If SOCK_CLOEXEC exists, then set the flag on socket creation.
Otherwise, use fcntl() (or SetHandleInformation() on Windows) to set the
flag afterwards.
This avoids the sockets to be inherited in child processes.
Refs #2783 <https://github.com/Genymobile/scrcpy/pull/2783>
On Linux, socket functions are unblocked by shutdown(), but on Windows
they are unblocked by closesocket().
Expose net_interrupt() and net_close() to abstract these differences:
- net_interrupt() calls shutdown() on Linux and closesocket() on
Windows (if not already called);
- net_close() calls close() on Linux and closesocket() on Windows (if
not already called).
This simplifies the server code, and prevents a data race on close
(reported by TSAN) on Linux (but does not fix it on Windows):
WARNING: ThreadSanitizer: data race (pid=836124)
Write of size 8 at 0x7ba0000000d0 by main thread:
#0 close ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1690 (libtsan.so.0+0x359d8)
#1 net_close ../app/src/util/net.c:211 (scrcpy+0x1c76b)
#2 close_socket ../app/src/server.c:330 (scrcpy+0x19442)
#3 server_stop ../app/src/server.c:522 (scrcpy+0x19e33)
#4 scrcpy ../app/src/scrcpy.c:532 (scrcpy+0x156fc)
#5 main ../app/src/main.c:92 (scrcpy+0x622a)
Previous read of size 8 at 0x7ba0000000d0 by thread T6:
#0 recv ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:6603 (libtsan.so.0+0x4f4a6)
#1 net_recv ../app/src/util/net.c:167 (scrcpy+0x1c5a7)
#2 run_receiver ../app/src/receiver.c:76 (scrcpy+0x12819)
#3 <null> <null> (libSDL2-2.0.so.0+0x84f40)
The platform-specific code for net.c was implemented in sys/*/net.c.
But the differences are quite limited, so use ifdef-blocks in the single
net.c instead.
