Extract Workhorse <-> GitLab authentication to make it reusable61927-internal-api-jwt-auth

Introduce JWTAutheticatable module that can be reused for ai=uthtication
between Pages and GitLab (the same way we use do now for Workhorse).

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/61927.

2 files changed, 44 insertions, 26 deletions

diff --git a/lib/gitlab/jwt_authenticatable.rb b/lib/gitlab/jwt_authenticatable.rb
new file mode 100644
index 00000000000..1270a148e8d
--- /dev/null
+++ b/lib/gitlab/jwt_authenticatable.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module JwtAuthenticatable
+    # Supposedly the effective key size for HMAC-SHA256 is 256 bits, i.e. 32
+    # bytes https://tools.ietf.org/html/rfc4868#section-2.6
+    SECRET_LENGTH = 32
+
+    def self.included(base)
+      base.extend(ClassMethods)
+    end
+
+    module ClassMethods
+      include Gitlab::Utils::StrongMemoize
+
+      def decode_jwt_for_issuer(issuer, encoded_message)
+        JWT.decode(
+          encoded_message,
+          secret,
+          true,
+          { iss: issuer, verify_iss: true, algorithm: 'HS256' }
+        )
+      end
+
+      def secret
+        strong_memoize(:secret) do
+          Base64.strict_decode64(File.read(secret_path).chomp).tap do |bytes|
+            raise "#{secret_path} does not contain #{SECRET_LENGTH} bytes" if bytes.length != SECRET_LENGTH
+          end
+        end
+      end
+
+      def write_secret
+        bytes = SecureRandom.random_bytes(SECRET_LENGTH)
+        File.open(secret_path, 'w:BINARY', 0600) do |f|
+          f.chmod(0600) # If the file already existed, the '0600' passed to 'open' above was a no-op.
+          f.write(Base64.strict_encode64(bytes))
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/workhorse.rb b/lib/gitlab/workhorse.rb
index 29087d26007..139ec6e384a 100644
--- a/lib/gitlab/workhorse.rb
+++ b/lib/gitlab/workhorse.rb
@@ -15,9 +15,7 @@ module Gitlab
     ALLOWED_GIT_HTTP_ACTIONS = %w[git_receive_pack git_upload_pack info_refs].freeze
     DETECT_HEADER = 'Gitlab-Workhorse-Detect-Content-Type'.freeze
 
-    # Supposedly the effective key size for HMAC-SHA256 is 256 bits, i.e. 32
-    # bytes https://tools.ietf.org/html/rfc4868#section-2.6
-    SECRET_LENGTH = 32
+    include JwtAuthenticatable
 
     class << self
       def git_http_ok(repository, repo_type, user, action, show_all_refs: false)
@@ -187,34 +185,12 @@ module Gitlab
         path.readable? ? path.read.chomp : 'unknown'
       end
 
-      def secret
-        @secret ||= begin
-          bytes = Base64.strict_decode64(File.read(secret_path).chomp)
-          raise "#{secret_path} does not contain #{SECRET_LENGTH} bytes" if bytes.length != SECRET_LENGTH
-
-          bytes
-        end
-      end
-
-      def write_secret
-        bytes = SecureRandom.random_bytes(SECRET_LENGTH)
-        File.open(secret_path, 'w:BINARY', 0600) do |f|
-          f.chmod(0600) # If the file already existed, the '0600' passed to 'open' above was a no-op.
-          f.write(Base64.strict_encode64(bytes))
-        end
-      end
-
       def verify_api_request!(request_headers)
         decode_jwt(request_headers[INTERNAL_API_REQUEST_HEADER])
       end
 
       def decode_jwt(encoded_message)
-        JWT.decode(
-          encoded_message,
-          secret,
-          true,
-          { iss: 'gitlab-workhorse', verify_iss: true, algorithm: 'HS256' }
-        )
+        decode_jwt_for_issuer('gitlab-workhorse', encoded_message)
       end
 
       def secret_path