Logfill bounded tranlog stream

[markhannum]

Problem

A replicant running sql_logfill destroys one connection to the master per gap-fill, and each of those is a sockpool miss. Observed in production at roughly 890 log-requests/sec across seven replicants, sustained for hours.

Cause

request_logs_from_master() asks for an unbounded stream (NULL stop LSN, TRANLOG_FLAGS_BLOCK), so it never ends on its own. logfill consumes only as far as the gap and abandons the rest — in production the master averaged 636 rows per query against a gap of a couple of records. newsql_disconnect() will not donate a handle whose last response was not LAST_ROW, and cdb2_discard_unread_socket_data defaults to off, so the socket is closed rather than pooled and the next request misses.

This happens on both sides of 0ed0d3f3b: with it, logfill hangs up and reconnects explicitly; without it, consume_previous_query() drains at most 10 rows and then disconnects from inside cdb2_run_statement.

Fix

Pass gap_lsn as the stop LSN. tranlogEof() already terminates on it, so the result set ends with LAST_ROW, the existing consumed path skips the disconnect, and the connection persists across gap-fills.

A second hunk stops tranlogNext() from entering the blocking poll once it has returned maxLsn — that loop checks only blockLsn and timeout, so otherwise the bound would trade connection churn for a stall.

Test

tests/sql_logfill_socket_reuse.test drops every Nth rep message on one replicant to force out-of-order arrivals, then counts log-requests against master disconnects. A gap is an out-of-order arrival rather than simply being behind — gap_lsn is repdb's waiting_lsn — so dropping messages is what reproduces the production condition. Unbounded, disconnects track requests one-for-one; the test allows a tenth. Verified 8 consecutive passes with the fix, and confirmed failing without it.

[roborivers]

Cbuild submission: Error ⚠.
Regression testing: Success ✓.

The first 10 failing tests are:
logfill [db unavailable at finish] **quarantined**
sc_resume_logicalsc_generated **quarantined**
sp_snapshot_generated
comdb2sys_queueodh_generated
consumer_non_atomic_default_consumer_generated **quarantined**
sc_downgrade [timeout] **quarantined**

[roborivers]

Cbuild submission: Error ⚠.
Regression testing: Success ✓.

The first 10 failing tests are:
logfill [db unavailable at finish] **quarantined**
ssl_san
consumer_non_atomic_default_consumer_generated **quarantined**
ssl_set_cmd
ssl_prefer
ssl_dbname
sc_downgrade [timeout] **quarantined**