sysctl: set kernel.core_pipe_limit=16

We need to make sure that our coredump pattern handler manages to read
process metadata from /proc/$PID/ before the kernel reaps the crashed
process. By default the kernel will reap the process as soon as it can.
By setting kernel.core_pipe_limit to a non-zero the kernel will wait for
userspace to finish before reaping.

We'll set the value to 16, which allows 16 crashes to be
processed in parallel. This matches the MaxConnections= setting in
systemd-coredump.socket.

See: #17301

(This doesn't close 17301, since we probably should also gracefully
handle if /proc/$PID/ vanished already while our coredump handler runs,
just in case people loclly set the sysctl back to zero. i.e. we should
collect what we can and rather issue an incomplete log record than
none.)

sysctl.d/50-coredump.conf.in

@@ -15,6 +15,17 @@
 # See systemd-coredump(8) and core(5).
 kernel.core_pattern=|@rootlibexecdir@/systemd-coredump %P %u %g %s %t %c %h
 
+# Allow that 16 coredumps are dispatched in parallel by the kernel. We want to
+# be able to collect process metadata from /proc/%P/ while processing
+# coredumps, and thus need to make sure the crashed processes are not reaped
+# until we finished collecting what we need. The kernel default for this sysctl
+# is "0" which means the kernel doesn't wait for userspace processes to finish
+# processing before reaping the crashed processes — by setting this higher the
+# kernel will delay reaping until we are done, but only for the specified
+# number of crashes in parallel. The value of 16 is chosen to match
+# systemd-coredump.socket's MaxConnections= value.
+kernel.core_pipe_limit=16
+
 # Also dump processes executing a set-user-ID/set-group-ID program that is
 # owned by a user/group other than the real user/group ID of the process, or
 # a program that has file capabilities. ("2" is called "suidsafe" in core(5)).