mapcache_seed: Refactor seeder for robust multiprocessing and shutdown

[marisn]

This commit is a major overhaul of the mapcache_seed utility, addressing critical bugs in signal handling, multiprocessing, and thread synchronization that could cause deadlocks or unresponsive behavior.

The key changes are:

1. Robust Signal Handling and Shutdown:

The Ctrl+C (SIGINT) handling has been completely rewritten to ensure a prompt and clean shutdown in all modes, fixing numerous deadlocks and race conditions.

• For Multiprocessing:

  • pop_queue() now checks a sig_int_received flag at the start to ensure child processes exit immediately after finishing their current task.
  • push_queue()'s EINTR retry loop now also checks the signal flag, preventing the parent process from deadlocking on a full message queue after a signal has been received.

• For Multithreading:

  • On SIGINT, the feed_worker now calls apr_queue_interrupt_all() to wake all blocked threads. pop_queue() correctly handles the resulting APR_EOF to stop the worker.
  • A race condition where a thread could start a new job just as a signal was received has been fixed by re-checking the signal flag after a work item is successfully popped from the queue.

• Graceful vs. Urgent Shutdown:

  • The final loop in feed_worker() that sends STOP commands is now wrapped in an if(!sig_int_received) block. This ensures it only runs during a normal, graceful shutdown and is skipped on Ctrl+C, preventing a major deadlock.

2. Multiprocessing Enhancements:

The -p mode has been significantly improved for stability and correctness.

• IPC-based Logging: A new, dedicated System V message queue (log_msqid) has been implemented for logging in multiprocessing mode. This decouples logging from the main work queue, improving performance and stability. log_thread_fn and seed_worker were updated to use this new mechanism.
• -p 1 Bugfix: A long-standing bug that caused the seeder to stall when using -p 1 has been fixed by changing all relevant logic guards from nprocesses > 1 to nprocesses >= 1.
• IPC Creation: Message queues are now created using IPC_PRIVATE instead of ftok, which is more robust.

3. Code Quality and Minor Fixes:

• child_init is correctly called at start of each worker process after fork().
• sig_int_received is now correctly typed as volatile sig_atomic_t.
• memset() is now used to initialize structs before use.
• The help text for the -p option has been clarified.

Co-authored with gemini-2.5-pro

[marisn]

Sorry for large blob, but it was hard to understand causes and best fixes for all dead-locking or use of garbage values cases.