/*

 * "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 "builtin.h"

#include "abspath.h"

#include "config.h"

#include "copy.h"

#include "run-command.h"

#include "environment.h"

#include "gettext.h"

#include "hex.h"

#include "parse-options.h"

#include "read-cache-ll.h"

#include "sparse-index.h"

#include "strvec.h"

#include "strbuf.h"

#include "lockfile.h"

#include "object-file.h"

#include "object-store-ll.h"

#include "dir.h"

#include "entry.h"

#include "setup.h"

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,

         const struct config_context *ctx, void *cb)

{

  if (!strcmp(var, "difftool.trustexitcode")) {

    trust_exit_code = git_config_bool(var, value);

    return 0;

  }

  return git_default_config(var, value, ctx, cb);

}

static int print_tool_help(void)

{

  struct child_process cmd = CHILD_PROCESS_INIT;

  cmd.git_cmd = 1;

  strvec_pushl(&cmd.args, "mergetool", "--tool-help=diff", NULL);

  return run_command(&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 (parse_oid_hex(++p, oid1, (const char **)&p))

    return error("expected object ID, got '%s'", p);

  if (*p != ' ')

    return error("expected ' ', got '%c'", *p);

  if (parse_oid_hex(++p, oid2, (const char **)&p))

    return error("expected object ID, got '%s'", p);

  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(the_repository->index, &wt_oid, fd, &st, OBJ_BLOB, name, 0)) {

      if (is_null_oid(oid)) {

        oidcpy(oid, &wt_oid);

        use = 1;

      } else if (oideq(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(const void *cmp_data UNUSED,

          const struct hashmap_entry *eptr,

          const struct hashmap_entry *entry_or_key,

          const void *keydata UNUSED)

{

  const struct working_tree_entry *a, *b;

  a = container_of(eptr, const struct working_tree_entry, entry);

  b = container_of(entry_or_key, const struct working_tree_entry, entry);

  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(const void *cmp_data UNUSED,

        const struct hashmap_entry *eptr,

        const struct hashmap_entry *entry_or_key,

        const void *keydata UNUSED)

{

  const struct pair_entry *a, *b;

  a = container_of(eptr, const struct pair_entry, entry);

  b = container_of(entry_or_key, const struct pair_entry, entry);

  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->entry, strhash(path));

  existing = hashmap_get_entry(map, e, entry, NULL);

  if (existing) {

    free(e);

    e = existing;

  } else {

    e->left[0] = e->right[0] = '\0';

    hashmap_add(map, &e->entry);

  }

  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(const void *cmp_data UNUSED,

        const struct hashmap_entry *eptr,

        const struct hashmap_entry *entry_or_key,

        const void *key)

{

  const struct path_entry *a, *b;

  a = container_of(eptr, const struct path_entry, entry);

  b = container_of(entry_or_key, const struct path_entry, entry);

  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 buf = STRBUF_INIT;

  const char *git_dir = absolute_path(get_git_dir());

  FILE *fp;

  strvec_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;

  strvec_pushf(&update_index.env, "GIT_INDEX_FILE=%s", index_path);

  /* Ignore any errors of update-index */

  run_command(&update_index);

  strvec_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;

  strvec_pushf(&diff_files.env, "GIT_INDEX_FILE=%s", index_path);

  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->entry, strhash(buf.buf));

    hashmap_add(result, &entry->entry);

  }

  fclose(fp);

  if (finish_command(&diff_files))

    die("diff-files did not exit properly");

  strbuf_release(&buf);

}

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);

  }

}

/*

 * Unconditional writing of a plain regular file is what

 * "git difftool --dir-diff" wants to do for symlinks.  We are preparing two

 * temporary directories to be fed to a Git-unaware tool that knows how to

 * show a diff of two directories (e.g. "diff -r A B").

 *

 * Because the tool is Git-unaware, if a symbolic link appears in either of

 * these temporary directories, it will try to dereference and show the

 * difference of the target of the symbolic link, which is not what we want,

 * as the goal of the dir-diff mode is to produce an output that is logically

 * equivalent to what "git diff" produces.

