success PERL 'difftool --no-symlinks does not overwrite working tree file ' '
+test_expect_success 'difftool --no-symlinks does not overwrite working tree file ' '
 	echo "orig content" >file &&
 	git difftool --dir-diff --no-symlinks --extcmd "$PWD/modify-file" branch &&
 	echo "new content" >expect &&
@@ -558,7 +558,7 @@ echo "tmp content" >"$2/file" &&
 echo "$2" >tmpdir
 EOF
 
-test_expect_success PERL 'difftool --no-symlinks detects conflict ' '
+test_expect_success 'difftool --no-symlinks detects conflict ' '
 	(
 		TMPDIR=$TRASH_DIRECTORY &&
 		export TMPDIR &&
@@ -571,7 +571,7 @@ test_expect_success PERL 'difftool --no-symlinks detects conflict ' '
 	)
 '
 
-test_expect_success PERL 'difftool properly honors gitlink and core.worktree' '
+test_expect_success 'difftool properly honors gitlink and core.worktree' '
 	git submodule add ./. submod/ule &&
 	test_config -C submod/ule diff.tool checktrees &&
 	test_config -C submod/ule difftool.checktrees.cmd '\''
@@ -585,7 +585,7 @@ test_expect_success PERL 'difftool properly honors gitlink and core.worktree' '
 	)
 '
 
-test_expect_success PERL,SYMLINKS 'difftool --dir-diff symlinked directories' '
+test_expect_success SYMLINKS 'difftool --dir-diff symlinked directories' '
 	git init dirlinks &&
 	(
 		cd dirlinks &&