feat(signals): honored SA_RESTART for interrupted syscalls

[FlareCoding]

Summary

• Handlers installed through musl's signal() set SA_RESTART, but interrupted blocking calls always surfaced EINTR, breaking ported programs that never expect it.
• Restartable waits (read/write, sockets, futex, proc_wait) return an internal ERESTARTSYS that the delivery boundary rewinds for re-execution under SA_RESTART, converts to EINTR otherwise, or restarts transparently when no handler ran; poll/select and nanosleep keep EINTR, matching Linux.
• Handled signals no longer interrupt or wake elevated tasks, which cannot run a handler, closing a restart livelock and spurious poll/futex results.

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Connect restart fails after signal

High Severity · Logic Bug

Mapping socket ERR_INTR to ERESTARTSYS makes blocking connect restartable under SA_RESTART, but tcp_connect only accepts CLOSED sockets. An interrupt during the SYN_SENT wait leaves the socket in SYN_SENT, so the restarted call returns EINVAL instead of finishing the handshake. With musl signal() (which sets SA_RESTART), a signal during connect turns a normal interrupt into a hard failure.

 

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