Use net-imap's SASL implementation 🚧[WIP]🚧

This commit converts `#authenticate` to use `net-imap` as a generic
fallback for mechanisms that haven't otherwise been added (as subclasses
of `Authenticator`).  In this commit, the original implementation is
still used by `#authenticate` for the `PLAIN`, `LOGIN`, and `CRAM-MD5`
mechanisms.  Every other mechanism supported by `net-imap` v0.4.0 is
added here:
* `ANONYMOUS`
* `DIGEST-MD5` _(deprecated)_
* `EXTERNAL`
* `OAUTHBEARER`
* `SCRAM-SHA-1` and `SCRAM-SHA-256`
* `XOAUTH`

**TODO:** Ideally, `net-smtp` and `net-imap` should both depend on a
shared `sasl` or `net-sasl` gem, rather than keep the SASL
implementation inside one or the other.  See
ruby/net-imap#23.

**TODO:** since we already know the authenticator arguments up-front, we
can validate authenticator arguments by simply creating the
authenticator object and rely on the its initializer to raise
ArgumentError for missing args.

.github/workflows/test.yml

@@ -19,4 +19,4 @@ jobs:
     - name: Install dependencies
       run: bundle install
     - name: Run test
-      run: rake test
+      run: bundle exec rake test

lib/net/smtp.rb

@@ -174,18 +174,56 @@ class SMTPUnsupportedCommand < ProtocolError
   # === \SMTP Authentication
   #
   # The Net::SMTP class supports the \SMTP extension for SASL Authentication
-  # [RFC4954[https://www.rfc-editor.org/rfc/rfc4954.html]].  To use SMTP
-  # authentication, pass extra arguments to SMTP.start or SMTP#start.
+  # [RFC4954[https://www.rfc-editor.org/rfc/rfc4954.html]] and the following
+  # SASL mechanisms: +ANONYMOUS+, +EXTERNAL+, +OAUTHBEARER+, +PLAIN+,
+  # +SCRAM-SHA-1+, +SCRAM-SHA-256+, and +XOAUTH2+.
+  #
+  # To use \SMTP authentication, pass extra arguments to
+  # SMTP.start or SMTP#start.
   #
   #     # PLAIN
   #     Net::SMTP.start('your.smtp.server', 25,
   #                     user: 'Your Account', secret: 'Your Password', authtype: :plain)
+  #     Net::SMTP.start("your.smtp.server", 25,
+  #                     auth: {type:     :plain,
+  #                            username: "authentication identity",
+  #                            password: password,
+  #                            authzid:  "authorization identity"}) # optional
+  #
+  #     # SCRAM-SHA-256
+  #     Net::SMTP.start("your.smtp.server", 25,
+  #                     user: "authentication identity", secret: password,
+  #                     authtype: :scram_sha_256)
+  #     Net::SMTP.start("your.smtp.server", 25,
+  #                     auth: {type:     :scram_sha_256,
+  #                            username: "authentication identity",
+  #                            password: password,
+  #                            authzid:  "authorization identity"}) # optional
+  #
+  #     # OAUTHBEARER
+  #     Net::SMTP.start("your.smtp.server", 25,
+  #                     auth: {type:         :oauthbearer,
+  #                            oauth2_token: oauth2_access_token,
+  #                            authzid:      "authorization identity", # optional
+  #                            host:         "your.smtp.server",       # optional
+  #                            port:         25})                      # optional
+  #
+  #     # XOAUTH2
+  #     Net::SMTP.start("your.smtp.server", 25,
+  #                     user: "username", secret: oauth2_access_token, authtype: :xoauth2)
+  #     Net::SMTP.start("your.smtp.server", 25,
+  #                     auth: {type:         :xoauth2,
+  #                            username:     "username",
+  #                            oauth2_token: oauth2_token})
   #
-  # Support for other SASL mechanisms—such as +EXTERNAL+, +OAUTHBEARER+,
-  # +SCRAM-SHA-256+, and +XOAUTH2+—will be added in a future release.
+  #     # EXTERNAL
+  #     Net::SMTP.start("your.smtp.server", 587,
+  #                     starttls: :always, ssl_context_params: ssl_ctx_params,
+  #                     authtype: "external")
   #
-  # The +LOGIN+ and +CRAM-MD5+ mechanisms are still available for backwards
-  # compatibility, but are deprecated and should be avoided.
+  # +DIGEST-MD5+, +LOGIN+, and +CRAM-MD5+ are still available for backwards
+  # compatibility, but are deprecated and should be avoided.  <em>Using a
+  # deprecated authentication mechanisms will print a warning.</em>
   #
   class SMTP < Protocol
     VERSION = "0.4.0"
@@ -912,14 +950,19 @@ def authenticate(*args, **kwargs, &block)
     # include +authcid+ for authentication identity, +authzid+ for authorization
     # identity, +username+ for either "authentication identity" or
     # "authorization identity" depending on the +mechanism+, and +password+.
-    def auth(authtype = nil, *args, type: nil, **kwargs, &blk)
-      type ||= authtype || DEFAULT_AUTH_TYPE
-      authenticator = Authenticator.auth_class(type).new(self)
-      if kwargs.empty?
-        # TODO: remove this, unless it is needed for 2.6/2.7/3.0 compatibility
-        critical { authenticator.auth(*args, &blk) }
-      else
-        critical { authenticator.auth(*args, **kwargs, &blk) }
+    #
+    # Keyword arguments that do not apply to the +mechanism+ may be silently
+    # ignored.
+    def auth(*args, **kwargs, &blk)
+      # TODO: continue to use v0.4.0 compatible API from here, and move the
+      # following argument munging and exception handling into the SASLAdapter.
+      args, kwargs = backward_compatible_auth_args(*args, **kwargs)
+      critical do
+        Authenticator::SASLAdapter.new(self).authenticate(*args, **kwargs, &blk)
+      rescue SMTPServerBusy, SMTPSyntaxError, SMTPFatalError => error
+        raise SMTPAuthenticationError.new(error.response)
+      rescue SASL::AuthenticationIncomplete => error
+        raise error.response.exception_class.new(error.response)
       end
     end
 
