[PATCH pve-common v3 2/4] certificate: add helper for lifetime-based ACME renewal lead time

[David Riley <d.riley@proxmox.com>]

Add helper to calculate renewal lead time dynamically based on the
certificate's actual validity period.

The scaling mirrors the logic already used in Proxmox Backup Server
[0]: certificates living for less than 10 days are renewed once 1/2 of
their lifetime has elapsed, while longer-lived certificates are
renewed once 2/3 has elapsed [1]. A 3-day minimum floor is present to
ensure the daily update service has multiple chances to retry
transient failures.

[0] https://lore.proxmox.com/pbs-devel/20260423134607.105229-2-m.federanko@proxmox.com/
[1] https://letsencrypt.org/docs/integration-guide/#when-to-renew

Signed-off-by: David Riley <d.riley@proxmox.com>
Reviewed-by: Michael Köppl <m.koeppl@proxmox.com>
Tested-by: Michael Köppl <m.koeppl@proxmox.com>
---
 src/PVE/Certificate.pm | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/src/PVE/Certificate.pm b/src/PVE/Certificate.pm
index 8406949..d2d0847 100644
--- a/src/PVE/Certificate.pm
+++ b/src/PVE/Certificate.pm
@@ -5,6 +5,7 @@ use warnings;
 
 use Date::Parse;
 use Encode qw(decode encode);
+use List::Util qw(max);
 use MIME::Base64 qw(decode_base64 encode_base64);
 use Net::SSLeay;
 
@@ -366,6 +367,28 @@ sub check_expiry {
     return ($not_after < $timestamp) ? 1 : 0;
 }
 
+# Long-lived certs are renewed once 2/3 of their lifetime has elapsed; short-lived ones (under
+# ten days) already at 1/2, following Let's Encrypt's integration guide. A 3-day floor still
+# applies so the daily-update service has a couple of chances to retry transient failures.
+sub get_cert_renew_lead_time {
+    my ($cert_path) = @_;
+
+    my ($not_before, $not_after) = get_validity_as_epoch($cert_path);
+    my $lifetime = $not_after - $not_before;
+
+    if ($lifetime <= 0) {
+        warn "Unexpected validity period ($lifetime s) for '$cert_path', falling back to a 30 day"
+            . " renewal lead time\n";
+
+        return 30 * 24 * 60 * 60;
+    }
+
+    my $scale = $lifetime < (10 * 24 * 60 * 60) ? 2 : 3;
+    my $lead = int($lifetime / $scale);
+
+    return max($lead, 3 * 24 * 60 * 60); # 3 day minimum
+}
+
 # Create a CSR and certificate key for a given order
 # returns path to CSR file or path to CSR and key files
 sub generate_csr {
-- 
2.47.3