runtime, cmd/gc, cmd/ld: ignore method wrappers in recover

Bug #1:

Issue 5406 identified an interesting case:
        defer iface.M()
may end up calling a wrapper that copies an indirect receiver
from the iface value and then calls the real M method. That's
two calls down, not just one, and so recover() == nil always
in the real M method, even during a panic.

[For the purposes of this entire discussion, a wrapper's
implementation is a function containing an ordinary call, not
the optimized tail call form that is somtimes possible. The
tail call does not create a second frame, so it is already
handled correctly.]

Fix this bug by introducing g->panicwrap, which counts the
number of bytes on current stack segment that are due to
wrapper calls that should not count against the recover
check. All wrapper functions must now adjust g->panicwrap up
on entry and back down on exit. This adds slightly to their
expense; on the x86 it is a single instruction at entry and
exit; on the ARM it is three. However, the alternative is to
make a call to recover depend on being able to walk the stack,
which I very much want to avoid. We have enough problems
walking the stack for garbage collection and profiling.
Also, if performance is critical in a specific case, it is already
faster to use a pointer receiver and avoid this kind of wrapper
entirely.

Bug #2:

The old code, which did not consider the possibility of two
calls, already contained a check to see if the call had split
its stack and so the panic-created segment was one behind the
current segment. In the wrapper case, both of the two calls
might split their stacks, so the panic-created segment can be
two behind the current segment.

Fix this by propagating the Stktop.panic flag forward during
stack splits instead of looking backward during recover.

Fixes #5406.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13367052

1 files changed, 1 insertions, 0 deletions

diff --git a/src/pkg/runtime/proc.c b/src/pkg/runtime/proc.c
index d37014f3a5..0edd7e0ac9 100644
--- a/src/pkg/runtime/proc.c
+++ b/src/pkg/runtime/proc.c
@@ -1773,6 +1773,7 @@ runtime·newproc1(FuncVal *fn, byte *argp, int32 narg, int32 nret, void *callerp
 	newg->gopc = (uintptr)callerpc;
 	newg->status = Grunnable;
 	newg->goid = runtime·xadd64(&runtime·sched.goidgen, 1);
+	newg->panicwrap = 0;
 	if(raceenabled)
 		newg->racectx = runtime·racegostart((void*)callerpc);
 	runqput(m->p, newg);