backport fix for #4368 use while loop rather than recursion to locate…

… next line to process

CHANGELOG.adoc

@@ -30,6 +30,7 @@ Compliance::
 
 Bug Fixes::
 
+  * Use while loop rather than recursion to locate next line to process; prevents stack limit error (#4368)
   * Avoid matching numeric character references when searching for # in xref target (#4393)
   * Use correct selector to collapse margin on first and last child of sidebar
   * Don't allow target of include directive to start with a space (to distinguish it from a dlist item) or to end with a space

lib/asciidoctor/reader.rb

@@ -125,16 +125,21 @@ def next_line_empty?
   # Returns the next line of the source data as a String if there are lines remaining.
   # Returns nothing if there is no more data.
   def peek_line direct = false
-    if direct || @look_ahead > 0
-      @unescape_next_line ? ((line = @lines[-1]).slice 1, line.length) : @lines[-1]
-    elsif @lines.empty?
-      @look_ahead = 0
-      nil
-    else
-      # FIXME the problem with this approach is that we aren't
-      # retaining the modified line (hence the @unescape_next_line tweak)
-      # perhaps we need a stack of proxied lines
-      (process_line @lines[-1]) || peek_line
+    while true
+      next_line = @lines[-1]
+      if direct || @look_ahead > 0
+        return @unescape_next_line ? (next_line.slice 1, next_line.length) : next_line
+      elsif next_line
+        # FIXME the problem with this approach is that we aren't
+        # retaining the modified line (hence the @unescape_next_line tweak)
+        # perhaps we need a stack of proxied lines
+        if (line = process_line next_line)
+          return line
+        end
+      else
+        @look_ahead = 0
+        return
+      end
     end
   end
 

test/reader_test.rb

@@ -145,6 +145,10 @@ class ReaderTest < Minitest::Test
         assert reader.next_line_empty?
       end
 
+      test 'peek_line should return nil if next entry is nil' do
+        assert_nil (Asciidoctor::Reader.new [nil]).peek_line
+      end
+
       test 'peek_line should return next line if there are lines remaining' do
         reader = Asciidoctor::Reader.new SAMPLE_DATA
         assert_equal SAMPLE_DATA.first, reader.peek_line
@@ -2841,6 +2845,39 @@ def process reader, target, attributes
           assert_empty logger
         end
       end
+
+      test 'should not fail to process preprocessor directive that evaluates to false and has a large number of lines' do
+        lines = (%w(data) * 5000) * ?\n
+        input = <<~EOS
+        before
+
+        ifdef::attribute-not-set[]
+        #{lines}
+        endif::attribute-not-set[]
+
+        after
+        EOS
+
+        doc = Asciidoctor.load input
+        assert_equal 2, doc.blocks.size
+        assert_equal 'before', doc.blocks[0].source
+        assert_equal 'after', doc.blocks[1].source
+      end
+
+      test 'should not fail to process lines if reader contains a nil entry' do
+        input = ['before', '', '', '', 'after']
+        doc = Asciidoctor.load input, extensions: proc {
+          preprocessor do
+            process do |_, reader|
+              reader.source_lines[2] = nil
+              nil
+            end
+          end
+        }
+        assert_equal 2, doc.blocks.size
+        assert_equal 'before', doc.blocks[0].source
+        assert_equal 'after', doc.blocks[1].source
+      end
     end
   end
 end