Do not close all file descriptors on parent process

[renecannao]

We got a report about proxysql crashing with the following backtrace:

Error: signal 6: 
proxysql(_Z13crash_handleri+0x2d)[0x4ae7fd] 
/lib/x86_64-linux-gnu/libc.so.6(+0x354b0)[0x7f17816804b0] 
/lib/x86_64-linux-gnu/libc.so.6(gsignal+0x38)[0x7f1781680428] 
/lib/x86_64-linux-gnu/libc.so.6(abort+0x16a)[0x7f178168202a] 
/usr/lib/x86_64-linux-gnu/libgnutls.so.30(+0xe9f2e)[0x7f17829c8f2e] 
/usr/lib/x86_64-linux-gnu/libhogweed.so.4(nettle_mpz_random_size+0x36)[0x7f1780926156] 
/usr/lib/x86_64-linux-gnu/libhogweed.so.4(nettle_mpz_random+0x34)[0x7f17809261c4] 
/usr/lib/x86_64-linux-gnu/libhogweed.so.4(nettle_rsa_compute_root_tr+0x121)[0x7f17809277b1] 
/usr/lib/x86_64-linux-gnu/libhogweed.so.4(nettle_rsa_pkcs1_sign_tr+0x7e)[0x7f1780927a3e] 
/usr/lib/x86_64-linux-gnu/libgnutls.so.30(+0xec3b3)[0x7f17829cb3b3] 
/usr/lib/x86_64-linux-gnu/libgnutls.so.30(gnutls_privkey_sign_data+0xbc)[0x7f178293193c] 
/usr/lib/x86_64-linux-gnu/libgnutls.so.30(+0x4422d)[0x7f178292322d] 
/usr/lib/x86_64-linux-gnu/libgnutls.so.30(gnutls_certificate_set_x509_key_mem2+0x56)[0x7f178292dd56] 
proxysql(MHD_start_daemon_va+0x1371)[0x92d511] 
proxysql(MHD_start_daemon+0x8a)[0x92d75a] 
proxysql(_ZN14ProxySQL_Admin43flush_admin_variables___database_to_runtimeEP9SQLite3DBb+0x5d3)[0x536cb3] 
proxysql(_ZN14ProxySQL_Admin4initEv+0xb53)[0x55d433] 
proxysql(_Z31ProxySQL_Main_init_Admin_modulev+0x82)[0x4a9a52] 
proxysql(_Z39ProxySQL_Main_init_phase2___not_startedv+0x148)[0x4aa428] 
proxysql(main+0x5a3)[0x4a5713] 
2020-03-19 20:55:26 main.cpp:1538:ProxySQL_daemonize_phase3(): [ERROR] ProxySQL crashed. Restarting!

After a long investigation, we found some clue in these two issues:

• Closing fd created by gnutls_global_init when ran with --daemon nzbget/nzbget#271
• exception when running without -foreground charybdis-ircd/charybdis#71

Those gave us some hint to finally be able to reproduce it.
Indeed, the crash happens only when proxysql is executed in background, as a daemon.
If proxysql is running in foreground, the crash doesn't happen.

If we do not call daemon_close_all() , proxysql doesn't crash also when running in background.

These are the file-descriptors that the parent process has after the fork():

proxysql 30059 rcannao    0r   CHR    1,3      0t0       6 /dev/null
proxysql 30059 rcannao    3r   CHR    1,9      0t0      11 /dev/urandom
proxysql 30059 rcannao    4r   REG   0,51 39530704  867685 /home/rcannao/proxysql/src/proxysql

Therefore, the same FDs are present on the child process.
if daemon_close_all() is called, the child process will lose a file-descriptor to /dev/urandom , and apparently never opens it again.
That brings me back to this comment: charybdis-ircd/charybdis#71 (comment)
Probably gnutls isn't able to pass a good random function because /dev/urandom is closed!

Although the backtrace in #2607 is different, and at today I am still unable to reproduce it, I won't be surprised if the root cause is the same: file descriptor to /dev/urandom being closed.

[renecannao]

PR merged