diff --git a/NEWS.md b/NEWS.md
index 3ba44ddeb87e75..c666338ffafbe0 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -92,6 +92,7 @@ releases.
 * prism 1.9.0
   * 1.7.0 to [v1.8.0][prism-v1.8.0], [v1.8.1][prism-v1.8.1], [v1.9.0][prism-v1.9.0]
 * psych 5.4.0
+  * 5.3.1 to [v5.4.0][psych-v5.4.0]
 * resolv 0.7.1
   * 0.7.0 to [v0.7.1][resolv-v0.7.1]
 * stringio 3.2.1.dev
@@ -250,6 +251,7 @@ A lot of work has gone into making Ractors more stable, performant, and usable.
 [prism-v1.8.0]: https://github.com/ruby/prism/releases/tag/v1.8.0
 [prism-v1.8.1]: https://github.com/ruby/prism/releases/tag/v1.8.1
 [prism-v1.9.0]: https://github.com/ruby/prism/releases/tag/v1.9.0
+[psych-v5.4.0]: https://github.com/ruby/psych/releases/tag/v5.4.0
 [resolv-v0.7.1]: https://github.com/ruby/resolv/releases/tag/v0.7.1
 [strscan-v3.1.7]: https://github.com/ruby/strscan/releases/tag/v3.1.7
 [strscan-v3.1.8]: https://github.com/ruby/strscan/releases/tag/v3.1.8
diff --git a/ext/json/lib/json/ext.rb b/ext/json/lib/json/ext.rb
index 5bacc5e371212f..28500f1492396d 100644
--- a/ext/json/lib/json/ext.rb
+++ b/ext/json/lib/json/ext.rb
@@ -41,5 +41,31 @@ def parse
     end
   end
 
+  if defined?(ResumableParser) # Not yet available on JRuby
+    class ResumableParser
+      # Returns whether the parser is entirely done: no unconsumed bytes in
+      # the buffer, no document under construction and no parsed value
+      # awaiting retrieval.
+      #
+      # The main use case is detecting a truncated stream once the input is
+      # exhausted:
+      #
+      #   loop do
+      #     begin
+      #       parser << socket.readpartial(4096)
+      #     rescue EOFError
+      #       break
+      #     end
+      #     while parser.parse
+      #       process(parser.value)
+      #     end
+      #   end
+      #   warn "stream was truncated" unless parser.empty?
+      def empty?
+        eos? && !partial_value? && !value?
+      end
+    end
+  end
+
   JSON_LOADED = true unless defined?(JSON::JSON_LOADED)
 end
diff --git a/ext/json/parser/parser.c b/ext/json/parser/parser.c
index a60f6a3c95511b..58dd281884f711 100644
--- a/ext/json/parser/parser.c
+++ b/ext/json/parser/parser.c
@@ -779,11 +779,18 @@ json_eat_comments(JSON_ParserState *state, JSON_ParserConfig *config)
 
     switch (peek(state)) {
         case '/': {
-            state->cursor = memchr(state->cursor, '\n', state->end - state->cursor);
-            if (!state->cursor) {
+            const char *newline = memchr(state->cursor, '\n', state->end - state->cursor);
+            if (!newline) {
+                // state->parser marks resumable mode, where the buffer end is only a
+                // chunk boundary: the terminating newline may still arrive, so leave
+                // the comment unterminated instead of consuming to end as a one-shot
+                // parse would.
+                if (state->parser) {
+                    raise_eos_error_at("unterminated comment, expected end of line", state, start);
+                }
                 state->cursor = state->end;
             } else {
-                state->cursor++;
+                state->cursor = newline + 1;
             }
             break;
         }
@@ -2737,6 +2744,41 @@ static VALUE cResumableParser_eos_p(VALUE self)
     return eos(&parser->state) ? Qtrue : Qfalse;
 }
 
