code review 140950043: runtime: make entersyscall/exitsyscall safe for stack splits

src/pkg/runtime/stack.c

 // Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 #include "runtime.h"
 #include "arch_GOARCH.h"
 #include "malloc.h"
 #include "stack.h"
 #include "funcdata.h"
 #include "typekind.h"
@@ skipping 858 matching lines @@
         bool newstackcall;
 
         if(g->m->forkstackguard)
                 runtime·throw("split stack after fork");
         if(g->m->morebuf.g != g->m->curg) {
                 runtime·printf("runtime: newstack called from g=%p\n"
                         "\tm=%p m->curg=%p m->g0=%p m->gsignal=%p\n",
                         g->m->morebuf.g, g->m, g->m->curg, g->m->g0, g->m->gsignal);
                 runtime·throw("runtime: wrong goroutine in newstack");
         }
+        if(g->throwsplit)
+                runtime·throw("runtime: stack split at bad time");
 
         // The goroutine must be executing in order to call newstack, so the possible states are
         // Grunning and Gsyscall (and, due to GC, also Gscanrunning and Gscansyscall).··
 
         // gp->status is usually Grunning, but it could be Gsyscall if a stack overflow
         // happens during a function call inside entersyscall.
         gp = g->m->curg;
         oldstatus = runtime·readgstatus(gp) & ~Gscan;
         framesize = g->m->moreframesize;
         argsize = g->m->moreargsize;
@@ skipping 226 matching lines @@
         if(gp->m != nil && gp->m->libcallsp != 0)
                 return;
 #endif
         if(StackDebug > 0)
                 runtime·printf("shrinking stack %D->%D\n", (uint64)oldsize, (uint64)newsize);
         nframes = copyabletopsegment(gp);
         if(nframes == -1)
                 return;
         copystack(gp, nframes, newsize);
 }