Re: [PATCH] difftool: handle changing symlinks in dir-diff mode

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi David,

Thank you very much for picking this up!

On Mon, 13 Mar 2017, David Aguilar wrote:

> Detect the null object ID for symlinks in dir-diff so that difftool can
> prepare temporary files that matches how git handles symlinks.

Maybe a description is needed how the OID can be null in that case. I have
to admit that I failed to wrap my head around this so far.

> Previously, a null object ID would crash difftool.  We now detect null
> object IDs and write the symlink's content into the temporary symlink
> stand-in file.
> 
> Original-patch-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> Signed-off-by: David Aguilar <davvid@gmail.com>
> ---
> diff --git a/builtin/difftool.c b/builtin/difftool.c
> index d13350ce83..6c20e20b45 100644
> --- a/builtin/difftool.c
> +++ b/builtin/difftool.c
> @@ -254,6 +254,31 @@ static int ensure_leading_directories(char *path)
>  	}
>  }
>  
> +static int create_symlink_file(struct cache_entry* ce, struct checkout* state)
> +{
> +	/*
> +	 * Dereference a worktree symlink and writes its contents
> +	 * into the checkout state's path.
> +	 */
> +	struct strbuf path = STRBUF_INIT;
> +	struct strbuf link = STRBUF_INIT;
> +
> +	int ok = 0;

It would appear that the convention in Git's C code is for functions to
return 0 upon success and -1 upon error, and to use `int ret` for that
purpose.

> +	if (strbuf_readlink(&link, ce->name, ce_namelen(ce)) == 0) {

Looking at the calling site, I would have expected the code to read the
contents as per ce->hash... After all, we are calling this in the
!use_wt_file() case.

But that is exactly that null hash, isn't it?

> @@ -414,7 +439,12 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,

The lines before this hunk read:

>                       if (!use_wt_file(workdir, dst_path, &roid)) {
>                               ce->ce_mode = rmode;

... and then follow these lines:

>  				oidcpy(&ce->oid, &roid);
>  				strcpy(ce->name, dst_path);
>  				ce->ce_namelen = dst_path_len;
> -				if (checkout_entry(ce, &rstate, NULL))
> +
> +				if (S_ISLNK(rmode) && is_null_oid(&roid)) {
> +					if (!create_symlink_file(ce, &rstate))
> +						return error("unable to create symlink file %s",
> +							     dst_path);
> +				} else if (checkout_entry(ce, &rstate, NULL))
>  					return error("could not write '%s'",
>  						     dst_path);
>  			} else if (!is_null_oid(&roid)) {

Given that we explicitly ask use_wt_file() whether we can use the
worktree's file, and we get the answer "no" before we enter the modified
code block, I would really expect us *not* to want to read the link from
disk at all.

Further, reading the code of use_wt_file(), there seems to be another case
where roid is left alone: when the file could not be lstat()ed. So I
wonder whether the create_symlink_file() is the correct solution, or
whether we could somehow fill roid correctly instead, and keep the
checkout_entry() call?

Ciao,
Dscho