 *

 * Most importantly, we want to get textual comparison of the result of the

 * readlink(2).  get_symlink() provides that---it returns the contents of

 * the symlink that gets written to a regular file to force the external tool

 * to compare the readlink(2) result as text, even on a filesystem that is

 * capable of doing a symbolic link.

 */

static char *get_symlink(const struct object_id *oid, const char *path)

{

  char *data;

  if (is_null_oid(oid)) {

    /* The symlink is unknown to Git so read from the filesystem */

    struct strbuf link = STRBUF_INIT;

    if (has_symlinks) {

      if (strbuf_readlink(&link, path, strlen(path)))

        die(_("could not read symlink %s"), path);

    } else if (strbuf_read_file(&link, path, 128))

      die(_("could not read symlink file %s"), path);

    data = strbuf_detach(&link, NULL);

  } else {

    enum object_type type;

    unsigned long size;

    data = repo_read_object_file(the_repository, oid, &type,

               &size);

    if (!data)

      die(_("could not read object %s for symlink %s"),

        oid_to_hex(oid), path);

  }

  return data;

}

static int checkout_path(unsigned mode, struct object_id *oid,

       const char *path, const struct checkout *state)

{

  struct cache_entry *ce;

  int ret;

  ce = make_transient_cache_entry(mode, oid, path, 0, NULL);

  ret = checkout_entry(ce, state, NULL, NULL);

  discard_cache_entry(ce);

  return ret;

}

static void write_file_in_directory(struct strbuf *dir, size_t dir_len,

      const char *path, const char *content)

{

  add_path(dir, dir_len, path);

  ensure_leading_directories(dir->buf);

  unlink(dir->buf);

  write_file(dir->buf, "%s", content);

}

/* Write the file contents for the left and right sides of the difftool

 * dir-diff representation for submodules and symlinks. Symlinks and submodules

 * are written as regular text files so that external diff tools can diff them

 * as text files, resulting in behavior that is analogous to to what "git diff"

 * displays for symlink and submodule diffs.

 */

static void write_standin_files(struct pair_entry *entry,

      struct strbuf *ldir, size_t ldir_len,

      struct strbuf *rdir, size_t rdir_len)

{

  if (*entry->left)

    write_file_in_directory(ldir, ldir_len, entry->path, entry->left);

  if (*entry->right)

    write_file_in_directory(rdir, rdir_len, entry->path, entry->right);

}

static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,

      struct child_process *child)

{

  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;

  struct strbuf tmpdir = STRBUF_INIT;

  char *lbase_dir = NULL, *rbase_dir = NULL;

  size_t ldir_len, rdir_len, wtdir_len;

  const char *workdir, *tmp;

  int ret = 0, i;

  FILE *fp = NULL;

  struct hashmap working_tree_dups = HASHMAP_INIT(working_tree_entry_cmp,

              NULL);

  struct hashmap submodules = HASHMAP_INIT(pair_cmp, NULL);

  struct hashmap symlinks2 = HASHMAP_INIT(pair_cmp, NULL);

  struct hashmap_iter iter;

  struct pair_entry *entry;

  struct index_state wtindex = INDEX_STATE_INIT(the_repository);

  struct checkout lstate, rstate;

  int err = 0;

  struct child_process cmd = CHILD_PROCESS_INIT;

  struct hashmap wt_modified, tmp_modified;

  int indices_loaded = 0;

  workdir = get_git_work_tree();

  /* Setup temp directories */

  tmp = getenv("TMPDIR");

  strbuf_add_absolute_path(&tmpdir, tmp ? tmp : "/tmp");

  strbuf_trim_trailing_dir_sep(&tmpdir);

  strbuf_addstr(&tmpdir, "/git-difftool.XXXXXX");

  if (!mkdtemp(tmpdir.buf)) {

    ret = error("could not create '%s'", tmpdir.buf);

    goto finish;