@@ -940,6 +983,39 @@ def check_auth_args(user, secret, authtype = DEFAULT_AUTH_TYPE)
       end
     end
 
+    # Temporarily adapt for differences between SMTP and Net::IMAP::SASL:
+    #
+    # * Net::IMAP::SASL doesn't handle the `type` keyword.
+    # * Net::IMAP::SASL adapters don't handle the +secret+ keyword.
+    # * Net::IMAP::SASL::LoginAdapter and CramMd5Adapter don't accept kwargs!
+    #
+    # These are reasonable changes and Net::IMAP::SASL can be updated.
+    #
+    # However, Net::IMAP::SASL will probably never support the `user` keyword.
+    def backward_compatible_auth_args(typearg = nil,
+                                      *args,
+                                      type: nil,
+                                      user: nil,
+                                      secret: nil,
+                                      **kwargs)
+      type ||= typearg || DEFAULT_AUTH_TYPE
+      check_auth_method(type)
+      kwargs[:username] ||= user if user
+      if secret
+        secret_type = type.match?(/\AX?OAUTH/i) ? :oauth2_token : :password
+        kwargs[secret_type] ||= secret
+      end
+      auth_class = Authenticator.auth_class(type)
+      if type.match?(/\A(?:LOGIN|CRAM[-_]MD5)\z/i)
+        usernames = [kwargs.delete(:authcid), kwargs.delete(:username)]
+        secrets   = [kwargs.delete(:password)]
+        args[0] ||= usernames.compact.first
+        args[1] ||= secrets.compact.first
+        check_auth_args(args[0], args[1], type)
+      end
+      [[type, *args], kwargs]
+    end
+
     #
     # SMTP command dispatcher
     #
@@ -1048,6 +1124,27 @@ def get_response(reqline)
       recv_response()
     end
 
