fix: buffer writes during disconnect to prevent programs in terminal from freezing

[ejc3]

Summary

When a client disconnects, writePacket() would loop forever waiting for the socket, causing the entire pipe chain to back up. This freezes any program writing to the terminal (e.g. Claude Code would no longer make progress).

This fix buffers data during disconnect (up to 4MB) before entering the blocking loop. On reconnect, buffered data is replayed via ET's existing recovery mechanism.

Problem

The root cause was in BackedWriter::write():

1. When socketFd < 0, it returned SKIPPED immediately without buffering
2. Connection::write() returned false on SKIPPED
3. writePacket() looped forever with 100ms sleeps waiting for reconnect
4. This blocked the terminal output pipe, filling the PTY buffer
5. Programs writing to stdout would block on write() and freeze

Solution

• Buffer packets before checking socket state
• Add BUFFERED_ONLY return state for successful buffering without socket
• Add DISCONNECT_BUFFER_BYTES constant (4MB) to control when to start blocking
• Keep MAX_BACKUP_BYTES at 64MB for normal connected operation

Behavior

State	Action
Connected	Buffer + send (trim old at 64MB)
Disconnected, buffer < 4MB	Buffer data, return success (no blocking)
Disconnected, buffer >= 4MB	Return SKIPPED → writePacket() blocks until reconnect

The blocking loop still exists but now only triggers after 4MB is buffered. This preserves data for replay on reconnect while still providing backpressure for extended outages.

Changes

• BackedWriter.hpp: Add BUFFERED_ONLY enum, add DISCONNECT_BUFFER_BYTES constant
• BackedWriter.cpp: Buffer before socket check, respect disconnect limit
• Connection.cpp: Handle BUFFERED_ONLY as success
• BackedIOTest.cpp: Update existing test, add new disconnect buffer test

Test plan

• All 111 existing tests pass (942 assertions)
• New unit test fills 4MB buffer and verifies SKIPPED is returned
• Manual testing: disconnect/reconnect with active terminal output

Evidence from production logs

After deploying this fix, the server logs show successful buffering during disconnects:

2026-02-04T06:30:23,822 Write buffered (no socket)
2026-02-04T06:30:23,932 Write buffered (no socket)
2026-02-04T06:00:23,821 Write buffered (no socket)
2026-02-04T06:00:23,934 Write buffered (no socket)
...

These writes completed successfully (returned BUFFERED_ONLY) instead of blocking. The terminal remained responsive during the disconnect, and output was replayed on reconnect.

[codecov]

Codecov Report

❌ Patch coverage is 93.65079% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.96%. Comparing base (ee8ddc2) to head (7814e0c).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
test/unit_tests/BackedIOTest.cpp	93.61%	3 Missing ⚠️
src/base/BackedWriter.cpp	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #731      +/-   ##
==========================================
- Coverage   87.05%   86.96%   -0.09%     
==========================================
  Files          70       70              
  Lines        5161     5210      +49     
  Branches      483      487       +4     
==========================================
+ Hits         4493     4531      +38     
- Misses        667      679      +12     
+ Partials        1        0       -1     

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[MisterTea]

Thanks!