Land PSE51 realtime signals

[jserv]

Wire the wait-for-signal API set and bounded sigqueue payload path that _POSIX_REALTIME_SIGNALS expects, on top of the existing kill/sigaction core. SYS_SIGSUSPEND, SYS_SIGTIMEDWAIT, and SYS_SIGQUEUE round out the PSE51 signal surface; SYS_SIGQUEUE is appended at slot 100 so the rest of the syscall numbering stays stable across the branch.

Process-directed pending state splits into two distinct sources so that plain kill() and queued sigqueue() can coexist on the same signo without silently swallowing each other:

• proc_pending_plain tracks kill-style instances (one bit per signo).
• queued[signo] is a bounded FIFO of sigqueue payloads.
• proc_pending is the OR summary refreshed under sig_lock so the lockless return-to-user fast path stays coherent.

signal_claim_proc_pending_locked consumes exactly one source per call (queue first, then plain), so consuming a queued payload never clears a sibling plain instance.

The per-signo ring is sized SIGQUEUE_MAX_PER_SIGNO + 1. Producers cap at SIGQUEUE_MAX_PER_SIGNO (EAGAIN behavior unchanged for user space); the extra slot is reserved for the rollback path. If sys_sigtimedwait dequeues a payload and copy_to_user later faults, the new signal_restore_proc_pending_locked helper re-inserts the payload at the queue head and always succeeds for a single in-flight consumer. The multi-consumer race past the reserved slot drops the payload but keeps the signal observable via the plain bit, as defense-in-depth.

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Summary by cubic

Adds PSE51 realtime signals with wait-for-signal APIs and bounded sigqueue payloads. Introduces SYS_SIGQUEUE, wires SYS_SIGSUSPEND, and extends SYS_SIGTIMEDWAIT to optionally return a queued value.

• New Features

  • Split process-directed pending into proc_pending_plain (bitmask) and per-signo FIFO queued[], with proc_pending as the OR summary for the fast path.
  • Bounded queue per signal (SIGQUEUE_MAX_PER_SIGNO = 4) with an internal +1 slot to guarantee lossless rollback after copy_to_user faults; overflow during multi-consumer rollback drops the payload but sets the plain bit to keep the signal observable.
  • signal_claim_proc_pending_locked consumes exactly one source (queue first, then plain) so kill() and sigqueue() on the same signal coexist; SYS_SIGTIMEDWAIT returns the signo and, when present, writes the queued value to the optional payload-out pointer; handlers still receive only the signo.

• Migration

  • New syscall: SYS_SIGQUEUE is added at slot 100; all existing syscall numbers remain unchanged.
  • _POSIX_REALTIME_SIGNALS stays 1; payloads are exposed via a Mazu-specific ABI (not full siginfo_t/SA_SIGINFO).
  • No changes required for existing handlers; use SYS_SIGTIMEDWAIT (with payload-out) and SYS_SIGQUEUE to send/receive values.

^{Written for commit 7b872a8. Summary will update on new commits.}

[cubic-dev-ai]

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