os/signal: document signal handling

This is an attempt to document the current state of signal handling.
It's not intended to describe the best way to handle signals.  Future
changes to signal handling should update these docs as appropriate.

update #9896.

Change-Id: I3c50af5cc641357b57dfe90ae1c7883a7e1ec059
Reviewed-on: https://go-review.googlesource.com/17877
Reviewed-by: Russ Cox <rsc@golang.org>

2 files changed, 183 insertions, 1 deletions

diff --git a/src/os/signal/doc.go b/src/os/signal/doc.go
new file mode 100644
index 0000000000..dca7d72660
--- /dev/null
+++ b/src/os/signal/doc.go
@@ -0,0 +1,183 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package signal implements access to incoming signals.
+
+Signals are primarily used on Unix-like systems. For the use of this
+package on Windows and Plan 9, see below.
+
+Types of signals
+
+The signals SIGKILL and SIGSTOP may not be caught by a program, and
+therefore can not be affected by this package.
+
+Synchronous signals are signals triggered by errors in program
+execution: SIGBUS, SIGFPE, and SIGSEGV. These are only considered
+synchronous when caused by program execution, not when sent using
+os.Process.Kill or the kill program or some similar mechanism. In
+general, except as discussed below, Go programs will convert a
+synchronous signal into a run-time panic.
+
+The remaining signals are asynchronous signals. They are not
+triggered by program errors, but are instead sent from the kernel or
+from some other program.
+
+Of the asynchronous signals, the SIGHUP signal is sent when a program
+loses its controlling terminal. The SIGINT signal is sent when the
+user at the controlling terminal presses the interrupt character,
+which by default is ^C (Control-C). The SIGQUIT signal is sent when
+the user at the controlling terminal presses the quit character, which
+by default is ^\ (Control-Backslash). In general you can cause a
+program to simply exit by pressing ^C, and you can cause it to exit
+with a stack dump by pressing ^\.
+
+Default behavior of signals in Go programs
+
+By default, a synchronous signal is converted into a run-time panic. A
+SIGHUP, SIGINT, or SIGTERM signal causes the program to exit. A
+SIGQUIT, SIGILL, SIGTRAP, SIGABRT, SIGSTKFLT, SIGEMT, or SIGSYS signal
+causes the program to exit with a stack dump. A SIGTSTP, SIGTTIN, or
+SIGTTOU signal gets the system default behavior (these signals are
+used by the shell for job control). The SIGPROF signal is handled
+directly by the Go runtime to implement runtime.CPUProfile. Other
+signals are ignored.
+
+If the Go program is started with either SIGHUP or SIGINT ignored,
+they will remain ignored. Go always registers a handler for the other
+signals.
+
+If the Go program is started with a non-empty signal mask, that will
+generally be honored. However, some signals are explicitly unblocked:
+the synchronous signals, SIGILL, SIGTRAP, SIGSTKFLT, SIGCHLD, SIGPROF,
+and, on GNU/Linux, signals 32 (SIGCANCEL) and 33 (SIGSETXID)
+(SIGCANCEL and SIGSETXID are used internally by glibc).
+
+Changing the behavior of signals in Go programs
+
+The functions in this package allow a program to change the way Go
+programs handle signals.
+
+Notify disables the default behavior for a given set of asynchronous
+signals and instead delivers them over one or more registered
+channels. Specifically, it applies to the signals SIGHUP, SIGINT,
+SIGQUIT, SIGABRT, and SIGTERM. It also applies to the job control
+signals SIGTSTP, SIGTTIN, and SIGTTOU, in which case the system
+default behavior does not occur. It also applies to some signals that
+are otherwise ignored: SIGUSR1, SIGUSR2, SIGPIPE, SIGALRM, SIGCHLD,
+SIGURG, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGWINCH, SIGIO, SIGPWR, SIGSYS,
+SIGINFO, SIGTHR, SIGWAITING, SIGLWP, SIGFREEZE, SIGTHAW, SIGLOST,
+SIGXRES, SIGJVM1, SIGJVM2, and any real time signals used on the
+system. Note that not all of these signals are available on all
+systems.
+
+If the program was started with SIGHUP or SIGINT ignored, and Notify
+is called for either signal, a signal handler will be installed for
+that signal and it will no longer be ignored. If, later, Reset or
+Ignore is called for that signal, or Stop is called on all channels
+passed to Notify for that signal, the signal will once again be
+ignored. Reset will restore the system default behavior for the
+signal, while Ignore will cause the system to ignore the signal
+entirely.
+
+If the program is started with a non-empty signal mask, some signals
+will be explicitly unblocked as described above. If Notify is called
+for a blocked signal, it will be unblocked. If, later, Reset is
+called for that signal, or Stop is called on all channels passed to
+Notify for that signal, the signal will once again be blocked.
+
+Go programs that use cgo or SWIG
+
