[PATCH pve-common v2 1/2] certificate: add helpers for lifetime-based ACME renewal

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

Add helpers to calculate a renewal lead time based on the actual
lifetime of an X.509 certificate.

Previously, certificate renewals relied on a hardcoded threshold.
To prepare for dynamic thresholds (#8006), introduce calculations
that scale the renewal lead time proportionally to the certificate's
validity period.

The scaling mirrors the logic already used in Proxmox Backup Server
[1]: 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 [2]. A 3-day minimum floor is present to
ensure the daily update service has multiple chances to retry
transient failures.

Link: https://bugzilla.proxmox.com/show_bug.cgi?id=8006

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

Signed-off-by: David Riley <d.riley@proxmox.com>
---
 src/PVE/Certificate.pm | 39 +++++++++++++++++++++++++++++++++++++--
 1 file changed, 37 insertions(+), 2 deletions(-)

diff --git a/src/PVE/Certificate.pm b/src/PVE/Certificate.pm
index b8415e2..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;
 
@@ -331,16 +332,28 @@ sub get_certificate_info {
 # Obtain the expiration timestamp of a X.509 certificate as a UNIX epoch.
 sub get_expiration_as_epoch {
     my ($cert_path) = @_;
+    my (undef, $not_after) = get_validity_as_epoch($cert_path);
+    return $not_after;
+}
+
+# Obtain the notBefore and notAfter timestamps of a X.509 certificate as UNIX epochs.
+sub get_validity_as_epoch {
+    my ($cert_path) = @_;
 
     my $cert = $read_certificate->($cert_path);
-    my $not_after = eval { convert_asn1_to_epoch(Net::SSLeay::X509_get_notAfter($cert)) };
+    my ($not_before, $not_after) = eval {
+        (
+            convert_asn1_to_epoch(Net::SSLeay::X509_get_notBefore($cert)),
+            convert_asn1_to_epoch(Net::SSLeay::X509_get_notAfter($cert)),
+        );
+    };
     my $err = $@;
 
     Net::SSLeay::X509_free($cert);
 
     die $err if $err;
 
-    return $not_after;
+    return ($not_before, $not_after);
 }
 
 # Checks whether certificate expires before $timestamp (UNIX epoch)
@@ -354,6 +367,28 @@ sub check_expiry {
     return ($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