runtime: add GODEBUG=asyncpreemptoff=1

This doesn't do anything yet, but it will provide a way to disable
non-cooperative preemption.

For #10958, #24543.

Change-Id: Ifdef303f103eabd0922ced8d9bebbd5f0aa2cda4
Reviewed-on: https://go-review.googlesource.com/c/go/+/201757
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/extern.go

@@ -127,6 +127,13 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	IDs will refer to the ID of the goroutine at the time of creation; it's possible for this
 	ID to be reused for another goroutine. Setting N to 0 will report no ancestry information.
 
+	asyncpreemptoff: asyncpreemptoff=1 disables signal-based
+	asynchronous goroutine preemption. This makes some loops
+	non-preemptible for long periods, which may delay GC and
+	goroutine scheduling. This is useful for debugging GC issues
+	because it also disables the conservative stack scanning used
+	for asynchronously preempted goroutines.
+
 The net, net/http, and crypto/tls packages also refer to debugging variables in GODEBUG.
 See the documentation for those packages for details.
 

src/runtime/runtime1.go

@@ -315,6 +315,7 @@ var debug struct {
 	scheddetail        int32
 	schedtrace         int32
 	tracebackancestors int32
+	asyncpreemptoff    int32
 }
 
 var dbgvars = []dbgVar{
@@ -334,6 +335,7 @@ var dbgvars = []dbgVar{
 	{"scheddetail", &debug.scheddetail},
 	{"schedtrace", &debug.schedtrace},
 	{"tracebackancestors", &debug.tracebackancestors},
+	{"asyncpreemptoff", &debug.asyncpreemptoff},
 }
 
 func parsedebugvars() {