Support deferred content (footnotes, links, and images) with slicing

Content that is produced lazily using deferred resolution of references
needs to use the state of the writer at the time of resolution attempt,
not at the time of the actual resolution.

markdown.dtx

@@ -14105,7 +14105,8 @@ M.writer = {}
 %
 % The objects produced by the \luam{writer.new} method expose instance methods
 % and variables of their own. As a convention, I will refer to these
-% \meta{member}s as `writer->`\meta{member}.
+% \meta{member}s as `writer->`\meta{member}. All member variables are
+% immutable unless explicitly stated otherwise.
 %
 % \end{markdown}
 %  \begin{macrocode}
@@ -14127,7 +14128,8 @@ function M.writer.new(options)
 % \begin{markdown}
 %
 % Parse the \Opt{slice} option and define \luamdef{writer->slice\_begin}
-% \luamdef{writer->slice\_end}, and \luamdef{writer->is\_writing}.
+% \luamdef{writer->slice\_end}, and \luamdef{writer->is\_writing}. The
+% \luamdef{writer->is\_writing} member variable is mutable.
 %
 % \end{markdown}
 %  \begin{macrocode}
@@ -14704,8 +14706,9 @@ function M.writer.new(options)
 % \par
 % \begin{markdown}
 %
-% Define \luamdef{writer->active_headings} as a stack of identifiers
-% of the headings that are currently active.
+% Define \luamdef{writer->active\_headings} as a stack of identifiers
+% of the headings that are currently active. The
+% \luamdef{writer->active\_headings} member variable is mutable.
 %
 % \end{markdown}
 %  \begin{macrocode}
@@ -14825,6 +14828,57 @@ function M.writer.new(options)
     end
     return buffer
   end
+%    \end{macrocode}
+% \par
+% \begin{markdown}
+%
+% Define \luamdef{writer->get_state} as a function that returns the current
+% state of the writer, where the state of a writer are its mutable member
+% variables.
+%
+% \end{markdown}
+%  \begin{macrocode}
+  function self.get_state()
+    return {
+      is_writing=self.is_writing,
+      active_headings={table.unpack(self.active_headings)},
+    }
+  end
+%    \end{macrocode}
+% \par
+% \begin{markdown}
+%
+% Define \luamdef{writer->set_state} as a function that restores the input
+% state `s` and returns the previous state of the writer.
+%
+% \end{markdown}
+%  \begin{macrocode}
+  function self.set_state(s)
+    previous_state = self.get_state()
+    for key, value in pairs(state) do
+      self[key] = value
+    end
+    return previous_state
+  end
+%    \end{macrocode}
+% \par
+% \begin{markdown}
+%
+% Define \luamdef{writer->defer_call} as a function that will encapsulate the
+% input function `f`, so that `f` is called with the state of the writer at the
+% time of calling \luam{writer->defer_call}.
+%
+% \end{markdown}
+%  \begin{macrocode}
+  function self.defer_call(f)
+    state = self.get_state()
+    return function(...)
+      state = self.set_state(state)
+      local return_value = f(...)
+      self.set_state(state)
+      return return_value
+    end
+  end
 
   return self
 end
@@ -15830,14 +15884,14 @@ function M.reader.new(writer, options)
 
   -- like indirect_link
   local function lookup_note(ref)
-    return function()
+    return writer.defer_call(function()
       local found = rawnotes[normalize_tag(ref)]
       if found then
         return writer.note(parse_blocks_toplevel(found))
       else
         return {"[", parse_inlines("^" .. ref), "]"}
       end
-    end
+    end)
   end
 
   local function register_note(ref,rawnote)
@@ -15929,27 +15983,27 @@ larsers.PipeTable = Ct(larsers.table_row * parsers.newline
   -- lookup link reference and return a link, if the reference is found,
   -- or a bracketed label otherwise.
   local function indirect_link(label,sps,tag)
-    return function()
+    return writer.defer_call(function()
       local r,fallback = lookup_reference(label,sps,tag)
       if r then
         return writer.link(parse_inlines_no_link(label), r.url, r.title)
       else
         return fallback
       end
-    end
+    end)
   end
 
   -- lookup image reference and return an image, if the reference is found,
   -- or a bracketed label otherwise.
   local function indirect_image(label,sps,tag)
-    return function()
+    return writer.defer_call(function()
       local r,fallback = lookup_reference(label,sps,tag)
       if r then
         return writer.image(writer.string(label), r.url, r.title)
       else
         return {"!", fallback}
       end
-    end
+    end)
   end
 %    \end{macrocode}
 % \par

tests/testfiles/lunamark-markdown/slice-with-deferred-content.test

@@ -0,0 +1,31 @@
+\def\markdownOptionFootnotes{true}
+\def\markdownOptionHeaderAttributes{true}
+\def\markdownOptionSlice{section-with-deferred-content}
+<<<
+# This is a section
+# This is another section {#section-with-deferred-content}
+Here is a [^footnote], a [link][], and an ![image][].
+
+# This is yet another section
+Here are the definitions:
+
+ [^footnote]: This is the text of the footnote.
+
+ [link]: http://link (This is the title of the link.)
+
+ [image]: http://image (This is the title of the image.)
+>>>
+headingOne: This is another section
+interblockSeparator
+footnote: This is the text of the footnote.
+BEGIN link
+- label: link
+- URI: http://link
+- title: This is the title of the link.
+END link
+BEGIN image
+- label: image
+- URI: http://image
+- title: This is the title of the image.
+END image
+interblockSeparator