  }

  strbuf_addf(&ldir, "%s/left/", tmpdir.buf);

  strbuf_addf(&rdir, "%s/right/", tmpdir.buf);

  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(&lstate, 0, sizeof(lstate));

  lstate.base_dir = lbase_dir = xstrdup(ldir.buf);

  lstate.base_dir_len = ldir.len;

  lstate.force = 1;

  memset(&rstate, 0, sizeof(rstate));

  rstate.base_dir = rbase_dir = xstrdup(rdir.buf);

  rstate.base_dir_len = rdir.len;

  rstate.force = 1;

  ldir_len = ldir.len;

  rdir_len = rdir.len;

  wtdir_len = wtdir.len;

  child->no_stdin = 1;

  child->git_cmd = 1;

  child->use_shell = 0;

  child->clean_on_exit = 1;

  child->dir = prefix;

  child->out = -1;

  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;

    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;

    i++;

    if (status != 'C' && status != 'R') {

      dst_path = src_path;

    } else {

      if (strbuf_getline_nul(&rpath, fp))

        break;

      dst_path = rpath.buf;

    }

    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 (oideq(&loid, &roid))

        strbuf_addstr(&buf, "-dirty");

      add_left_or_right(&submodules, dst_path, buf.buf, 1);

      continue;

    }

    if (S_ISLNK(lmode)) {

      char *content = get_symlink(&loid, src_path);

      add_left_or_right(&symlinks2, src_path, content, 0);

      free(content);

    }

    if (S_ISLNK(rmode)) {

      char *content = get_symlink(&roid, dst_path);

      add_left_or_right(&symlinks2, dst_path, content, 1);

      free(content);

    }

    if (lmode && status != 'C') {

      if (checkout_path(lmode, &loid, src_path, &lstate)) {

        ret = error("could not write '%s'", src_path);

        goto finish;

      }

    }

    if (rmode && !S_ISLNK(rmode)) {

      struct working_tree_entry *entry;

      /* Avoid duplicate working_tree entries */

      FLEX_ALLOC_STR(entry, path, dst_path);

      hashmap_entry_init(&entry->entry, strhash(dst_path));

      if (hashmap_get(&working_tree_dups, &entry->entry,

          NULL)) {

        free(entry);

        continue;

      }

      hashmap_add(&working_tree_dups, &entry->entry);

      if (!use_wt_file(workdir, dst_path, &roid)) {

        if (checkout_path(rmode, &roid, dst_path,

              &rstate)) {

          ret = error("could not write '%s'",

                dst_path);

          goto finish;

        }

      } 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(&wtindex, rmode, &roid,

               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)) {

          ret = error("could not create "

                "directory for '%s'",

                dst_path);

          goto finish;

        }

        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;

          }

        }

      }

    }

  }

  fclose(fp);

  fp = NULL;

  if (finish_command(child)) {

    ret = error("error occurred running diff --raw");

    goto finish;

  }

  if (!i)

    goto finish;

  /*

   * 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_for_each_entry(&submodules, &iter, entry,

        entry /* member name */) {

    write_standin_files(entry, &ldir, ldir_len, &rdir, rdir_len);

  }

  /*

   * 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_for_each_entry(&symlinks2, &iter, entry,

        entry /* member name */) {

    write_standin_files(entry, &ldir, ldir_len, &rdir, rdir_len);

  }

  strbuf_setlen(&ldir, ldir_len);

  strbuf_setlen(&rdir, rdir_len);

  if (extcmd) {

    strvec_push(&cmd.args, extcmd);

  } else {

    strvec_push(&cmd.args, "difftool--helper");

    cmd.git_cmd = 1;

    setenv("GIT_DIFFTOOL_DIRDIFF", "true", 1);

  }

  strvec_pushl(&cmd.args, ldir.buf, rdir.buf, NULL);

  ret = run_command(&cmd);

  /* TODO: audit for interaction with sparse-index. */

  ensure_full_index(&wtindex);

