caddytls: Support new tls.context module (#6369)

* caddytls: Support new tls.context module

This allows modules to manipulate the context passed into CertMagic's GetCertificate function, which can be useful for tracing/metrics, or other
custom logic.

This is experimental and may resolve the request of a sponsor, so we'll see how it goes!

* Derpy derp

modules/caddytls/connpolicy.go

@@ -15,6 +15,7 @@
 package caddytls
 
 import (
+	"context"
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/base64"
@@ -78,6 +79,14 @@ func (cp ConnectionPolicies) Provision(ctx caddy.Context) error {
 				cp[i].ClientAuthentication.verifiers = append(cp[i].ClientAuthentication.verifiers, validator.(ClientCertificateVerifier))
 			}
 		}
+
+		if len(pol.HandshakeContextRaw) > 0 {
+			modIface, err := ctx.LoadModule(pol, "HandshakeContextRaw")
+			if err != nil {
+				return fmt.Errorf("loading handshake context module: %v", err)
+			}
+			cp[i].handshakeContext = modIface.(HandshakeContext)
+		}
 	}
 
 	return nil
@@ -137,6 +146,7 @@ type ConnectionPolicy struct {
 	// How to match this policy with a TLS ClientHello. If
 	// this policy is the first to match, it will be used.
 	MatchersRaw caddy.ModuleMap `json:"match,omitempty" caddy:"namespace=tls.handshake_match"`
+	matchers    []ConnectionMatcher
 
 	// How to choose a certificate if more than one matched
 	// the given ServerName (SNI) value.
@@ -192,15 +202,28 @@ type ConnectionPolicy struct {
 	// This feature is EXPERIMENTAL and subject to change or removal.
 	InsecureSecretsLog string `json:"insecure_secrets_log,omitempty"`
 
+	// A module that can manipulate the context passed into CertMagic's
+	// certificate management functions during TLS handshakes.
+	// EXPERIMENTAL - subject to change or removal.
+	HandshakeContextRaw json.RawMessage `json:"handshake_context,omitempty" caddy:"namespace=tls.context inline_key=module"`
+	handshakeContext    HandshakeContext
+
 	// TLSConfig is the fully-formed, standard lib TLS config
 	// used to serve TLS connections. Provision all
 	// ConnectionPolicies to populate this. It is exported only
 	// so it can be minimally adjusted after provisioning
 	// if necessary (like to adjust NextProtos to disable HTTP/2),
 	// and may be unexported in the future.
 	TLSConfig *tls.Config `json:"-"`
+}
 
-	matchers []ConnectionMatcher
+type HandshakeContext interface {
+	// HandshakeContext returns a context to pass into CertMagic's
+	// GetCertificate function used to serve, load, and manage certs
+	// during TLS handshakes. Generally you'll start with the context
+	// from the ClientHelloInfo, but you may use other information
+	// from it as well. Return an error to abort the handshake.
+	HandshakeContext(*tls.ClientHelloInfo) (context.Context, error)
 }
 
 func (p *ConnectionPolicy) buildStandardTLSConfig(ctx caddy.Context) error {
@@ -240,7 +263,18 @@ func (p *ConnectionPolicy) buildStandardTLSConfig(ctx caddy.Context) error {
 			}
 			cfg.DefaultServerName = p.DefaultSNI
 			cfg.FallbackServerName = p.FallbackSNI
-			return cfg.GetCertificate(hello)
+
+			// TODO: experimental: if a handshake context module is configured, allow it
+			// to modify the context before passing it into CertMagic's GetCertificate
+			ctx := hello.Context()
+			if p.handshakeContext != nil {
+				ctx, err = p.handshakeContext.HandshakeContext(hello)
+				if err != nil {
+					return nil, fmt.Errorf("handshake context: %v", err)
+				}
+			}
+
+			return cfg.GetCertificateWithContext(ctx, hello)
 		},
 		MinVersion: tls.VersionTLS12,
 		MaxVersion: tls.VersionTLS13,