Re: [PATCH 1/2] difftool: add the builtin

> This adds a builtin difftool that represents a conversion of the current
> Perl script version of the difftool.
> 
> The motivation is that Perl scripts are not at all native on Windows,
> and that `git difftool` therefore is pretty slow on that platform, when
> there is no good reason for it to be slow.
> 
> In addition, Perl does not really have access to Git's internals. That
> means that any script will always have to jump through unnecessary
> hoops.

> The current version of the builtin difftool does not, however, make full
> use of the internals but instead chooses to spawn a couple of Git
> processes, still, to make for an easier conversion. There remains a lot
> of room for improvement, left for a later date.
> 
> Note: the original difftool is still called by `git difftool`. To get the
> new, experimental version, call `git builtin-difftool`. The reason: this
> new, experimental, builtin difftool will be shipped as part of Git for
> Windows v2.11.0, to allow for easier large-scale testing.

> diff --git a/builtin/builtin-difftool.c b/builtin/builtin-difftool.c
> new file mode 100644
> index 0000000..9feefcd
> --- /dev/null
> +++ b/builtin/builtin-difftool.c
> @@ -0,0 +1,680 @@
> +/*
> + * "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 "parse-options.h"
> +#include "run-command.h"
> +#include "argv-array.h"
> +#include "strbuf.h"
> +#include "lockfile.h"
> +
> +static char *diff_gui_tool;
> +static int trust_exit_code;
> +
> +static const char * const builtin_difftool_usage[] = {
> +	N_("git add [<options>] [--] <pathspec>..."),
> +	NULL
> +};

> [...]
> +static void changed_files(struct hashmap *result, const char *index_path,
> +			  const char *workdir)
> +{
> +[...]
> +}
> +
> +#include "dir.h"

> +static int run_dir_diff(const char *extcmd, int symlinks,
> +			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;
> +
> +	setup_work_tree();
> +	workdir = get_git_work_tree();
> +
> +	/* Setup temp directories */
> +	tmp = getenv("TMPDIR");
> +	sprintf(tmpdir, "%s/git-difftool.XXXXXX", tmp ? tmp : "/tmp");

> +	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, 0777);
> +	mkdir(rdir.buf, 0777);

> [...]
> +int cmd_builtin_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()
> +	};
> +
> +	symlinks = has_symlinks;
> +
> +	git_config(difftool_config, NULL);
> +
> +	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, argc, argv);
> +	return run_file_diff(prompt, argc, argv);
> +}
> diff --git a/git.c b/git.c
> index efa1059..eaa0f67 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 },
> +	{ "builtin-difftool", cmd_builtin_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 },