+    # Returns a successful Response.
+    #
+    # Yields continuation data.
+    #
+    # This method may raise:
+    #
+    # * Net::SMTPAuthenticationError
+    # * Net::SMTPServerBusy
+    # * Net::SMTPSyntaxError
+    # * Net::SMTPFatalError
+    # * Net::SMTPUnknownError
+    def send_command_with_continuations(*args)
+      server_resp = get_response args.join(" ")
+      while server_resp.continue?
+        client_resp = yield server_resp.string.strip.split(nil, 2).last
+        server_resp = get_response client_resp
+      end
+      server_resp.success? or raise server_resp.exception_class.new(server_resp)
+      server_resp
+    end
+
     private
 
     def validate_line(line)

lib/net/smtp/auth_sasl_adapter.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require "net/imap"
+
+module Net
+  class SMTP
+    SASL = Net::IMAP::SASL
+
+    class Authenticator
+
+      # Experimental
+      #
+      # Initialize with a block that runs a command, yielding for continuations.
+      class SASLAdapter < SASL::ClientAdapter
+        include SASL::ProtocolAdapters::SMTP
+
+        RESPONSE_ERRORS = [
+          SMTPAuthenticationError,
+          SMTPServerBusy,
+          SMTPSyntaxError,
+          SMTPFatalError,
+        ].freeze
+
+        def initialize(...)
+          super
+          @command_proc ||= client.method(:send_command_with_continuations)
+        end
+
+        def host;                     client.address                  end
+        def response_errors;          RESPONSE_ERRORS                 end
+        def sasl_ir_capable?;         true                            end # TODO
+        def auth_capable?(mechanism); client.auth_capable?(mechanism) end
+        def drop_connection;          client.finish                   end
+        def drop_connection!;         client.finish                   end # TODO
+      end
+
+    end
+  end
+end

lib/net/smtp/auth_sasl_compatibility_adapter.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module Net
+  class SMTP
+
+    # Curries arguments to SMTP#auth, using the Authenticator API.
+    #
+    # Net::SMTP#authenticate still supports the v0.4.0 Authenticator API, so
+    # Authenticator subclasses can still be added and used with it.  This class
+    # will be used as the default, when no matching Authenticator subclass
+    # exists.
+    class CompatibilityAdapter
+      def initialize(mechanism) @mechanism = mechanism end
+      def new(smtp) @smtp = smtp; self end
+      def auth(*args, **kwargs, &block)
+        args.pop while args.any? && args.last.nil?
+        @smtp.auth(@mechanism, *args, **kwargs, &block)
+      end
+    end
+
+    Authenticator.auth_classes.default_proc = ->_, mechanism {
+      CompatibilityAdapter.new(mechanism)
+    }
+
+  end
+end

net-smtp.gemspec

@@ -26,4 +26,5 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency "net-protocol"
+  spec.add_dependency "net-imap", ">= 0.4.1" # experimental SASL support
 end

test/net/smtp/test_smtp.rb

@@ -530,16 +530,6 @@ def test_start_auth_cram_md5
       assert_raise Net::SMTPAuthenticationError do
         Net::SMTP.start('localhost', port, user: 'account', password: 'password', authtype: :cram_md5){}
       end
-
-      port = fake_server_start(auth: 'CRAM-MD5')
-      smtp = Net::SMTP.new('localhost', port)
-      auth_cram_md5 = Net::SMTP::AuthCramMD5.new(smtp)
-      auth_cram_md5.define_singleton_method(:digest_class) { raise '"openssl" or "digest" library is required' }
-      Net::SMTP::AuthCramMD5.define_singleton_method(:new) { |_| auth_cram_md5 }
-      e = assert_raise RuntimeError do
-        smtp.start(user: 'account', password: 'password', authtype: :cram_md5){}
-      end
-      assert_equal('"openssl" or "digest" library is required', e.message)
     end
 
     def test_start_instance