✨ Add support for GMail's X-GM-EXT-1 extension

Adds support for fetching `X-GM-MSGID`, `X-GM-THRID`, and `X-GM-LABELS`
on GMail servers.  Note that the `X-GM-RAW` search attribute is
documented but it does not need any special code to support it.

See specs at https://developers.google.com/gmail/imap/imap-extensions.

lib/net/imap.rb

@@ -483,9 +483,17 @@ module Net
   #   which arranges the results into ordered groups or threads according to a
   #   chosen algorithm.
   #
-  # ==== +XLIST+ (non-standard, deprecated)
+  # ==== +X-GM-EXT-1+
+  # +X-GM-EXT-1+ is a non-standard Gmail extension.  See {Google's
+  # documentation}[https://developers.google.com/gmail/imap/imap-extensions].
+  # - Updates #fetch and #uid_fetch with support for +X-GM-MSGID+ (unique
+  #   message ID), +X-GM-THRID+ (thread ID), and +X-GM-LABELS+ (Gmail labels).
+  # - Updates #search with the +X-GM-RAW+ search attribute.
   # - #xlist: replaced by +SPECIAL-USE+ attributes in #list responses.
   #
+  # *NOTE:* The +OBJECTID+ extension should replace +X-GM-MSGID+ and
+  # +X-GM-THRID+, although neither Gmail nor Net::IMAP support it yet.
+  #
   # ==== RFC6851: +MOVE+
   # Folded into IMAP4rev2[https://tools.ietf.org/html/rfc9051] and also included
   # above with {Core IMAP commands}[rdoc-ref:Net::IMAP@Core+IMAP+commands].

lib/net/imap/fetch_data.rb

@@ -53,6 +53,12 @@ class IMAP < Protocol
     # * <b><tt>"RFC822.TEXT"</tt></b> --- See #rfc822_text or replace with
     #   <tt>"BODY[TEXT]"</tt> and #text.
     #
+    # Net::IMAP supports dynamic attributes defined by the following extensions:
+    # * +X-GM-EXT-1+ {[non-standard Gmail
+    #   extension]}[https://developers.google.com/gmail/imap/imap-extensions]
+    #   * <b><tt>"X-GM-MSGID"</tt></b> --- unique message ID.  Access via #attr.
+    #   * <b><tt>"X-GM-THRID"</tt></b> --- Thread ID.  Access via #attr.
+    #
     # [Note:]
     #   >>>
     #     Additional static fields are defined in other \IMAP extensions, but
@@ -73,6 +79,9 @@ class IMAP < Protocol
     #
     # * +CONDSTORE+ {[RFC7162]}[https://www.rfc-editor.org/rfc/rfc7162.html]:
     #   * <b><tt>"MODSEQ"</tt></b> --- See #modseq.
+    # * +X-GM-EXT-1+ {[non-standard Gmail
+    #   extension]}[https://developers.google.com/gmail/imap/imap-extensions]
+    #   * <b><tt>"X-GM-LABELS"</tt></b> --- Gmail labels.  Access via #attr.
     #
     # [Note:]
     #   >>>

lib/net/imap/response_parser.rb

@@ -513,6 +513,12 @@ def case_insensitive__nstring
       #                          ; Strictly ascending
       alias uniqueid    nz_number
 
+      # valid number ranges are not enforced by parser
+      #
+      # a 64-bit unsigned integer and is the decimal equivalent for the ID hex
+      # string used in the web interface and the Gmail API.
+      alias x_gm_id     number
+
       # [RFC3501 & RFC9051:]
       #   response        = *(continue-req / response-data) response-done
       #
@@ -765,6 +771,9 @@ def msg_att(n)
             when "RFC822.HEADER"        then nstring            # not in rev2
             when "RFC822.TEXT"          then nstring            # not in rev2
             when "MODSEQ"               then parens__modseq     # CONDSTORE
+            when "X-GM-MSGID"           then x_gm_id            # GMail
+            when "X-GM-THRID"           then x_gm_id            # GMail
+            when "X-GM-LABELS"          then x_gm_labels        # GMail
             else parse_error("unknown attribute `%s' for {%d}", name, n)
             end
           attr[name] = val
@@ -1765,6 +1774,19 @@ def parens__mbx_list_flags
           .split(nil).map! { _1.capitalize.to_sym }
       end
 
