[PATCH pve-common 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 (the difference between its notBefore
and notAfter dates [0]).

This commit is in preparation for fixing #8006. The scaling mirrors
the logic already used in Proxmox Backup Server [1]: certificates
living for less than 10 days are renewed at half their lifetime,
while longer-lived certificates use a third of their lifetime.

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

[0] https://www.cs.auckland.ac.nz/~pgut001/pubs/x509guide.txt
[1] https://lore.proxmox.com/pbs-devel/20260423134607.105229-2-m.federanko@proxmox.com/

Signed-off-by: David Riley <d.riley@proxmox.com>
---
 src/PVE/Certificate.pm | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/src/PVE/Certificate.pm b/src/PVE/Certificate.pm
index b8415e2..45c9bc7 100644
--- a/src/PVE/Certificate.pm
+++ b/src/PVE/Certificate.pm
@@ -328,6 +328,21 @@ sub get_certificate_info {
     return $info;
 }
 
+# Obtain the notBefore timestamp of a X.509 certificate as a UNIX epoch.
+sub get_not_before_as_epoch {
+    my ($cert_path) = @_;
+
+    my $cert = $read_certificate->($cert_path);
+    my $not_before = eval { convert_asn1_to_epoch(Net::SSLeay::X509_get_notBefore($cert)) };
+    my $err = $@;
+
+    Net::SSLeay::X509_free($cert);
+
+    die $err if $err;
+
+    return $not_before;
+}
+
 # Obtain the expiration timestamp of a X.509 certificate as a UNIX epoch.
 sub get_expiration_as_epoch {
     my ($cert_path) = @_;
@@ -354,6 +369,22 @@ sub check_expiry {
     return ($not_after < $timestamp) ? 1 : 0;
 }
 
+sub get_cert_renew_lead_time {
+    my ($cert_path) = @_;
+
+    my $default_lead = 30 * 24 * 60 * 60;
+
+    my $not_before = get_not_before_as_epoch($cert_path);
+    my $not_after = get_expiration_as_epoch($cert_path);
+
+    my $lifetime = $not_after - $not_before;
+    return $default_lead if $lifetime <= 0;
+
+    my $scale = $lifetime < (10 * 24 * 60 * 60) ? 2 : 3;
+
+    return int($lifetime / $scale);
+}
+
 # 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