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Ian proposed an improved way of handling signals masks in Go, motivated by a problem where the Android java runtime expects certain signals to be blocked for all JVM threads. Discussion here https://groups.google.com/forum/#!topic/golang-dev/_TSCkQHJt6g Ian's text is used in the following: A Go program always needs to have the synchronous signals enabled. These are the signals for which _SigPanic is set in sigtable, namely SIGSEGV, SIGBUS, SIGFPE. A Go program that uses the os/signal package, and calls signal.Notify, needs to have at least one thread which is not blocking that signal, but it doesn't matter much which one. Unix programs do not change signal mask across execve. They inherit signal masks across fork. The shell uses this fact to some extent; for example, the job control signals (SIGTTIN, SIGTTOU, SIGTSTP) are blocked for commands run due to backquote quoting or $(). Our current position on signal masks was not thought out. We wandered into step by step, e.g., http://golang.org/cl/7323067 . This CL does the following: Introduce a new platform hook, msigsave, that saves the signal mask of the current thread to m.sigsave. Call msigsave from needm and newm. In minit grab set up the signal mask from m.sigsave and unblock the essential synchronous signals, and SIGILL, SIGTRAP, SIGPROF, SIGSTKFLT (for systems that have it). In unminit, restore the signal mask from m.sigsave. The first time that os/signal.Notify is called, start a new thread whose only purpose is to update its signal mask to make sure signals for signal.Notify are unblocked on at least one thread. The effect on Go programs will be that if they are invoked with some non-synchronous signals blocked, those signals will normally be ignored. Previously, those signals would mostly be ignored. A change in behaviour will occur for programs started with any of these signals blocked, if they receive the signal: SIGHUP, SIGINT, SIGQUIT, SIGABRT, SIGTERM. Previously those signals would always cause a crash (unless using the os/signal package); with this change, they will be ignored if the program is started with the signal blocked (and does not use the os/signal package). ./all.bash completes successfully on linux/amd64. OpenBSD is missing the implementation. Change-Id: I188098ba7eb85eae4c14861269cc466f2aa40e8c Reviewed-on: https://go-review.googlesource.com/10173 Reviewed-by: Ian Lance Taylor <iant@golang.org> |
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