code review 11920044: runtime: fix potential deadlock in netpoll on windows

src/pkg/runtime/netpoll_windows.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 "defs_GOOS_GOARCH.h"
 #include "os_GOOS.h"
 
 #define DWORD_MAX 0xffffffff
 
@@ skipping 53 matching lines @@
 G*
 runtime·netpoll(bool block)
 {
         uint32 wait, qty, key;
         int32 mode, errno;
         net_anOp *o;
         G *gp;
 
         if(iocphandle == INVALID_HANDLE_VALUE)
                 return nil;
+        gp = nil;
+retry:
         o = nil;
         errno = 0;
         qty = 0;
         wait = INFINITE;
         if(!block)
                 // TODO(brainman): should use 0 here instead, but scheduler hogs CPU
                 wait = 1;
         // TODO(brainman): Need a loop here to fetch all pending notifications
         // (or at least a batch). Scheduler will behave better if is given
         // a batch of newly runnable goroutines.
@@ skipping 13 matching lines @@
         }
         if(o == nil)
                 runtime·throw("netpoll: GetQueuedCompletionStatus returned o == nil");
         mode = o->mode;
         if(mode != 'r' && mode != 'w') {
                 runtime·printf("netpoll: GetQueuedCompletionStatus returned invalid mode=%d\n", mode);
                 runtime·throw("netpoll: GetQueuedCompletionStatus returned invalid mode");
         }
         o->errno = errno;
         o->qty = qty;
-        gp = nil;
         runtime·netpollready(&gp, (void*)o->runtimeCtx, mode);
+        if(block && gp == nil)
+                goto retry;
         return gp;
 }