tsan: t5400: set_try_to_free_routine

["Martin Ågren" <martin.agren@gmail.com>]

Using SANITIZE=thread made t5400-send-pack.sh hit the potential race
below.

This is set_try_to_free_routine in wrapper.c. The race relates to the
reading of the "old" value. The caller doesn't care about the "old"
value, so this should be harmless right now. But it seems that using
this mechanism from multiple threads and restoring the earlier value
will probably not work out every time. (Not necessarily because of the
race in set_try_to_free_routine, but, e.g., because callers might not
restore the function pointer in the reverse order of how they
originally set it.)

Properly "fixing" this for thread-safety would probably require some
redesigning, which at the time might not be warranted. I'm just posting
this for completeness.

Martin

WARNING: ThreadSanitizer: data race (pid=21382)
  Read of size 8 at 0x000000979970 by thread T1:
    #0 set_try_to_free_routine wrapper.c:35 (git+0x0000006cde1c)
    #1 prepare_trace_line trace.c:105 (git+0x0000006a3bf0)
    #2 trace_strbuf_fl trace.c:185 (git+0x0000006a3bf0)
    #3 packet_trace pkt-line.c:80 (git+0x0000005f9f43)
    #4 packet_read pkt-line.c:309 (git+0x0000005fbe10)
    #5 recv_sideband sideband.c:37 (git+0x000000684c5e)
    #6 sideband_demux send-pack.c:216 (git+0x00000065a38c)
    #7 run_thread run-command.c:933 (git+0x000000655a93)
    #8 <null> <null> (libtsan.so.0+0x0000000230d9)

  Previous write of size 8 at 0x000000979970 by main thread:
    #0 set_try_to_free_routine wrapper.c:38 (git+0x0000006cde39)
    #1 prepare_trace_line trace.c:105 (git+0x0000006a3bf0)
    #2 trace_strbuf_fl trace.c:185 (git+0x0000006a3bf0)
    #3 packet_trace pkt-line.c:80 (git+0x0000005f9f43)
    #4 packet_read pkt-line.c:324 (git+0x0000005fc0bb)
    #5 packet_read_line_generic pkt-line.c:332 (git+0x0000005fc0bb)
    #6 packet_read_line pkt-line.c:342 (git+0x0000005fc0bb)
    #7 receive_unpack_status send-pack.c:149 (git+0x00000065c1e4)
    #8 send_pack send-pack.c:581 (git+0x00000065c1e4)
    #9 git_transport_push transport.c:574 (git+0x0000006ab2c1)
    #10 transport_push transport.c:1068 (git+0x0000006ae5d5)
    #11 push_with_options builtin/push.c:336 (git+0x00000049d580)
    #12 do_push builtin/push.c:419 (git+0x00000049f57d)
    #13 cmd_push builtin/push.c:591 (git+0x00000049f57d)
    #14 run_builtin git.c:330 (git+0x00000040949e)
    #15 handle_builtin git.c:538 (git+0x00000040949e)
    #16 run_argv git.c:590 (git+0x000000409a0e)
    #17 cmd_main git.c:667 (git+0x000000409a0e)
    #18 main common-main.c:43 (git+0x0000004079d1)

  Location is global 'try_to_free_routine' of size 8 at 0x000000979970 (git+0x000000979970)

  Thread T1 (tid=21405, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x000000027577)
    #1 start_async run-command.c:1130 (git+0x0000006582e7)
    #2 send_pack send-pack.c:562 (git+0x00000065b7c8)
    #3 git_transport_push transport.c:574 (git+0x0000006ab2c1)
    #4 transport_push transport.c:1068 (git+0x0000006ae5d5)
    #5 push_with_options builtin/push.c:336 (git+0x00000049d580)
    #6 do_push builtin/push.c:419 (git+0x00000049f57d)
    #7 cmd_push builtin/push.c:591 (git+0x00000049f57d)
    #8 run_builtin git.c:330 (git+0x00000040949e)
    #9 handle_builtin git.c:538 (git+0x00000040949e)
    #10 run_argv git.c:590 (git+0x000000409a0e)
    #11 cmd_main git.c:667 (git+0x000000409a0e)
    #12 main common-main.c:43 (git+0x0000004079d1)

SUMMARY: ThreadSanitizer: data race wrapper.c:35 set_try_to_free_routine