  /*

   * 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, path_entry_cmp, NULL, wtindex.cache_nr);

  hashmap_init(&tmp_modified, path_entry_cmp, NULL, 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) {

      struct lock_file lock = LOCK_INIT;

      strbuf_reset(&buf);

      strbuf_addf(&buf, "%s/wtindex", tmpdir.buf);

      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);

        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.buf);

    warning(_("you may want to cleanup or recover these."));

    ret = 1;

  } else {

    remove_dir_recursively(&tmpdir, 0);

    if (ret)

      warning(_("failed: %d"), ret);

  }

finish:

  if (fp)

    fclose(fp);

  free(lbase_dir);

  free(rbase_dir);

  strbuf_release(&info);

  strbuf_release(&lpath);

  strbuf_release(&rpath);

  strbuf_release(&ldir);

  strbuf_release(&rdir);

  strbuf_release(&wtdir);

  strbuf_release(&buf);

  strbuf_release(&tmpdir);

  return (ret < 0) ? 1 : ret;

}

static int run_file_diff(int prompt, const char *prefix,

       struct child_process *child)

{

  strvec_push(&child->env, "GIT_PAGER=");

  strvec_push(&child->env, "GIT_EXTERNAL_DIFF=git-difftool--helper");

  if (prompt > 0)

    strvec_push(&child->env, "GIT_DIFFTOOL_PROMPT=true");

  else if (!prompt)

    strvec_push(&child->env, "GIT_DIFFTOOL_NO_PROMPT=true");

  child->git_cmd = 1;

  child->dir = prefix;

  return run_command(child);

}

int cmd_difftool(int argc, const char **argv, const char *prefix)

{

  int use_gui_tool = -1, dir_diff = 0, prompt = -1, symlinks = 0,

      tool_help = 0, no_index = 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")),

    OPT_SET_INT_F('y', "no-prompt", &prompt,

      N_("do not prompt before launching a diff tool"),

      0, PARSE_OPT_NONEG),

    OPT_SET_INT_F(0, "prompt", &prompt, NULL,

      1, PARSE_OPT_NONEG | PARSE_OPT_HIDDEN),

    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_BOOL(0, "no-index", &no_index, N_("passed to `diff`")),

    OPT_END()

  };

  struct child_process child = CHILD_PROCESS_INIT;

  git_config(difftool_config, NULL);

  symlinks = has_symlinks;

  argc = parse_options(argc, argv, prefix, builtin_difftool_options,

           builtin_difftool_usage, PARSE_OPT_KEEP_UNKNOWN_OPT |

           PARSE_OPT_KEEP_DASHDASH);

  if (tool_help)

    return print_tool_help();

  if (!no_index && !startup_info->have_repository)

    die(_("difftool requires worktree or --no-index"));

  if (!no_index){

    setup_work_tree();

    setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);

    setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);

  } else if (dir_diff)

    die(_("options '%s' and '%s' cannot be used together"), "--dir-diff", "--no-index");

  die_for_incompatible_opt3(use_gui_tool == 1, "--gui",

          !!difftool_cmd, "--tool",

          !!extcmd, "--extcmd");

  /*

   * Explicitly specified GUI option is forwarded to git-mergetool--lib.sh;

   * empty or unset means "use the difftool.guiDefault config or default to

   * false".

   */

  if (use_gui_tool == 1)

    setenv("GIT_MERGETOOL_GUI", "true", 1);

  else if (use_gui_tool == 0)

    setenv("GIT_MERGETOOL_GUI", "false", 1);

  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.

   */

  strvec_push(&child.args, "diff");

  if (no_index)

    strvec_push(&child.args, "--no-index");

  if (dir_diff)

    strvec_pushl(&child.args, "--raw", "--no-abbrev", "-z", NULL);

  strvec_pushv(&child.args, argv);

  if (dir_diff)

    return run_dir_diff(extcmd, symlinks, prefix, &child);

  return run_file_diff(prompt, prefix, &child);

}