gn format: make sure there's a blank line before comments, except at block start

tools/gn/command_format.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <sstream>
 
 #include "base/command_line.h"
 #include "tools/gn/commands.h"
 #include "tools/gn/input_file.h"
 #include "tools/gn/parser.h"
@@ skipping 54 matching lines @@
   void PrintMargin();
 
   void TrimAndPrintToken(const Token& token);
 
   // End the current line, flushing end of line comments.
   void Newline();
 
   // Remove trailing spaces from the current line.
   void Trim();
 
+  // Whether there's a blank separator line at the current position.
+  bool HaveBlankLine();
+
   // Get the 0-based x position on the current line.
   int CurrentColumn();
 
   // Print the expression to the output buffer. Returns the type of element
   // added to the output.
   ExprStyle Expr(const ParseNode* root);
 
   // Format a list of values using the given style.
   // |end| holds any trailing comments to be printed just before the closing
   // bracket.
@@ skipping 49 matching lines @@
   PrintMargin();
 }
 
 void Printer::Trim() {
   size_t n = output_.size();
   while (n > 0 && output_[n - 1] == ' ')
     --n;
   output_.resize(n);
 }
 
+bool Printer::HaveBlankLine() {
+  size_t n = output_.size();
+  while (n > 0 && output_[n - 1] == ' ')
+    --n;
+  return n > 2 && output_[n - 1] == '\n' && output_[n - 2] == '\n';
+}
+
 int Printer::CurrentColumn() {
   int n = 0;
   while (n < static_cast<int>(output_.size()) &&
          output_[output_.size() - 1 - n] != '\n') {
     ++n;
   }
   return n;
 }
 
 void Printer::Block(const ParseNode* root) {
@@ skipping 154 matching lines @@
       Print(" ");
     Expr(list[0]);
     CHECK(!list[0]->comments() || list[0]->comments()->after().empty());
     if (style != kSequenceStyleFunctionCall)
       Print(" ");
   } else {
     margin_ += kIndentSize;
     size_t i = 0;
     for (const auto& x : list) {
       Newline();
+      // If:
+      // - we're going to output some comments, and;
+      // - we haven't just started this multiline list, and;
+      // - there isn't already a blank line here;
+      // Then: insert one.
+      if (i != 0 && x->comments() && !x->comments()->before().empty() &&
+          !HaveBlankLine()) {
+        Newline();
+      }
       ExprStyle expr_style = Expr(x);
       CHECK(!x->comments() || x->comments()->after().empty());
       if (i < list.size() - 1 || style == kSequenceStyleList) {
         if ((style == kSequenceStyleList || kSequenceStyleFunctionCall) &&
             expr_style == kExprStyleRegular) {
           Print(",");
         } else {
           Newline();
         }
       }
@@ skipping 73 matching lines @@
   }
   std::string output_string;
   if (FormatFileToString(input_name, dump_tree, &output_string)) {
     printf("%s", output_string.c_str());
   }
 
   return 0;
 }
 
 }  // namespace commands