+/*
+ * call-seq: partial_value? -> true or false
+ *
+ * Returns whether a document is currently under construction: an unclosed
+ * container, a key awaiting its value, etc.
+ *
+ * It answers the same question as <tt>!partial_value.nil?</tt>, but as a
+ * cheap predicate on the parser's internal state, without materializing the
+ * partially parsed Ruby objects:
+ *   parser << '{"a":1,'
+ *   parser.parse # => false
+ *   parser.partial_value? # => true
+ *
+ * A fully parsed document whose value hasn't been retrieved yet is not under
+ * construction: #value? returns true and #partial_value? returns false.
+ */
+static VALUE cResumableParser_partial_value_p(VALUE self)
+{
+    JSON_ResumableParser *parser = cResumableParser_get(self);
+
+    // Mirror of #value?: values on the stack while the document isn't DONE
+    // belong to a partially built document. A container whose first key or
+    // element hasn't been parsed yet has no frame nor value registered (the
+    // tokenizer rewinds to the container start on EOS), so that state is
+    // observable through the buffer (#eos?/#rest) instead, keeping this
+    // predicate consistent with #partial_value returning nil.
+    if (parser->value_stack.head > 0) {
+        json_frame *frame = json_frame_stack_peek(&parser->frames);
+        if (frame->phase != JSON_PHASE_DONE) {
+            return Qtrue;
+        }
+    }
+    return Qfalse;
+}
+
 /*
  * call-seq: parsed_bytes -> integer
  *
@@ -2793,6 +2835,7 @@ void Init_parser(void)
     rb_define_method(cResumableParser, "value", cResumableParser_value, 0);
     rb_define_method(cResumableParser, "value?", cResumableParser_value_p, 0);
     rb_define_method(cResumableParser, "partial_value", cResumableParser_partial_value, 0);
+    rb_define_method(cResumableParser, "partial_value?", cResumableParser_partial_value_p, 0);
     rb_define_method(cResumableParser, "clear", cResumableParser_clear, 0);
     rb_define_method(cResumableParser, "rest", cResumableParser_rest, 0);
     rb_define_method(cResumableParser, "eos?", cResumableParser_eos_p, 0);
diff --git a/set.c b/set.c
index 2827df6c4f298e..0ea48a17e19ce7 100644
--- a/set.c
+++ b/set.c
@@ -2411,11 +2411,10 @@ rb_set_size(VALUE set)
  * === Methods for Creating a \Set
  *
  * - ::[]:
- *   Returns a new set containing the given objects.
+ *   Returns a new set populated with the given objects.
  * - ::new:
- *   Returns a new set containing either the given objects
- *   (if no block given) or the return values from the called block
- *   (if a block given).
+ *   Returns a new set based on the given object (if no block given),
+ *   or on the return values from the called block (if a block given).
  *
  * === Methods for \Set Operations
  *
diff --git a/test/json/json_parser_test.rb b/test/json/json_parser_test.rb
index 9000d1152f7ea8..2f79b87cc028e6 100644
--- a/test/json/json_parser_test.rb
+++ b/test/json/json_parser_test.rb
@@ -526,6 +526,7 @@ def test_parse_comments
     JSON
     assert_equal({ "key1" => "value1" }, parse(json, allow_comments: true))
     assert_equal({}, parse('{} /**/', allow_comments: true))
+    assert_equal({}, parse('{} // eol comment ending at eof', allow_comments: true))
     assert_raise(ParserError) { parse('{} /* comment not closed', allow_comments: true) }
     assert_raise(ParserError) { parse('{} /*/', allow_comments: true) }
     assert_raise(ParserError) { parse('{} /x wrong comment', allow_comments: true) }
diff --git a/test/json/resumable_parser_test.rb b/test/json/resumable_parser_test.rb
index 734d6e220b769d..357052c9a4035a 100644
--- a/test/json/resumable_parser_test.rb
+++ b/test/json/resumable_parser_test.rb
@@ -199,6 +199,48 @@ def test_incomplete_input_at_structural_positions_resumes
     assert_incomplete "{\"a\":1,"
   end
 
+  def test_line_comment_spanning_feed_boundary_is_not_terminated_early
+    # A `//` line comment is only terminated by a newline. When the newline
+    # has not arrived yet, the comment must stay incomplete rather than being
+    # treated as consumed -- otherwise its body, delivered in a later chunk,
+    # leaks out as parsed values.
+    values = []
+    parser = new_parser(allow_comments: true)
+    parser << '[1] //'
+    values << parser.value while parser.parse
+
+    parser << "[2]\n[3]" # [2] belongs to the comment, [3] is a real document
+    values << parser.value while parser.parse
+
+    assert_equal [[1], [3]], values
+  end
+
+  def test_line_comment_terminated_by_newline_across_feeds
+    values = []
+    parser = new_parser(allow_comments: true)
+    parser << '[1] //co'
+    values << parser.value while parser.parse
+
+    parser << "mment\n[2]"
+    values << parser.value while parser.parse
+
+    assert_equal [[1], [2]], values
+  end
+
+  def test_block_comment_spanning_feed_boundary_is_not_terminated_early
+    # A `/* */` block comment whose closing `*/` has not arrived yet must stay
+    # incomplete, mirroring the line-comment behaviour above.
+    values = []
+    parser = new_parser(allow_comments: true)
+    parser << '[1] /*'
+    values << parser.value while parser.parse
+
+    parser << '[2]*/[3]' # [2] belongs to the comment, [3] is a real document
+    values << parser.value while parser.parse
+
+    assert_equal [[1], [3]], values
+  end
+
   def test_rest
     @parser << '[1, 2, 3, "unterminated string'
     refute @parser.parse
@@ -239,6 +281,89 @@ def test_eos
     assert_predicate @parser, :eos?
   end
 