+In a Go program that includes non-Go code, typically C/C++ code
+accessed using cgo or SWIG, Go's startup code normally runs first. It
+configures the signal handlers as expected by the Go runtime, before
+the non-Go startup code runs. If the non-Go startup code wishes to
+install its own signal handlers, it must take certain steps to keep Go
+working well. This section documents those steps and the overall
+effect changes to signal handler settings by the non-Go code can have
+on Go programs. In rare cases, the non-Go code may run before the Go
+code, in which case the next section also applies.
+
+If the non-Go code called by the Go program does not change any signal
+handlers or masks, then the behavior is the same as for a pure Go
+program.
+
+If the non-Go code installs any signal handlers, it must use the
+SA_ONSTACK flag with sigaction. Failing to do so is likely to cause
+the program to crash if the signal is received. Go programs routinely
+run with a limited stack, and therefore set up an alternate signal
+stack. Also, the Go standard library expects that any signal handlers
+will use the SA_RESTART flag. Failing to do so may cause some library
+calls to return "interrupted system call" errors.
+
+If the non-Go code installs a signal handler for any of the
+synchronous signals (SIGBUS, SIGFPE, SIGSEGV), then it should record
+the existing Go signal handler. If those signals occur while
+executing Go code, it should invoke the Go signal handler (whether the
+signal occurs while executing Go code can be determined by looking at
+the PC passed to the signal handler). Otherwise some Go run-time
+panics will not occur as expected.
+
+If the non-Go code installs a signal handler for any of the
+asynchronous signals, it may invoke the Go signal handler or not as it
+chooses. Naturally, if it does not invoke the Go signal handler, the
+Go behavior described above will not occur. This can be an issue with
+the SIGPROF signal in particular.
+
+The non-Go code should not change the signal mask on any threads
+created by the Go runtime. If the non-Go code starts new threads of
+its own, it may set the signal mask as it pleases.
+
+If the non-Go code starts a new thread, changes the signal mask, and
+then invokes a Go function in that thread, the Go runtime will
+automatically unblock certain signals: the synchronous signals,
+SIGILL, SIGTRAP, SIGSTKFLT, SIGCHLD, SIGPROF, SIGCANCEL, and
+SIGSETXID. When the Go function returns, the non-Go signal mask will
+be restored.
+
+If the Go signal handler is invoked on a non-Go thread not running Go
+code, the handler generally forwards the signal to the non-Go code, as
+follows. If the signal is SIGPROF, the Go handler does
+nothing. Otherwise, the Go handler removes itself, unblocks the
+signal, and raises it again, to invoke any non-Go handler or default
+system handler. If the program does not exit, the Go handler then
+reinstalls itself and continues execution of the program.
+
+Non-Go programs that call Go code
+
+When Go code is built with options like -buildmode=c-shared, it will
+be run as part of an existing non-Go program. The non-Go code may
+have already installed signal handlers when the Go code starts (that
+may also happen in unusual cases when using cgo or SWIG; in that case,
+the discussion here applies).
+
+If the Go runtime sees an existing signal handler for the SIGCANCEL or
+SIGSETXID signals (which are used only on GNU/Linux), it will turn on
+the SA_ONSTACK flag and otherwise keep the signal handler.
+
+For other signals listed above, the Go runtime will install a signal
+handler. It will save any existing signal handler. If a synchronous
+signal arrives while executing non-Go code, the Go runtime will invoke
+the existing signal handler instead of the Go signal handler.
+
+If a signal is delivered to a non-Go thread, it will act as described
+above, except that if there is an existing non-Go signal handler, that
+handler will be installed before raising the signal.
+
+Windows
+
+On Windows a ^C (Control-C) or ^BREAK (Control-Break) normally cause
+the program to exit. If Notify is called for os.SIGINT, ^C or ^BREAK
+will cause os.SIGINT to be sent on the channel, and the program will
+not exit. If Reset is called, or Stop is called on all channels passed
+to Notify, then the default behavior will be restored.
+
+Plan 9
+
+On Plan 9, signals have type syscall.Note, which is a string. Calling
+Notify with a syscall.Note will cause that value to be sent on the
+channel when that string is posted as a note.
+
+*/
+package signal
diff --git a/src/os/signal/signal.go b/src/os/signal/signal.go
index 1625786d49..2e6f186658 100644
--- a/src/os/signal/signal.go
+++ b/src/os/signal/signal.go
@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package signal implements access to incoming signals.
 package signal
 
 import (