Merge branch 'builtin-difftool'

This topic branch brings the new, experimental builtin version of the
difftool into Git for Windows' master branch.

It still hands off to the legacy Perl script unless the feature flag is
flipped: only when the config setting difftool.useBuiltin is set to true
will `git difftool` actually use the experimental builtin. The idea is to
play it safe for the majority of users, but to allow heavy difftool users
to test early and to help make the builtin robust, before we actually
retire the Perl script.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.gitignore

@@ -76,6 +76,7 @@
 /git-init-db
 /git-interpret-trailers
 /git-instaweb
+/git-legacy-difftool
 /git-log
 /git-ls-files
 /git-ls-remote

Makefile

@@ -527,7 +527,7 @@ SCRIPT_LIB += git-sh-setup
 SCRIPT_LIB += git-sh-i18n
 
 SCRIPT_PERL += git-add--interactive.perl
-SCRIPT_PERL += git-difftool.perl
+SCRIPT_PERL += git-legacy-difftool.perl
 SCRIPT_PERL += git-archimport.perl
 SCRIPT_PERL += git-cvsexportcommit.perl
 SCRIPT_PERL += git-cvsimport.perl
@@ -888,6 +888,7 @@ 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

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);

builtin/difftool.c

@@ -0,0 +1,731 @@
+/*
+ * "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 || p[1])
+		return error("unexpected trailer: '%s'", p);
+	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 (!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);
+	}
+	strcpy(is_right ? e->right : e->left, content);
+}
+
+struct path_entry {
+	struct hashmap_entry entry;
+	char path[FLEX_ARRAY];
+};
+
+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);
+				ce_mode_from_stat(ce2, rmode);
+
+				add_index_entry(&wtindex, ce2,
+						ADD_CACHE_JUST_APPEND);
+
+				add_path(&wtdir, wtdir_len, dst_path);
+				add_path(&rdir, rdir_len, dst_path);
+				if (ensure_leading_directories(rdir.buf))
+					return error("could not create "
+						     "directory for '%s'",
+						     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("");
+				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);
+}
+
+/*
+ * NEEDSWORK: this function can go once the legacy-difftool Perl script is
+ * retired.
+ *
+ * We intentionally avoid reading the config directly here, to avoid messing up
+ * the GIT_* environment variables when we need to fall back to exec()ing the
+ * Perl script.
+ */
+static int use_builtin_difftool(void) {
+	struct child_process cp = CHILD_PROCESS_INIT;
+	struct strbuf out = STRBUF_INIT;
+	int ret;
+
+	argv_array_pushl(&cp.args,
+			 "config", "--bool", "difftool.usebuiltin", NULL);
+	cp.git_cmd = 1;
+	if (capture_command(&cp, &out, 6))
+		return 0;
+	strbuf_trim(&out);
+	ret = !strcmp("true", out.buf);
+	strbuf_release(&out);
+	return 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 the builtin difftool has been tested enough
+	 * and git-legacy-difftool.perl is retired to contrib/, this preamble
+	 * can be removed.
+	 */
+	if (!use_builtin_difftool()) {
+		const char *path = mkpath("%s/git-legacy-difftool",
+					  git_exec_path());
+
+		if (sane_execvp(path, (char **)argv) < 0)
+			die_errno("could not exec %s", path);
+
+		return 0;
+	}
+	prefix = setup_git_directory();
+	trace_repo_setup(prefix);
+	setup_work_tree();
+	/* 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);
+}

git.c

@@ -424,6 +424,12 @@ 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 },
+	/*
+	 * NEEDSWORK: Once the redirection to git-legacy-difftool.perl in
+	 * builtin/difftool.c has been removed, this entry should be changed to
+	 * RUN_SETUP | NEED_WORK_TREE
+	 */
+	{ "difftool", cmd_difftool },
 	{ "fast-export", cmd_fast_export, RUN_SETUP },
 	{ "fetch", cmd_fetch, RUN_SETUP },
 	{ "fetch-pack", cmd_fetch_pack, RUN_SETUP },

t/t7800-difftool.sh

@@ -23,6 +23,8 @@ prompt_given ()
 	test "$prompt" = "Launch 'test-tool' [Y/n]? branch"
 }
 
+# NEEDSWORK: lose all the PERL prereqs once legacy-difftool is retired.
+
 # Create a file on master and change it on branch
 test_expect_success PERL 'setup' '
 	echo master >file &&