cmd/tailscale: add --min-validity flag to the cert command (#12822)

Some users run "tailscale cert" in a cron job to renew their
certificates on disk. The time until the next cron job run may be long
enough for the old cert to expire with our default heristics.

Add a `--min-validity` flag which ensures that the returned cert is
valid for at least the provided duration (unless it's longer than the
cert lifetime set by Let's Encrypt).

Updates #8725

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/tailscale/cli/cert.go

@@ -16,6 +16,7 @@ import (
 	"net/http"
 	"os"
 	"strings"
+	"time"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
 	"software.sslmate.com/src/go-pkcs12"
@@ -34,14 +35,16 @@ var certCmd = &ffcli.Command{
 		fs.StringVar(&certArgs.certFile, "cert-file", "", "output cert file or \"-\" for stdout; defaults to DOMAIN.crt if --cert-file and --key-file are both unset")
 		fs.StringVar(&certArgs.keyFile, "key-file", "", "output key file or \"-\" for stdout; defaults to DOMAIN.key if --cert-file and --key-file are both unset")
 		fs.BoolVar(&certArgs.serve, "serve-demo", false, "if true, serve on port :443 using the cert as a demo, instead of writing out the files to disk")
+		fs.DurationVar(&certArgs.minValidity, "min-validity", 0, "ensure the certificate is valid for at least this duration; the output certificate is never expired if this flag is unset or 0, but the lifetime may vary; the maximum allowed min-validity depends on the CA")
 		return fs
 	})(),
 }
 
 var certArgs struct {
-	certFile string
-	keyFile  string
-	serve    bool
+	certFile    string
+	keyFile     string
+	serve       bool
+	minValidity time.Duration
 }
 
 func runCert(ctx context.Context, args []string) error {
@@ -102,7 +105,7 @@ func runCert(ctx context.Context, args []string) error {
 		certArgs.certFile = domain + ".crt"
 		certArgs.keyFile = domain + ".key"
 	}
-	certPEM, keyPEM, err := localClient.CertPair(ctx, domain)
+	certPEM, keyPEM, err := localClient.CertPairWithValidity(ctx, domain, certArgs.minValidity)
 	if err != nil {
 		return err
 	}