runtime: set g to gsignal before adjustSignalStack

When a signal is received, the runtime probes whether an
alternate signal stack is set, if so, adjust gsignal's stack to
point to the alternate signal stack. This is done in
adjustSignalStack, which calls sigaltstack "syscall", which is a
libc call on darwin through asmcgocall. asmcgocall decides
whether to do stack switch based on whether we're running on g0
stack, gsignal stack, or regular g stack. If g is not set to
gsignal, asmcgocall may make wrong decision. Set g first.
adjustSignalStack is recursively nosplit, so it is okay that
temporarily gsignal.stack doesn't match the stack we're running
on.

May fix #39079.

Change-Id: I59b2c5dc08c3c951f1098fff038bf2e06d7ca055
Reviewed-on: https://go-review.googlesource.com/c/go/+/238020
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: cherrymui

src/runtime/signal_unix.go

@@ -432,15 +432,15 @@ func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
 		return
 	}
 
+	setg(g.m.gsignal)
+
 	// If some non-Go code called sigaltstack, adjust.
 	var gsignalStack gsignalStack
 	setStack := adjustSignalStack(sig, g.m, &gsignalStack)
 	if setStack {
 		g.m.gsignal.stktopsp = getcallersp()
 	}
 
-	setg(g.m.gsignal)
-
 	if g.stackguard0 == stackFork {
 		signalDuringFork(sig)
 	}