Gccgo can only get a backtrace for the currently running thread, which
means that it can only get a backtrace for goroutines currently running
Go code. When a goroutine is running C code, gccgo has no way to stop
it and get the backtrace. This test is all about getting a backtrace
of goroutines running C code, so it can't work for gccgo.
Change-Id: I2dff4403841fb544da7396562ab1193875fc14c3
Reviewed-on: https://go-review.googlesource.com/1904
Reviewed-by: Minux Ma <minux@golang.org>
The test doesn't work with GOTRACEBACK != 2.
Diagnose that failure mode.
LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://golang.org/cl/152970043
Normally, the caller to runtime.entersyscall() must not return before
calling runtime.exitsyscall(), lest g->syscallsp become a dangling
pointer. runtime.cgocallbackg() violates this constraint. To work around
this, save g->syscallsp and g->syscallpc around cgo->Go callbacks, then
restore them after calling runtime.entersyscall(), which restores the
syscall stack frame pointer saved by cgocall. This allows the GC to
correctly trace a goroutine that is currently returning from a
Go->cgo->Go chain.
This also adds a check to proc.c that panics if g->syscallsp is clearly
invalid. It is not 100% foolproof, as it will not catch a case where the
stack was popped then pushed back beyond g->syscallsp, but it does catch
the present cgo issue and makes existing tests fail without the bugfix.
Fixes#7978.
LGTM=dvyukov, rsc
R=golang-codereviews, dvyukov, minux, bradfitz, iant, gobot, rsc
CC=golang-codereviews, rsc
https://golang.org/cl/131910043
