[libmicrohttpd] stop_daemon is not thread-safe in 0.9.22 with MHD_USE_THREAD_PER_CONNECTION

[Dilyan . Palauzov]

Hello,

I start MHD with MHD_USE_THREAD_PER_CONNECTION . version 0.9.22, so name 
libmicrohttpd.so.10.16.1, compiled with CFLAGS="-fno-strict-aliasing -O0 
-ggdb3" ./configure  --disable-curl --disable-https .

According to the messages below, either libmicrohttpd is not thread-safe, 
or I use the API is a wrong way.
 

When MHD_stop_daemon is called, valgrind+helgind 3.8.1 report:

---Thread-Announcement------------------------------------------

Thread #8 was created
   at 0x5F1AC7E: clone (clone.S:77)
   by 0x53F2F6F: do_clone.constprop.4 (createthread.c:75)
   by 0x53F457F: pthread_create@@GLIBC_2.2.5 (createthread.c:256)
   by 0x4C2F777: pthread_create_WRK (in 
/usr/local/lib/valgrind/vgpreload_helgri
   by 0x51DF44F: create_thread (daemon.c:816)
   by 0x51E21B0: MHD_start_daemon_va (daemon.c:2414)
   by 0x51E0D58: MHD_start_daemon (daemon.c:1675)
   by 0x46530C: main (xyz.cpp:457)

----------------------------------------------------------------

== first issue ==

Possible data race during write of size 4 at 0x6884174 by thread #1
Locks held: none
   at 0x51E276E: MHD_stop_daemon (daemon.c:2623)

This conflicts with a previous read of size 4 by thread #8
Locks held: none
   at 0x51E0C87: MHD_select_thread (daemon.c:1641)

Address 0x6884174 is 276 bytes inside a block of size 368 alloc'd
   at 0x4C2BF7B: malloc (in 
/usr/local/lib/valgrind/vgpreload_helgrind-amd64-lin
   by 0x51E19A7: MHD_start_daemon_va (daemon.c:2080)
   by 0x51E0D58: MHD_start_daemon (daemon.c:1675)

meaning, that two threads (the main thread 1 and the thread 8, created my 
MHD as described above) access daemon->shutdown, without synchronizing 
each other, one of the thread writes, the other reads.  This leads to 
undefined results.

Could you please insert some mutex, that ensures that no two threads 
access at the same time daemon->shutdown ?

== second issue==

Another warning is:

Possible data race during write of size 4 at 0x6884168 by thread #1
Locks held: none
   at 0x51E2789: MHD_stop_daemon (daemon.c:2625)

This conflicts with a previous read of size 4 by thread #8
Locks held: none
   at 0x51E0158: MHD_select (daemon.c:1340)
   by 0x51E0C63: MHD_select_thread (daemon.c:1644)
   by 0x4C2C36D: mythread_wrapper (in 
/usr/local/lib/valgrind/vgpreload_helgrind
   by 0x53F3E99: start_thread (pthread_create.c:308)
   by 0x5F1ACBC: clone (clone.S:112)

Address 0x6884168 is 264 bytes inside a block of size 368 alloc'd
   at 0x4C2BF7B: malloc (in 
/usr/local/lib/valgrind/vgpreload_helgrind-amd64-lin
   by 0x51E19A7: MHD_start_daemon_va (daemon.c:2080)
   by 0x51E0D58: MHD_start_daemon (daemon.c:1675)

meaning, that two threads access
  daemon->socket_fd
one read, one write, without synchronizing each other.  Could you please 
guard the access to daemon->socket_fd with some mutex?

== third and issue ==
is the same as the first issue, but here 
   MHD_select (daemon.c:1371), called from
   MHD_select_thread (daemon.c:1644)
and 
   MHD_stop_daemon (daemon.c:2623)
make concurrent access.

== fourth issue ==
is the same as the first issue, but here
   MHD_select_thread (daemon.c:1641)
and 
   MHD_stop_daemon (daemon.c:2623)
do concurrent access without synchronisation.

Shall I make use of the API is a special way, so that the above warnings 
do not come,or does valgrind+helgrind 3.8.1 function suboptimal, or would 
it be possible to fix the issues in libmicrohttpd?

I compile valgrind 3.8.1 with CFLAGS="-Wno-long-long -pipe -O3 -Wl,-O1 
-Wl,-S -Wl,--hash-style=gnu -march=native  -Wno-pointer-sign 
-fno-stack-protector" ./configure --enable-tls , libc is 2.15

Thanks in advance for your attention
  Dilyan Palauzov


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