runtime: disable crash on invalid pointer?

[rsc]

CL 148470043 will introduce code that causes the runtime to crash if the garbage
collector sees an invalid pointer. Because we believe that everything is precise, a
crash in this code indicates a bug: either the collector has freed something it should
not have, or something that looks like a pointer is being scanned conservatively, or
something is creating invalid pointers.

This should help shake out latent bugs in the runtime during the release freeze.

It is not yet clear to me whether we should keep this code in the actual release. This
issue is to decide between:

1) Leave crash.
2) Make crash on-by-default but disabled by a GODEBUG setting.
3) Remove crash, ignoring invalid pointer.

We can make the decision based on how much we see the crashes.

[gopherbot]

Comment 1:

CL https://golang.org/cl/148470043 mentions this issue.

[gopherbot]

Comment 2:

CL https://golang.org/cl/165780043 mentions this issue.

[rsc]

Comment 3:

This issue was closed by revision 8fcdc70.

Status changed to Fixed.