tls: Fix background certificate renewals that use TLS-SNI challenge

The loop which performs renewals in the background obtains a read lock
on the certificate cache map, so that it can be safely iterated. Before
this fix, it would obtain the renewals in the read lock. This has been
fine, except that the TLS-SNI challenge, when invoked after Caddy has
already started, requires adding a certificate to the cache. Doing this
requires an exclusive write lock. But it cannot obtain a write lock
because a read lock is obtained higher in the stack, while the loop
iterates. In other words, it's a deadlock.

I was able to reproduce this issue consistently locally, after jumping
through many hoops to force a renewal in a short time that bypasses
Let's Encrypt's authz caching. I was also able to verify that by queuing
renewals (like we do deletions and OCSP updates), lock contention is
relieved and the deadlock is avoided.

This only affects background renewals where the TLS-SNI(-01) challenge
are used. Users report seeing strange errors in the logs after this
happens ("tls: client offered an unsupported, maximum protocol version
of 301"), but I was not able to reproduce these locally. I was also not
able to reproduce the leak of sockets which are left in CLOSE_WAIT.
I am not sure if those are symptoms of running in production on Linux
and are related to this bug, or not.

Either way, this is an important fix. I do not yet know the ripple
effects this will have on other symptoms we've been chasing. But it
definitely resolves a deadlock during renewals.

caddytls/certificates.go

@@ -229,7 +229,7 @@ func cacheCertificate(cert Certificate) {
 	}
 	certCacheMu.Lock()
 	if _, ok := certCache[""]; !ok {
-		// use as default - must be *appended* to list, or bad things happen!
+		// use as default - must be *appended* to end of list, or bad things happen!
 		cert.Names = append(cert.Names, "")
 		certCache[""] = cert
 	}

caddytls/maintain.go

@@ -65,7 +65,7 @@ func maintainAssets(stopChan chan struct{}) {
 
 // RenewManagedCertificates renews managed certificates.
 func RenewManagedCertificates(allowPrompts bool) (err error) {
-	var renewed, deleted []Certificate
+	var renewQueue, deleteQueue []Certificate
 	visitedNames := make(map[string]struct{})
 
 	certCacheMu.RLock()
@@ -77,7 +77,7 @@ func RenewManagedCertificates(allowPrompts bool) (err error) {
 		// the list of names on this cert should never be empty...
 		if cert.Names == nil || len(cert.Names) == 0 {
 			log.Printf("[WARNING] Certificate keyed by '%s' has no names: %v - removing from cache", name, cert.Names)
-			deleted = append(deleted, cert)
+			deleteQueue = append(deleteQueue, cert)
 			continue
 		}
 
@@ -99,11 +99,21 @@ func RenewManagedCertificates(allowPrompts bool) (err error) {
 				continue
 			}
 
+			// queue for renewal when we aren't in a read lock anymore
+			// (the TLS-SNI challenge will need a write lock in order to
+			// present the certificate, so we renew outside of read lock)
+			renewQueue = append(renewQueue, cert)
+		}
+	}
+	certCacheMu.RUnlock()
+
+	// Perform renewals that are queued
+	for _, cert := range renewQueue {
 		// Get the name which we should use to renew this certificate;
 		// we only support managing certificates with one name per cert,
 		// so this should be easy. We can't rely on cert.Config.Hostname
 		// because it may be a wildcard value from the Caddyfile (e.g.
-			// *.something.com) which, as of 2016, is not supported by ACME.
+		// *.something.com) which, as of Jan. 2017, is not supported by ACME.
 		var renewName string
 		for _, name := range cert.Names {
 			if name != "" {
@@ -112,38 +122,33 @@ func RenewManagedCertificates(allowPrompts bool) (err error) {
 			}
 		}
 
+		// perform renewal
 		err := cert.Config.RenewCert(renewName, allowPrompts)
 		if err != nil {
-				if allowPrompts && timeLeft < 0 {
+			if allowPrompts && cert.NotAfter.Sub(time.Now().UTC()) < 0 {
 				// Certificate renewal failed, the operator is present, and the certificate
 				// is already expired; we should stop immediately and return the error. Note
 				// that we used to do this any time a renewal failed at startup. However,
 				// after discussion in https://github.com/mholt/caddy/issues/642 we decided to
 				// only stop startup if the certificate is expired. We still log the error
 				// otherwise. I'm not sure how permanent the change in #642 will be...
-					certCacheMu.RUnlock()
+				// TODO: Get rid of the expiration check... always break on error.
 				return err
 			}
 			log.Printf("[ERROR] %v", err)
 			if cert.Config.OnDemand {
-					deleted = append(deleted, cert)
+				deleteQueue = append(deleteQueue, cert)
 			}
 		} else {
-				renewed = append(renewed, cert)
-			}
-		}
-	}
-	certCacheMu.RUnlock()
-
-	// Apply changes to the cache
-	for _, cert := range renewed {
-		// TODO: Don't do these until we have valid OCSP for the new cert
+			// successful renewal, so update in-memory cache by loading
+			// renewed certificate so it will be used with handshakes
+			// TODO: Not until CA has valid OCSP response ready for the new cert... sigh.
 			if cert.Names[len(cert.Names)-1] == "" {
 				// Special case: This is the default certificate. We must
 				// flush it out of the cache so that we no longer point to
 				// the old, un-renewed certificate. Otherwise it will be
-			// renewed on every scan, which is too often. When we cache
-			// this certificate in a moment, it will be the default again.
+				// renewed on every scan, which is too often. The next cert
+				// to be cached (probably this one) will become the default.
 				certCacheMu.Lock()
 				delete(certCache, "")
 				certCacheMu.Unlock()
@@ -156,7 +161,10 @@ func RenewManagedCertificates(allowPrompts bool) (err error) {
 				log.Printf("[ERROR] %v", err)
 			}
 		}
-	for _, cert := range deleted {
+	}
+
+	// Apply queued deletion changes to the cache
+	for _, cert := range deleteQueue {
 		certCacheMu.Lock()
 		for _, name := range cert.Names {
 			delete(certCache, name)