cmd/ld, runtime/cgo: allow a symbol to be both cgo_export and cgo_import.

Fixes #4878.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7420052

src/cmd/ld/go.c

@@ -499,11 +499,16 @@ loadcgo(char *file, char *pkg, char *p, int n)
 				remote = local;
 			local = expandpkg(local, pkg);
 			s = lookup(local, 0);
+
+			// export overrides import, for openbsd/cgo.
+			// see issue 4878.
 			if(s->dynimplib != nil) {
-				fprint(2, "%s: symbol is both imported and exported: %s\n", argv0, local);
-				nerrors++;
+				s->dynimplib = nil;
+				s->extname = nil;
+				s->dynimpvers = nil;
+				s->type = 0;
 			}
-			
+
 			if(s->cgoexport == 0) {
 				if(strcmp(f[0], "cgo_export_static") == 0)
 					s->cgoexport |= CgoExportStatic;

src/pkg/runtime/cgo/gcc_openbsd_386.c

@@ -48,9 +48,9 @@ tcb_fixup(int mainthread)
 	bcopy(oldtcb, newtcb + TLS_SIZE, TCB_SIZE);
 	__set_tcb(newtcb + TLS_SIZE);
 
-	// The main thread TCB is a static allocation - do not try to free it.
-	if(!mainthread)
-		free(oldtcb);
+	// NOTE(jsing, minux): we can't free oldtcb without causing double-free
+	// problem. so newtcb will be memory leaks. Get rid of this when OpenBSD
+	// has proper support for PT_TLS.
 }
 
 static void *

src/pkg/runtime/cgo/gcc_openbsd_amd64.c

@@ -48,9 +48,9 @@ tcb_fixup(int mainthread)
 	bcopy(oldtcb, newtcb + TLS_SIZE, TCB_SIZE);
 	__set_tcb(newtcb + TLS_SIZE);
 
-	// The main thread TCB is a static allocation - do not try to free it.
-	if(!mainthread)
-		free(oldtcb);
+	// NOTE(jsing, minux): we can't free oldtcb without causing double-free
+	// problem. so newtcb will be memory leaks. Get rid of this when OpenBSD
+	// has proper support for PT_TLS.
 }
 
 static void *

src/run.bash

@@ -74,7 +74,6 @@ go run $GOROOT/test/run.go - .
 ) || exit $?
 
 [ "$CGO_ENABLED" != 1 ] ||
-[ "$GOHOSTOS" == openbsd ] || # issue 4878
 (xcd ../misc/cgo/test
 go test
 case "$GOHOSTOS-$GOARCH" in