handle return value of process method for block and block macro extension properly

* don't crash if value is an abstract block with context :compound that isn't of type Block (e.g., a list)
* ignore value if it matches parent argument

3 files changed, 129 insertions, 3 deletions

diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index 35d457c9..5447a386 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -37,6 +37,8 @@ Compliance::
 
 Bug Fixes::
 
+  * Don't crash if process method for custom block returns an abstract block with context `:compound` that isn't of type `Block` (e.g., a list)
+  * Ignore return value of process method for custom block or block macro if value matches parent argument
   * Remove unnamespaced selectors in Pygments stylesheet
   * Normalize output from Pygments to use `linenos` class for inline line numbering and trim space after number; update default stylesheet accordingly
   * Change `AbstractBlock#sections?` to return false when called on block that isn't a Section or Document (PR #3591) *@mogztter*
diff --git a/lib/asciidoctor/parser.rb b/lib/asciidoctor/parser.rb
index ad9d34cc..ccfee715 100644
--- a/lib/asciidoctor/parser.rb
+++ b/lib/asciidoctor/parser.rb
@@ -660,7 +660,7 @@ class Parser
                 if (default_attrs = ext_config[:default_attrs])
                   attributes.update(default_attrs) {|_, old_v| old_v }
                 end
-                if (block = extension.process_method[parent, target, attributes])
+                if (block = extension.process_method[parent, target, attributes]) && block != parent
                   attributes.replace block.attributes
                   break
                 else
@@ -1050,12 +1050,12 @@ class Parser
     if (extension = options[:extension])
       # QUESTION do we want to delete the style?
       attributes.delete 'style'
-      return unless (block = extension.process_method[parent, block_reader || (Reader.new lines), attributes.merge])
+      return unless (block = extension.process_method[parent, block_reader || (Reader.new lines), attributes.merge]) && block != parent
       attributes.replace block.attributes
       # FIXME if the content model is set to compound, but we only have simple in this context, then
       # forcefully set the content_model to simple to prevent parsing blocks from children
       # TODO document this behavior!!
-      if block.content_model == :compound && !(lines = block.lines).empty?
+      if block.content_model == :compound && Block === block && !(lines = block.lines).empty?
         content_model = :compound
         block_reader = Reader.new lines
       end
diff --git a/test/extensions_test.rb b/test/extensions_test.rb
index 5537925e..303f0abd 100644
--- a/test/extensions_test.rb
+++ b/test/extensions_test.rb
@@ -1572,6 +1572,75 @@ context 'Extensions' do
       end
     end
 
+    test 'should allow extension to replace custom block with a list' do
+      begin
+        Asciidoctor::Extensions.register do
+          block do
+            named :lst
+            on_context :paragraph
+            process do |parent, reader|
+              reader.read_lines.each_with_object create_list parent, :ulist do |line, list|
+                list << (create_list_item list, line)
+              end
+            end
+          end
+        end
+        input = <<~'EOS'
+        before
+
+        [lst]
+        a
+        b
+        c
+
+        after
+        EOS
+        doc = document_from_string input
+        assert_equal 3, doc.blocks.size
+        list = doc.blocks[1]
+        assert_equal :ulist, list.context
+        assert_equal 3, list.items.size
+        assert_equal 'a', list.items[0].text
+        assert_css 'li', doc.convert, 3
+      ensure
+        Asciidoctor::Extensions.unregister_all
+      end
+    end
+
+    test 'should allow extension to replace custom block with a section' do
+      begin
+        Asciidoctor::Extensions.register do
+          block do
+            named :sect
+            on_context :open
+            process do |parent, _, attrs|
+              create_section parent, attrs['title'], {}
+            end
+          end
+        end
+        input = <<~'EOS'
+        .Section Title
+        [sect]
+        --
+        a
+
+        b
+        --
+        EOS
+        doc = document_from_string input
+        assert_equal 1, doc.blocks.size
+        sect = doc.blocks[0]
+        assert_equal :section, sect.context
+        assert_equal 'Section Title', sect.title
+        assert_equal 2, sect.blocks.size
+        assert_equal :paragraph, sect.blocks[0].context
+        assert_equal :paragraph, sect.blocks[1].context
+        assert_css 'p', doc.convert, 2
+      ensure
+        Asciidoctor::Extensions.unregister_all
+      end
+    end
+
     test 'can use parse_content to append blocks to current parent' do
       begin
         Asciidoctor::Extensions.register do
@@ -1604,6 +1673,61 @@ context 'Extensions' do
       end
     end
 
+    test 'should ignore return value of custom block if value is parent' do
+      begin
+        Asciidoctor::Extensions.register do
+          block do
+            named :unwrap
+            on_context :open
+            process do |parent, reader|
+              parse_content parent, reader.read_lines
+            end
+          end
+        end
+        input = <<~'EOS'
+        [unwrap]
+        --
+        a
+
+        b
+
+        c
+        --
+        EOS
+        doc = document_from_string input
+        assert_equal 3, doc.blocks.size
+        doc.blocks.each do |block|
+          assert_equal :paragraph, block.context
+        end
+        assert_equal 'a', doc.blocks[0].source
+        assert_css 'p', doc.convert, 3
+      ensure
+        Asciidoctor::Extensions.unregister_all
+      end
+    end
+
+    test 'should ignore return value of custom block macro if value is parent' do
+      begin
+        Asciidoctor::Extensions.register do
+          block_macro :para do
+            process do |parent, target|
+              parse_content parent, target
+            end
+          end
+        end
+        input = <<~'EOS'
+        para::text[]
+        EOS
+        doc = document_from_string input
+        assert_equal 1, doc.blocks.size
+        assert_equal :paragraph, doc.blocks[0].context
+        assert_equal 'text', doc.blocks[0].source
+        assert_css 'p', doc.convert, 1
+      ensure
+        Asciidoctor::Extensions.unregister_all
+      end
+    end
+
     test 'parse_content should not share attributes between parsed blocks' do
       begin
         Asciidoctor::Extensions.register do