Ensure native library isn't actually loaded, and no IO on net thread

net/udp/udp_socket_posix.cc

Index: net/udp/udp_socket_posix.cc
diff --git a/net/udp/udp_socket_posix.cc b/net/udp/udp_socket_posix.cc
index f869256ca547b912ddd5853cbbdf0949f3e1db85..6b047f5c03c9e5131f347ea636489c72dcc90a29 100644
--- a/net/udp/udp_socket_posix.cc
+++ b/net/udp/udp_socket_posix.cc
@@ -20,6 +20,7 @@
 #include "base/metrics/sparse_histogram.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/rand_util.h"
+#include "base/threading/thread_restrictions.h"
 #include "base/trace_event/trace_event.h"
 #include "net/base/io_buffer.h"
 #include "net/base/ip_address.h"
@@ -352,6 +353,10 @@ int UDPSocketPosix::BindToNetwork(
     // space as it shims libc functions like connect().
     base::FilePath file(base::FilePath::FromUTF16Unsafe(
         base::GetNativeLibraryName(base::ASCIIToUTF16("netd_client"))));

[xunjieli, 2016/06/27 18:54:16]

If we already have netd_client loaded in Chromium's address space, why do we
need to load it here? In other words, when will setNetworkForSocket == nullptr?

[pauljensen, 2016/06/27 19:07:10]

setNetworkForSocket is nullptr when the executable starts executing.  It's set
by line 361 below.

libnetd_client.so will be loaded indirectly by the executable calling socket()
in UDPSocketPosix::Open().
See how socket() in libc.so calls into libnetd_client.so:
http://androidxref.com/6.0.1_r10/xref/bionic/libc/bionic/socket.cpp#22
http://androidxref.com/6.0.1_r10/xref/bionic/libc/bionic/NetdClient.cpp#37

I just need to get the base::NativeLibrary pointer here so I can pass it to
base::GetFunctionPointerFromNativeLibrary().  Normally dlopen() goes and loads
the library, but then the library is already loaded it just gives back the
handle to it.  dlopen() man page says "If  the same library is loaded again with
dlopen(), the same file handle is returned."  I've verified this by reading
Android's dlopen code and also I wrote an example and strace'd it to verify, see
my notes in the linked bug.

+    // Disable IO assertion in LoadNativeLibrary as the library will already
+    // be loaded by the socket() call (which goes through libnetd_client) that
+    // created |socket_|, so no IO will actually be performed. crbug.com/623555
+    base::ThreadRestrictions::ScopedAllowIO allow_io;
     base::NativeLibrary lib = base::LoadNativeLibrary(file, nullptr);
     setNetworkForSocket = reinterpret_cast<SetNetworkForSocket>(
         base::GetFunctionPointerFromNativeLibrary(lib, "setNetworkForSocket"));