Week 4 - M4: Ref exchange + single-branch push

[aman-a-shah]

Owner: @Shuhan6017
Files: minigit/remote.py, tests/test_remote.py
Depends on: #19, #22 (has_object), #24 (is_ancestor)
Branch: week4/m4-single-branch-push

Make minigit push <address> <branch> --token <token> copy missing objects and update one remote branch.

Steps

1. Start from updated main after Week 3 - M4: Multi-object transfer + reachable objects #19 merges. Construct real M1/M3 instances when dependencies are not injected.
2. Resolve the local branch tip and authenticate. Read the remote tip through REF; - means the branch does not exist.
   • Missing local branch: raise NetworkProtocolError.
   • Matching tips: report up to date.
   • For an existing remote tip, require it to be a known ancestor of the local tip. Otherwise reject the push.
3. Document and implement this sequence after AUTH/REF:
   • PUSH <branch> <old-hash|-> <new-hash>: server checks the expected old ref; replies OK or ERR <reason>.
   • HAVE <hash>: server calls has_object; replies YES or NO.
   • PUT <hash>: client follows with Week 3's OBJ <type> <len>\n and exactly <len> raw bytes. Server verifies, stores, then replies OK.
   • DONE: server validates the new history, updates the ref, replies OK, and closes the push session.
4. Use collect_reachable(branch) for the commit/tree/blob set. Ask HAVE for each unique hash and upload only missing objects.
   • Reuse the shared receive buffer and recv_exact.
   • Hash-check each upload before write_object. Route all object access through M1.
5. Before updating the ref, verify every referenced commit, tree, and blob exists with the expected type. Recheck the old ref and fast-forward condition.
   • Use M3 for ref reads, ancestry checks, and the final ref write.
   • Keep HEAD, index, and working files unchanged on the server.
6. Report success only after the final OK. Protocol, auth, object-validation, and connection failures become NetworkProtocolError.
   • A transfer failure leaves the old ref intact. Complete objects already received can stay for retry.
7. Run scripts/quality-check.sh. Open a PR linked to this issue and request a teammate review.

Tests

• Push nested files and multiple commits between two temporary repos.
• Push to an empty branch, then push one more commit.
• Repeat the same push; transfer no objects.
• Shared blobs transfer once; the receiver can read the full history through fresh M1/M3 instances.
• Bad auth, bad payload, or disconnect before DONE leaves the ref unchanged.
• Reject an unrelated remote tip and a ref changed during transfer.
• Use loopback, port 0, timeouts, and reliable server shutdown.

Pull and broader conflict handling remain in Weeks 5-6.

Done when

• First and incremental pushes update one remote branch.
• Only missing objects transfer.
• Ref moves after all required objects validate.
• Quality check passes; PR open and review requested.