Re: [PATCH v2 1/8] lib/printbuf: New data structure for heap-allocated strings

[Kent Overstreet <kent.overstreet@gmail.com>]

On Mon, Apr 25, 2022 at 03:44:34AM +0100, Matthew Wilcox wrote:
> On Sun, Apr 24, 2022 at 04:46:03PM -0700, Joe Perches wrote:
> > > + * pr_human_readable_u64, pr_human_readable_s64: Print an integer with human
> > > + * readable units.
> > 
> > Why not extend vsprintf for this using something like %pH[8|16|32|64] 
> > or %pH[c|s|l|ll|uc|us|ul|ull] ?
> 
> The %pX extension we have is _cute_, but ultimately a bad idea.  It
> centralises all kinds of unrelated things in vsprintf.c, eg bdev_name()
> and clock() and ip_addr_string().

And it's not remotely discoverable. I didn't realize we had bdev_name()
available as a format string until just now or I would've been using it!

> Really, it's working around that we don't have something like Java's
> StringBuffer (which I see both seq_buf and printbuf as attempting to
> be).  So we have this primitive format string hack instead of exposing
> methods like:
> 
> void dentry_string(struct strbuf *, struct dentry *);

Exactly!

> as an example,
>                 if (unlikely(ino == dir->i_ino)) {
>                         EXT4_ERROR_INODE(dir, "'%pd' linked to parent dir",
>                                          dentry);
>                         return ERR_PTR(-EFSCORRUPTED);
>                 }
> 
> would become something like:
> 
> 		if (unlikely(ino == dir->i_ino)) {
> 			struct strbuf strbuf;
> 			strbuf_char(strbuf, '\'');
> 			dentry_string(strbuf, dentry);
> 			strbuf_string(strbuf, "' linked to parent dir");
> 			EXT4_ERROR_INODE(dir, strbuf);
> 			return ERR_PTR(-EFSCORRUPTED);
> 		}
> 
> which isn't terribly nice, but C has sucky syntax for string
> construction.  Other languages have done this better, including Rust.

Over IRC just now you proposed "%p(%p)", dentry_name, dentry - I'm _really_
liking this idea, especially if we can get glibc to take it.

Then your ext4 example becomes just 

	if (unlikely(ino == dir->i_ino)) {
		EXT4_ERROR_INODE(dir, "'%p(%p)' linked to parent dir",
				 dentry_name, dentry);
		return ERR_PTR(-EFSCORRUPTED);
	}

And you can cscope to the pretty-printer! And dentry_name becomes just

void dentry_name(struct printbuf *out, struct dentry *dentry)
{
	...
}

Which is quite a bit simpler than the current definition.

Sweeeeeet.