[PATCH 5/9] refspec_ref_prefixes(): clean up refspec_item logic

The point of refspec_ref_prefixes() is to look over the set of refspecs and set up an appropriate list of "ref-prefix" strings to send to the server.

The logic for handling individual refspec_items has some confusing bits. The final part of our if/else cascade checks this:

  else if (item->src && !item->exact_sha1)
	prefix = item->src;

But we know that "item->exact_sha1" can never be true, because earlier we did:

  if (item->exact_sha1 || item->negative)
	continue;

This is due to 6c301adb0a (fetch: do not pass ref-prefixes for fetch by exact SHA1, 2018-05-31), which added the continue. So it is tempting to remove the extra exact_sha1 at the end of the cascade, leaving the one at the top of the loop.

But I don't think that's quite right. The full cascade is:

  if (rs->fetch == REFSPEC_FETCH)
	prefix = item->src;
  else if (item->dst)
	prefix = item->dst;
  else if (item->src && !item->exact_sha1)
	prefix = item->src;

which all comes from 6373cb598e (refspec: consolidate ref-prefix generation logic, 2018-05-16). That first "if" is supposed to handle fetches, where we care about the source name, since that is coming from the server. And the rest should be for pushes, where we care about the destination, since that's the name the server will use. And we get that either explicitly from "dst" (for something like "foo:bar") or implicitly from the source (a refspec like "foo" is treated as "foo:foo").

But how should exact_sha1 interact with those? For a fetch, exact_sha1 always means we do not care about sending a name to the server (there is no server refname at all). But pushing an exact sha1 should still care about the destination on the server! It is only if we have to fall back to the implicit source that we need to care if it is a real ref (though arguably such a push does not even make sense; where would the server store it?).

So I think that 6c301adb0a "broke" the push case by always skipping exact_sha1 items, even though a push should only care about the destination.

Of course this is all completely academic. We have still not implemented a v2 push protocol, so even though we do call this function for pushes, we'd never actually send these ref-prefix lines.

However, given the effort I spent to figure out what was going on here, and the overlapping exact_sha1 checks, I'd like to rewrite this to preemptively fix the bug, and hopefully make it less confusing.

This splits the "if" at the top-level into fetch vs push, and then each handles exact_sha1 appropriately itself. The check for negative refspecs remains outside of either (there is no protocol support for them, so we never send them to the server, but rather use them only to reduce the advertisement we receive).

The resulting behavior should be identical for fetches, but hopefully sets us up better for a potential future v2 push.

Signed-off-by: Jeff King <peff@peff.net>
---
This could be dropped without affecting the rest of the series if it's
too churn-y.

 refspec.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)