caddytls: Add `load_storage` module (#4055)

An idea that came up in https://caddy.community/t/save-internally-issued-wildcard-certificate-in-consul/11740, this a simple module that might be useful for anyone who uses storage modules that aren't filesystem, to let them load certs/keys externally issued for use by Caddy.

Bit goofy, since we need to fetch the certmagic.Storage during provisioning, it needs a wrapping struct instead of just being an array like `load_files`.

Future work might involve adding Caddyfile support via a subdirective of the `tls` directive maybe?

modules/caddytls/storageloader.go

@@ -0,0 +1,89 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package caddytls
+
+import (
+	"crypto/tls"
+	"fmt"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/certmagic"
+)
+
+func init() {
+	caddy.RegisterModule(StorageLoader{})
+}
+
+// StorageLoader loads certificates and their associated keys
+// from the globally configured storage module.
+type StorageLoader struct {
+	// A list of pairs of certificate and key file names along with their
+	// encoding format so that they can be loaded from storage.
+	Pairs []CertKeyFilePair `json:"pairs,omitempty"`
+
+	// Reference to the globally configured storage module.
+	storage certmagic.Storage
+}
+
+// CaddyModule returns the Caddy module information.
+func (StorageLoader) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "tls.certificates.load_storage",
+		New: func() caddy.Module { return new(StorageLoader) },
+	}
+}
+
+// Provision loads the storage module for sl.
+func (sl *StorageLoader) Provision(ctx caddy.Context) error {
+	sl.storage = ctx.Storage()
+	return nil
+}
+
+// LoadCertificates returns the certificates to be loaded by sl.
+func (sl StorageLoader) LoadCertificates() ([]Certificate, error) {
+	certs := make([]Certificate, 0, len(sl.Pairs))
+	for _, pair := range sl.Pairs {
+		certData, err := sl.storage.Load(pair.Certificate)
+		if err != nil {
+			return nil, err
+		}
+		keyData, err := sl.storage.Load(pair.Key)
+		if err != nil {
+			return nil, err
+		}
+
+		var cert tls.Certificate
+		switch pair.Format {
+		case "":
+			fallthrough
+		case "pem":
+			cert, err = tls.X509KeyPair(certData, keyData)
+		default:
+			return nil, fmt.Errorf("unrecognized certificate/key encoding format: %s", pair.Format)
+		}
+		if err != nil {
+			return nil, err
+		}
+
+		certs = append(certs, Certificate{Certificate: cert, Tags: pair.Tags})
+	}
+	return certs, nil
+}
+
+// Interface guard
+var (
+	_ CertificateLoader = (*StorageLoader)(nil)
+	_ caddy.Provisioner = (*StorageLoader)(nil)
+)