don't require explicit empty blob uploads for HTTP either

[mostynb]

The gRPC API implementation has a hack to handle clients (like bazel?) which in some cases don't upload the empty blob, but do reference it. It is kind of OK not to upload the empty blob, since it can be recognised by hash and trivially regenerated.

Let's imlpement the hack for the HTTP API also.

Implements #233.

[mostynb]

@brentleyjones: would you like to cherry-pick + test this with your setup?

[brentleyjones]

@mostynb Yes! I'll report back soon-ish. Thanks!

[brentleyjones]

@mostynb This patch worked. BuildBarn is now able to validate AC's for HTTP mode with this patch.

Sadly infrastructure issues are preventing me from continuing to use HTTP mode, so I'll also have to followup on #231 testing for you.

[brentleyjones]

gRPC-instance1.txt
gRPC-instance2.txt
HTTP-instance1.txt
HTTP-instance2.txt

I inserted a newline between the two builds. As you can see in instance 2 for gRPC, it can't find ed11bb857931deee5257fae1ec0aeb51fc2e8be493c4af44315108c485613543, but in instance 2 for HTTP it can.

I started these with --disable_http_ac_validation true --disable_grpc_ac_deps true.

[mostynb]

Are the logs above from a test branch with both this change and the commit from #231? If so, then the logic for disabling the gRPC AC dependency check must be broken- I can't see any GRPC NODEPS AC GET ... OK log lines.

[brentleyjones]

Yes, it has both commits from this and #231. This PR worked perfectly and can be merged, IMO. (I thought I was commenting in #231.)