postgresql: Default to io_method=io_uring on PostgreSQL 18.

This is more performant than the PostgreSQL 18 default of
`io_method=workers`, but requires kernel 5.1.  All supported OSes of
Zulip have at least that -- however, it may not be available inside
containers, so add a puppet fact to check the syscall before enabling
it in PostgreSQL.

puppet/zulip/lib/facter/io_uring_available.rb

@@ -0,0 +1,16 @@
+Facter.add(:io_uring_available) do
+  confine :kernel => 'Linux'
+  setcode do
+    # Syscall 425 is io_uring_setup
+    # https://www.chromium.org/chromium-os/developer-library/reference/linux-constants/syscalls/#x86_64_425
+    # https://manpages.debian.org/trixie/liburing-dev/io_uring_setup.2.en.html
+    #
+    # We get ENOSYS if the kernel doesn't have the syscall, or EPERM
+    # if it's disabled or restricted via seccomp (i.e. in Docker)
+    result = Facter::Core::Execution.execute(
+      "perl -MErrno=EPERM,ENOSYS -e 'syscall(425); exit((\$! != EPERM and \$! != ENOSYS) ? 0 : 1)'",
+      :on_fail => :failed
+    )
+    result != :failed && $?.success?
+  end
+end

puppet/zulip/templates/postgresql/zulip.conf.template.erb

@@ -42,6 +42,9 @@ max_wal_size = <%= @max_wal_size %>
 <% unless @random_page_cost.nil? -%>
 random_page_cost = <%= @random_page_cost %>
 <% end -%>
+<% if @version.to_i >= 18 && @io_uring_available -%>
+io_method = io_uring
+<% end -%>
 <% unless @effective_io_concurrency.nil? -%>
 effective_io_concurrency = <%= @effective_io_concurrency %>
 <% end -%>