Fix RecvMsgWithPid on broken socket pair

base/posix/unix_domain_socket_linux.cc

 // Copyright (c) 2011 The Chromium 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 "base/posix/unix_domain_socket_linux.h"
 
 #include <errno.h>
 #include <sys/socket.h>
 #include <sys/uio.h>
 #include <unistd.h>
@@ skipping 134 matching lines @@
     errno = EMSGSIZE;
     return -1;
   }
 
   if (wire_fds) {
     for (unsigned i = 0; i < wire_fds_len; ++i)
       fds->push_back(new base::ScopedFD(wire_fds[i]));
   }
 
   if (out_pid) {
-    DCHECK(pid != -1);
+    // |pid| will legitimately be -1 if we read EOF, so only DCHECK if we
+    // actually received a message.  Unfortunately, Linux allows sending zero
+    // length messages, which are indistinguishable from EOF, so this check
+    // has false negatives.
+    if (r > 0 || msg.msg_controllen > 0)
+      DCHECK_GE(pid, 0);
+
     *out_pid = pid;
   }
 
   return r;
 }
 
 // static
 ssize_t UnixDomainSocket::SendRecvMsg(int fd,
                                       uint8_t* reply,
                                       unsigned max_reply_len,
@@ skipping 43 matching lines @@
   if (recv_fds.size() > (result_fd != NULL ? 1 : 0)) {
     NOTREACHED();
     return -1;
   }
 
   if (result_fd)
     *result_fd = recv_fds.empty() ? -1 : recv_fds[0]->release();
 
   return reply_len;
 }