[Bug debuginfod/29976] New: webapi connection pool eats all file handles

["ross at burtonini dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29976

            Bug ID: 29976
           Summary: webapi connection pool eats all file handles
           Product: elfutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: debuginfod
          Assignee: unassigned at sourceware dot org
          Reporter: ross at burtonini dot com
                CC: elfutils-devel at sourceware dot org
  Target Milestone: ---

If I start debuginfod without any concurrency limits:

[Mon Jan  9 17:40:14 2023] (2356243/2356243): libmicrohttpd error: Failed to
create worker inter-thread communication channel: Too many open files

My machine has 256 cores, and stracing debuginfod shows that it fails to open
more files after creating 510 epoll fds (twice):

epoll_create1(EPOLL_CLOEXEC)            = 1021
epoll_ctl(1021, EPOLL_CTL_ADD, 3, {events=EPOLLIN, data={u32=4027013664,
u64=187651148175904}}) = 0
epoll_ctl(1021, EPOLL_CTL_ADD, 1020, {events=EPOLLIN, data={u32=2965961632,
u64=281473647704992}}) = 0
mmap(NULL, 8454144, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) =
0xfff6b97b0000
mprotect(0xfff6b97c0000, 8388608, PROT_READ|PROT_WRITE) = 0
rt_sigprocmask(SIG_BLOCK, ~[], [], 8)   = 0
clone(child_stack=0xfff6b9fbea00,
flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID,
parent_tid=[2361982], tls=0xfff6b9fbf880, child_tidptr=0xfff6b9fbf210) =
2361982
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
eventfd2(0, EFD_CLOEXEC|EFD_NONBLOCK)   = 1022
epoll_create1(EPOLL_CLOEXEC)            = 1023
epoll_ctl(1023, EPOLL_CTL_ADD, 3, {events=EPOLLIN, data={u32=4027014456,
u64=187651148176696}}) = 0
epoll_ctl(1023, EPOLL_CTL_ADD, 1022, {events=EPOLLIN, data={u32=2965961632,
u64=281473647704992}}) = 0
mmap(NULL, 8454144, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) =
0xfff6b8fa0000
mprotect(0xfff6b8fb0000, 8388608, PROT_READ|PROT_WRITE) = 0
rt_sigprocmask(SIG_BLOCK, ~[], [], 8)   = 0
clone(child_stack=0xfff6b97aea00,
flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID,
parent_tid=[2361983], tls=0xfff6b97af880, child_tidptr=0xfff6b97af210) =
2361983
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
eventfd2(0, EFD_CLOEXEC|EFD_NONBLOCK)   = -1 EMFILE (Too many open files)

ulimit -n is 1024, do I really need more just to start debuginfod if I have 256
cores?  As the web connections is 2xthreads and it appears to be using two fds
per connection, maybe I do.

Should the connection pool have a hard limit when using the default? I doubt
512 incoming connections would be usual, and if that is needed then the user
can specify -C.

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