+      # See https://developers.google.com/gmail/imap/imap-extensions
+      def x_gm_label; accept(T_BSLASH) ? atom.capitalize.to_sym : astring end
+
+      # See https://developers.google.com/gmail/imap/imap-extensions
+      def x_gm_labels
+        lpar; return [] if rpar?
+        labels = []
+        labels << x_gm_label
+        labels << x_gm_label while SP?
+        rpar
+        labels
+      end
+
       # See https://www.rfc-editor.org/errata/rfc3501
       #
       # charset = atom / quoted

test/net/imap/fixtures/response_parser/fetch_responses.yml

@@ -123,6 +123,71 @@
           RFC822: "foo\r\n"
       raw_data: "* 123 FETCH (RFC822 {5}\r\nfoo\r\n)\r\n"
 
+  test_fetch_msg_att_X-GM-MSGID:
+    :comments: |
+      Example copied from https://developers.google.com/gmail/imap/imap-extensions
+    :response: "* 1 FETCH (X-GM-MSGID 1278455344230334865)\r\n"
+    :expected: !ruby/struct:Net::IMAP::UntaggedResponse
+      name: FETCH
+      data: !ruby/struct:Net::IMAP::FetchData
+        seqno: 1
+        attr:
+          X-GM-MSGID: 1278455344230334865
+      raw_data: "* 1 FETCH (X-GM-MSGID 1278455344230334865)\r\n"
+
+  test_fetch_msg_att_X-GM-THRID:
+    :comments: |
+      Example copied from https://developers.google.com/gmail/imap/imap-extensions
+    :response: "* 4 FETCH (X-GM-THRID 1266894439832287888)\r\n"
+    :expected: !ruby/struct:Net::IMAP::UntaggedResponse
+      name: FETCH
+      data: !ruby/struct:Net::IMAP::FetchData
+        seqno: 4
+        attr:
+          X-GM-THRID: 1266894439832287888
+      raw_data: "* 4 FETCH (X-GM-THRID 1266894439832287888)\r\n"
+
+  test_fetch_msg_att_X-GM-LABELS_1:
+    :comments: |
+      Example copied from https://developers.google.com/gmail/imap/imap-extensions
+    :response: "* 1 FETCH (X-GM-LABELS (\\Inbox \\Sent Important \"Muy Importante\"))\r\n"
+    :expected: !ruby/struct:Net::IMAP::UntaggedResponse
+      name: FETCH
+      data: !ruby/struct:Net::IMAP::FetchData
+        seqno: 1
+        attr:
+          X-GM-LABELS:
+          - :Inbox
+          - :Sent
+          - Important
+          - Muy Importante
+      raw_data: "* 1 FETCH (X-GM-LABELS (\\Inbox \\Sent Important \"Muy Importante\"))\r\n"
+
+  test_fetch_msg_att_X-GM-LABELS_2:
+    :comments: |
+      Example copied from https://developers.google.com/gmail/imap/imap-extensions
+    :response: "* 2 FETCH (X-GM-LABELS (foo))\r\n"
+    :expected: !ruby/struct:Net::IMAP::UntaggedResponse
+      name: FETCH
+      data: !ruby/struct:Net::IMAP::FetchData
+        seqno: 2
+        attr:
+          X-GM-LABELS:
+          - foo
+      raw_data: "* 2 FETCH (X-GM-LABELS (foo))\r\n"
+
+  test_fetch_msg_att_X-GM-LABELS_3:
+    :comments: |
+      Example copied from https://developers.google.com/gmail/imap/imap-extensions
+    :response: "* 3 FETCH (X-GM-LABELS ())\r\n"
+    :expected: !ruby/struct:Net::IMAP::UntaggedResponse
+      name: FETCH
+      data: !ruby/struct:Net::IMAP::FetchData
+        seqno: 3
+        attr:
+          X-GM-LABELS: []
+      raw_data: "* 3 FETCH (X-GM-LABELS ())\r\n"
+
   test_invalid_fetch_msg_att_rfc822_with_brackets:
     :response: "* 123 FETCH (RFC822[] {5}\r\nfoo\r\n)\r\n"
     :expected: !ruby/struct:Net::IMAP::UntaggedResponse