Fix Cells receive-pack server-first handshake

Vasilii Iakliushinrequested to merge
defer-cells-receive-pack-stream into main

What does this MR do and why?

Contributes to #874

Problem

Cells pushes keep a receive-pack request idle while the client's pre-push hook runs (for example Git LFS uploads). Cloudflare closes the idle connection after about 15 seconds and the push fails.

Solution

Split the SSH-over-HTTP receive-pack into two requests:

  1. Send a command-less request (0000 body) and relay its ref advertisement to the client.
  2. Wait for the client to start sending, then open the streaming request and relay its response, skipping the repeated advertisement.

No request is open toward the cell while the client is silent. This adds one short request per push (0.1–0.6 s against a GDK cell). Pull is unchanged.

References

How to set up and validate locally

  1. go test -mod=mod ./internal/command/githttp/... ./internal/pktline/...
  2. Optionally push to a GDK cell with an LFS or pre-push delay.

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Edited by Vasilii Iakliushin

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