+  def test_empty_predicate
+    # empty? is defined on the state left after parsing everything that
+    # could be parsed from the fed bytes, so drain with parse/value first.
+    {
+      ''               => true,  # nothing fed: vacuously empty
+      '{"a":1}'        => true,
+      '{"a":1}{"b":2}' => true,
+      '{"a":1} '       => true,  # trailing whitespace
+      '{"a":1}{"b":2'  => false, # inside a number token
+      '{"a":1}{"b":'   => false, # right after a colon (token boundary)
+      '{"a":1}{'       => false, # right after an object open
+      '{"a":1,'        => false, # right after a comma (token boundary)
+      '"abc'           => false, # inside a string token
+      '[1,2'           => false, # unclosed array
+    }.each do |json, expected|
+      parser = new_parser
+      parser << json
+      parser.value while parser.parse
+      assert_equal expected, parser.empty?, "expected #{json.inspect} to be empty? == #{expected}"
+    end
+  end
+
+  def test_empty_predicate_with_undrained_buffer
+    @parser << '{"a":1}{"b":2}'
+    assert @parser.parse
+    refute_predicate @parser, :empty? # second document still in the buffer
+    assert_equal({ "a" => 1 }, @parser.value)
+    assert @parser.parse
+    assert_equal({ "b" => 2 }, @parser.value)
+    assert_predicate @parser, :empty?
+  end
+
+  def test_empty_predicate_with_pending_value
+    # A fully parsed document awaiting retrieval with #value is not empty.
+    @parser << '{"a":1}'
+    assert @parser.parse
+    refute_predicate @parser, :empty?
+    assert_equal({ "a" => 1 }, @parser.value)
+    assert_predicate @parser, :empty?
+  end
+
+  def test_empty_predicate_across_feeds
+    @parser << '{"a' # chunk boundary inside a string literal
+    refute @parser.parse
+    refute_predicate @parser, :empty?
+
+    @parser << '":1'
+    refute @parser.parse
+    refute_predicate @parser, :empty?
+
+    @parser << '}'
+    assert @parser.parse
+    refute_predicate @parser, :empty? # value not retrieved yet
+    assert_equal({ "a" => 1 }, @parser.value)
+    assert_predicate @parser, :empty?
+  end
+
+  def test_partial_value_predicate
+    {
+      ''               => false,
+      '{"a":1}'        => false,
+      '{"a":1}{"b":2}' => false,
+      '{"a":1} '       => false,
+      '{"a":1}{"b":2'  => true,  # inside a number token
+      '{"a":1}{"b":'   => true,  # right after a colon (token boundary)
+      # The tokenizer rewinds to the token start on EOS, so nothing is
+      # registered yet for a lone '{' or an unterminated top-level string:
+      # partial_value returns nil and partial_value? agrees. The truncation
+      # is still observable through the buffer: eos? is false, rest isn't
+      # empty.
+      '{"a":1}{'       => false, # right after an object open
+      '"abc'           => false, # inside a string token
+      '{"a":1,'        => true,  # right after a comma (token boundary)
+      '[1,2'           => true,  # unclosed array
+    }.each do |json, expected|
+      parser = new_parser
+      parser << json
+      parser.value while parser.parse
+      assert_equal expected, parser.partial_value?, "expected #{json.inspect} to be partial_value? == #{expected}"
+      assert_equal !parser.partial_value.nil?, parser.partial_value?, "partial_value?/partial_value mismatch for #{json.inspect}"
+    end
+  end
+
   def test_partial_value
     assert_nil @parser.partial_value
     assert_partial_value [1, 2, 3], '[1, 2, 3, "unterminated string'