fix hard line break round-trip in commonmark renderer

Author: zayeem06

api_test/main.c

@@ -773,6 +773,37 @@ static void render_commonmark(test_batch_runner *runner) {
   free(commonmark);
 
   cmark_node_free(empty_item_doc);
+
+  // Hard line breaks are written as a backslash break in every position.
+  // Two trailing spaces would be stripped as insignificant whitespace at the
+  // start of a line, or after a space, losing the break on a round trip.
+  static const char lead_break[] = "\\\nfoo\n";
+  cmark_node *lead_break_doc =
+      cmark_parse_document(lead_break, sizeof(lead_break) - 1, CMARK_OPT_DEFAULT);
+  commonmark = cmark_render_commonmark(lead_break_doc, CMARK_OPT_DEFAULT, 0);
+  STR_EQ(runner, commonmark, "\\\nfoo\n",
+         "render hard break at start of line");
+  free(commonmark);
+  cmark_node_free(lead_break_doc);
+
+  static const char space_break[] = "foo \\\nbar\n";
+  cmark_node *space_break_doc =
+      cmark_parse_document(space_break, sizeof(space_break) - 1, CMARK_OPT_DEFAULT);
+  commonmark = cmark_render_commonmark(space_break_doc, CMARK_OPT_DEFAULT, 0);
+  STR_EQ(runner, commonmark, "foo \\\nbar\n",
+         "render hard break after a space");
+  free(commonmark);
+  cmark_node_free(space_break_doc);
+
+  static const char plain_break[] = "foo\\\nbar\n";
+  cmark_node *plain_break_doc =
+      cmark_parse_document(plain_break, sizeof(plain_break) - 1, CMARK_OPT_DEFAULT);
+  commonmark = cmark_render_commonmark(plain_break_doc, CMARK_OPT_DEFAULT, 0);
+  STR_EQ(runner, commonmark, "foo\\\nbar\n",
+         "render ordinary hard break");
+  free(commonmark);
+  cmark_node_free(plain_break_doc);
+
   cmark_node_free(text);
   cmark_node_free(doc);
 }

src/commonmark.c

@@ -331,7 +331,11 @@ static int S_render_node(cmark_renderer *renderer, cmark_node *node,
 
   case CMARK_NODE_LINEBREAK:
     if (!(CMARK_OPT_HARDBREAKS & options)) {
-      LIT("  ");
+      // A backslash line break is unambiguous in every position. Two trailing
+      // spaces are only recognized as a hard break when they follow text on
+      // the line; at the start of a line, or after a space, they are stripped
+      // as insignificant whitespace and the break is lost on a round trip.
+      LIT("\\");
     }
     CR();
     break;