✨ Gather ESEARCH response to #search/#uid_search

* Return empty ESearchResult when no result
* Clear SEARCH responses before new search.  Note that we don't need to
  do this for ESEARCH responses, since they are tagged.

lib/net/imap.rb

@@ -1930,7 +1930,7 @@ def uid_expunge(uid_set)
     end
 
     # :call-seq:
-    #   search(criteria, charset = nil) -> result
+    #   search(criteria, charset = nil, esearch: false) -> result
     #
     # Sends a {SEARCH command [IMAP4rev1 §6.4.4]}[https://www.rfc-editor.org/rfc/rfc3501#section-6.4.4]
     # to search the mailbox for messages that match the given search +criteria+,
@@ -1966,6 +1966,11 @@ def uid_expunge(uid_set)
     #                 "CHARSET", "UTF-8",
     #                 %w(OR UNSEEN FLAGGED)])
     #
+    # +esearch+ controls the return type when the server does not return any
+    # search results.  If +esearch+ is +true+ or +criteria+ begins with
+    # +RETURN+, an empty ESearchResult will be returned.  When +esearch+ is
+    # +false+, an empty SearchResult will be returned.
+    #
     # Related: #uid_search
     #
     # ===== Search criteria
@@ -2974,12 +2979,24 @@ def enforce_logindisabled?
       end
     end
 
-    def search_internal(cmd, keys, charset = nil)
+    def search_internal(cmd, keys, charset = nil, esearch: false)
       keys = normalize_searching_criteria(keys)
       args = charset ? ["CHARSET", charset, *keys] : keys
       synchronize do
-        send_command(cmd, *args)
-        clear_responses("SEARCH").last || []
+        clear_responses("SEARCH")
+        result = nil
+        send_command(cmd, *args) do |response, tag|
+          if response in data: ESearchResult(tag: ^tag) => result
+            responses("ESEARCH") { _1.delete(result) }
+          end
+        end
+        if result
+          result
+        elsif esearch || keys in RawData[/\ARETURN /] | Array[/\ARETURN\z/i, *]
+          ESearchResult.new
+        else
+          clear_responses("SEARCH").last || []
+        end
       end
     end
 

test/net/imap/test_imap.rb

@@ -1234,6 +1234,45 @@ def test_unselect
     end
   end
 
+  test("#search/#uid_search with ESEARCH or IMAP4rev2") do
+    with_fake_server do |server, imap|
+      # Example from RFC9051, 6.4.4:
+      #   C: A282 SEARCH RETURN (MIN COUNT) FLAGGED
+      #       SINCE 1-Feb-1994 NOT FROM "Smith"
+      #   S: * ESEARCH (TAG "A282") MIN 2 COUNT 3
+      #   S: A282 OK SEARCH completed
+      server.on "SEARCH" do |cmd|
+        cmd.untagged "ESEARCH", "(TAG \"unrelated1\") MIN 1 COUNT 2"
+        cmd.untagged "ESEARCH", "(TAG %p) MIN 2 COUNT 3" % [cmd.tag]
+        cmd.untagged "ESEARCH", "(TAG \"unrelated2\") MIN 222 COUNT 333"
+        cmd.done_ok
+      end
+      result = imap.search(
+        'RETURN (MIN COUNT) FLAGGED SINCE 1-Feb-1994 NOT FROM "Smith"'
+      )
+      cmd = server.commands.pop
+      assert_equal Net::IMAP::ESearchResult.new(
+        cmd.tag, false, [["MIN", 2], ["COUNT", 3]]
+      ), result
+      esearch_responses = imap.clear_responses("ESEARCH")
+      assert_equal 2, esearch_responses.count
+      refute esearch_responses.include?(result)
+    end
+  end
+
+  test("missing server ESEARCH response") do
+    with_fake_server do |server, imap|
+      # Example from RFC9051, 6.4.4:
+      #   C: A282 SEARCH RETURN (SAVE) FLAGGED SINCE 1-Feb-1994 NOT FROM "Smith"
+      #   S: A282 OK SEARCH completed, result saved
+      server.on "SEARCH" do |cmd| cmd.done_ok "result saved" end
+      result = imap.search(
+        'RETURN (SAVE) FLAGGED SINCE 1-Feb-1994 NOT FROM "Smith"'
+      )
+      assert_equal Net::IMAP::ESearchResult.new, result
+    end
+  end
+
   test("missing server SEARCH response") do
     with_fake_server do |server, imap|
       server.on "SEARCH",     &:done_ok