fix: stop dropping other threads' database log lines during queries

[nabbi]

Automated PR, tested in a zm-holodeck sandbox. An AI agent (Claude Opus 5.5) reproduced the bug on a disposable ZoneMinder built from upstream master 6c0f976c2, wrote this fix and ran the checks below on the same lab. Minimal human review before opening: please review it as you would any contributor's PR.

TL;DR

• Fixes: lines logged while another thread runs a database query never reach the Logs table, and the database log queue can deadlock threads that log.
• Change: only the thread running a query is kept out of database logging (a thread_local count instead of setting the process-wide level to NOLOG), and the queue's backlog warning is logged after releasing the queue's mutex; 5 files, +206/-18 (173 of them tests).
• Risk: every daemon logs through logPrint, and every Logs row goes through zmDbDo. The debug feedback loop the old toggle prevented is covered by a test. Lock order is now db_mutex → log_mutex → queue mutex everywhere.
• Tested: reproduced before, passes after; the checks covering the diff are green.

Bug: reproduction and cause

Reproduction

Lab: upstream master 6c0f976c2, clean, Debian trixie, MariaDB, simulated cameras.
A Catch2 test (in the PR, [notCI][dblive]): thread A runs zmDbDo("DO SLEEP(2)"); 500 ms in, the main thread checks the logger's database level and logs an Info line, then waits up to 5 s for it in Logs.

/src/tests/zm_db_logging_live.cpp:97: FAILED:
  CHECK( Logger::fetch()->databaseLevel() == Logger::INFO )
with expansion:
  -6 == 0
/src/tests/zm_db_logging_live.cpp:101: FAILED:
  CHECK( WaitForLog(Marker("during"), std::chrono::seconds(5)) )
with expansion:
  false

First seen in a running zmc: its log file had a Warning and an Info line at 17:58:31.9644 and .9668; the Logs table had only the Info line logged 0.05 ms before them.
Deadlock: a thread holds db_mutex for 1 s so rows queue up while 4 threads log: dbQueue deadlock: 0 of 4 logging threads finished.

Cause

• zmDbDo() sets the logger's database level to NOLOG for the whole mysql_query loop (src/zm_db.cpp:275-277). The level is one value for the process, and logPrint queues a Logs row only if (level <= mDatabaseLevel) (src/zm_logger.cpp:544). Since c56d23a5a (2024-06-20) the window is the full query.
• The toggle keeps the "Success running sql query INSERT INTO Logs" line from queueing another insert at a debug database level (43e7e612c, c56d23a5a); only the thread running the query needs that.
• mDatabaseLevel is written from zmDbDo and zmDbQueue::process under different locks and read in logPrint: a data race.
• zmDbQueue::process logs its backlog warning while holding the queue's mutex (src/zm_db.cpp:390, :401); logPrint holds log_mutex while dbQueue.push() takes the queue's mutex (src/zm_db.cpp:417). Opposite order: deadlock.

Change and checks

Change

• src/zm_logger.h, src/zm_logger.cpp: Logger::DatabaseLogPause, an RAII guard over a static thread_local count; logPrint skips the database while the calling thread holds one.
• src/zm_db.cpp: zmDbDo and the backlog warning use the guard instead of saving, lowering and restoring mDatabaseLevel, which is now written only by Logger::initialise(). zmDbQueue::process takes the backlog size under its mutex and warns after unlocking.
• tests/zm_db_logging_live.cpp: three [notCI][dblive] cases (need a database, like zm_db_contention_live.cpp).

Behaviour changes

Change	Who sees it	Intended / collateral	Tested by
Lines from other threads during a query reach Logs	Log view, api/logs, clients reading logs	intended	dblive test 1
zmDbDo's own lines stay out of Logs, now for the calling thread only	same	no change	dblive test 2
Backlog warning logged after the queue's mutex is released	nobody	side fix (deadlock)	dblive test 3

After

fix/b3 at 522dc4e68 (base 6c0f976c2): All tests passed (291 assertions in 6 test cases); unit-tests 14716 assertions / 249 cases; script-tests PASS=28; smoke PASS=11 XFAIL=1 (known). Not run: clients, contract (no API/web change), multi-server. 1.38: same bug; zm_logger needs small context edits.

🤖 Generated with Claude Code