Re: [PATCH v4 03/16] index-helper: new daemon for caching index and related stuff

> +static int try_shm(struct index_state *istate)
> +{
> +       void *new_mmap = NULL;
> +       size_t old_size = istate->mmap_size;
> +       ssize_t new_size;
> +       const unsigned char *sha1;
> +       struct stat st;
> +       int fd;
> +
> +       if (!is_main_index(istate) ||
> +           old_size <= 20 ||
> +           stat(git_path("index-helper.path"), &st))
> +               return -1;
> +       if (poke_daemon(istate, &st, 0))
> +               return -1;
> +       sha1 = (unsigned char *)istate->mmap + old_size - 20;
> +
> +       fd = open(index_helper_path("git-index-%s", sha1_to_hex(sha1)),
> +                 O_RDONLY);
> +       if (fd < 0)
> +               goto fail;
> +
> +       if (fstat(fd, &st))
> +               goto fail;
> +
> +       new_size = st.st_size;
> +       new_mmap = mmap(NULL, new_size, PROT_READ, MAP_SHARED, fd, 0);
> +       if (new_size <= 20 ||
> +           hashcmp((unsigned char *)istate->mmap + old_size - 20,
> +                   (unsigned char *)new_mmap + new_size - 20)) {
> +               if (new_mmap)
> +                       munmap(new_mmap, new_size);
> +               goto fail;

> +       }
> +
> +       /* The memory barrier here matches index-helper.c:share_index. */
> +       __sync_synchronize();
> +
> +       munmap(istate->mmap, istate->mmap_size);
> +       istate->mmap = new_mmap;
> +       istate->mmap_size = new_size;
> +       istate->from_shm = 1;
> +       return 0;
> +
> +fail: