On Fri, Sep 2, 2016 at 12:39 PM, Shawn Pearce [off-list ref] wrote:

On Fri, Sep 2, 2016 at 10:15 AM, Jonathan Tan [off-list ref] wrote:

quoted

+               if (is_null_oid(&old_oid)) {
+                       if (strcmp(name, "capabilities^{}"))

Its not the zero ID that is special, its the "capabilities^{}" name
that is special when its the first entry in the stream. In the wire
protocol a "x^{}" line is a modifier to a prior "x" line to add a
peeled object to the prior line. But if we see "^{}" on the first line
that is non-sense, there is no prior line to modify with this
identifier.

Further ^{} is used here because its invalid in a name. A server
really cannot have a reference that ends with the sequence ^{}. And a
server should not have a reference named "capabilities" without a
"refs/" prefix on it.

So the entire "capabilities^{}" on the first line is a bunch of
contradictions that violate a number of things about the protocol,
which is why clients should ignore it.

I think the test should be about:

  !*list && !strcmp(name, "capabilities^{}")

quoted

+                               warning("zero object ID received that is not accompanied by a "
+                                       "capability declaration, ignoring and continuing anyway");

Annoyingly a zero object ID is sort of possible; with a probability of
1/2^160 or something. Its just a very very unlikely value. Slightly
stronger to test against the known invalid name.

That ship has sailed long ago I would think?
There are tests for null sha1 in the refs code, e.g. for
deletion/creation of a branch
we consider the null sha1 as the counterpart.

So while it may be possible to have SHA1 producing a "0"x40, you
cannot e.g. push it?