runtime: make module typemaps visible to the GC

The map[typeOff]*_type object is created at run time and stored in
the moduledata. The moduledata object is marked by the linker as
SNOPTRDATA, so the reference is ignored by the GC. Running
misc/cgo/testplugin/test.bash with GOGC=1 will eventually collect
the typemap and crash.

This bug probably comes up in -linkshared binaries in Go 1.7.
I don't know why we haven't seen a report about this yet.

Fixes #17680

Change-Id: I0e9b5c006010e8edd51d9471651620ba665248d3
Reviewed-on: https://go-review.googlesource.com/32430
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/symtab.go

@@ -222,6 +222,15 @@ type modulehash struct {
 	runtimehash  *string
 }
 
+// pinnedTypemaps are the map[typeOff]*_type from the moduledata objects.
+//
+// These typemap objects are allocated at run time on the heap, but the
+// only direct reference to them is in the moduledata, created by the
+// linker and marked SNOPTRDATA so it is ignored by the GC.
+//
+// To make sure the map isn't collected, we keep a second reference here.
+var pinnedTypemaps []map[typeOff]*_type
+
 var firstmoduledata moduledata  // linker symbol
 var lastmoduledatap *moduledata // linker symbol
 

src/runtime/type.go

@@ -497,7 +497,9 @@ func typelinksinit() {
 			// If any of this module's typelinks match a type from a
 			// prior module, prefer that prior type by adding the offset
 			// to this module's typemap.
-			md.typemap = make(map[typeOff]*_type, len(md.typelinks))
+			tm := make(map[typeOff]*_type, len(md.typelinks))
+			pinnedTypemaps = append(pinnedTypemaps, tm)
+			md.typemap = tm
 			for _, tl := range md.typelinks {
 				t := (*_type)(unsafe.Pointer(md.types + uintptr(tl)))
 				for _, candidate := range typehash[t.hash] {