Re: [PATCH 2/2] archive: avoid spawning `gzip`

On Fri, Apr 12, 2019 at 04:04:40PM -0700, Rohit Ashiwal via GitGitGadget wrote:

Very cool. It's nice to drop a dependency, and this should be a bit more efficient, too.

I wondered how you were going to kick this in, since users can define arbitrary filters. I think it's kind of neat to automagically convert "gzip -cn" (which also happens to be the default). But I think we should mention that in the Documentation, in case somebody tries to use a custom version of gzip and wonders why it isn't kicking in.

Likewise, it might make sense in the tests to put a poison gzip in the $PATH so that we can be sure we're using our internal code, and not just calling out to gzip (on platforms that have it, of course).

The alternative is that we could use a special token like ":zlib" or something to indicate that the internal implementation should be used (and then tweak the baked-in default, too). That might be less surprising for users, but most people would still get the benefit since they'd be using the default config.

This looks sufficiently magical that it might merit a comment. I had to look in the zlib header file to learn that this is just a normal stdio-style mode. But we can't just do:

  gzip = gzdopen(fd, "wb");

because we want to (maybe) append a compression level. It's also slightly confusing that it explicitly includes a NUL, but later:

we may overwrite that and assume that outmode[3] is also a NUL. Which it is, because of how C initialization works. But that means we also do not need the "\0" in the initializer.

Dropping that may make it slightly less jarring (any time I see a backslash escape in an initializer, I assume I'm in for some binary trickery, but this turns out to be much more mundane).

I'd also consider just using a strbuf:

  struct strbuf outmode = STRBUF_INIT;

  strbuf_addstr(&outmode, "wb");
  if (args->compression_level >= 0 && args->compression_level <= 9)
	strbuf_addch(&outmode, '0' + args->compression_level);

That's overkill in a sense, but it saves us having to deal with manually-counted offsets, and this code is only run once per program invocation, so the efficiency shouldn't matter.

Is there a way to get a more specific error from zlib? I'm less concerned about gzdopen here (which should never fail), and more about the writing and closing steps. I don't see anything good for gzdopen(), but...

...according to zlib.h, here the returned int is meaningful. And if Z_ERRNO, we should probably use die_errno() to give a better message.

That was a lot of little nits, but the overall shape of the patch looks good to me (and I think the goal is obviously good). Thanks for working on it.

-Peff