gn format: Add --dry-run flag for PRESUBMIT

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 "base/files/file_util.h"
 #include "base/strings/string_split.h"
 #include "tools/gn/commands.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/input_file.h"
 #include "tools/gn/parser.h"
 #include "tools/gn/scheduler.h"
 #include "tools/gn/setup.h"
 #include "tools/gn/source_file.h"
 #include "tools/gn/tokenizer.h"
 
 namespace commands {
 
+const char kSwitchDryRun[] = "dry-run";
 const char kSwitchDumpTree[] = "dump-tree";
 const char kSwitchInPlace[] = "in-place";
 const char kSwitchStdin[] = "stdin";
 
 const char kFormat[] = "format";
 const char kFormat_HelpShort[] =
     "format: Format .gn file.";
 const char kFormat_Help[] =
     "gn format [--dump-tree] [--in-place] [--stdin] BUILD.gn\n"
     "\n"
     "  Formats .gn file to a standard format.\n"
     "\n"
     "Arguments\n"
+    "  --dry-run\n"
+    "      Does not change or output anything, but sets the process exit code\n"
+    "      based on whether output would be different than what's on disk.\n"
+    "      This is useful for presubmit/lint-type checks.\n"
+    "      - Exit code 0: successful format, matches on disk.\n"
+    "      - Exit code 1: general failure (parse error, etc.)\n"
+    "      - Exit code 2: successful format, but differs from on disk.\n"
+    "\n"
     "  --dump-tree\n"
     "      For debugging only, dumps the parse tree.\n"
     "\n"
     "  --in-place\n"
     "      Instead of writing the formatted file to stdout, replace the input\n"
     "      file with the formatted output. If no reformatting is required,\n"
     "      the input file will not be touched, and nothing printed.\n"
     "\n"
     "  --stdin\n"
     "      Read input from stdin (and write to stdout). Not compatible with\n"
@@ skipping 893 matching lines @@
   if (err.has_error()) {
     err.PrintToStdout();
     return false;
   }
 
   DoFormat(parse_node.get(), dump_tree, output);
   return true;
 }
 
 int RunFormat(const std::vector<std::string>& args) {
+  bool dry_run =
+      base::CommandLine::ForCurrentProcess()->HasSwitch(kSwitchDryRun);
   bool dump_tree =
       base::CommandLine::ForCurrentProcess()->HasSwitch(kSwitchDumpTree);
-
   bool from_stdin =
       base::CommandLine::ForCurrentProcess()->HasSwitch(kSwitchStdin);
+  bool in_place =
+    base::CommandLine::ForCurrentProcess()->HasSwitch(kSwitchInPlace);
+
+  if (dry_run) {
+    // --dry-run only works with an actual file to compare to.
+    from_stdin = false;
+    in_place = true;
+  }
 
   if (from_stdin) {
     if (args.size() != 0) {
       Err(Location(), "Expecting no arguments when reading from stdin.\n")
           .PrintToStdout();
       return 1;
     }
     std::string input = ReadStdin();
     std::string output;
     if (!FormatStringToString(input, dump_tree, &output))
@@ skipping 10 matching lines @@
     return 1;
   }
 
   Setup setup;
   SourceDir source_dir =
       SourceDirForCurrentDirectory(setup.build_settings().root_path());
   SourceFile file = source_dir.ResolveRelativeFile(args[0]);
 
   std::string output_string;
   if (FormatFileToString(&setup, file, dump_tree, &output_string)) {
-    bool in_place =
-        base::CommandLine::ForCurrentProcess()->HasSwitch(kSwitchInPlace);
     if (in_place) {
       base::FilePath to_write = setup.build_settings().GetFullPath(file);
       std::string original_contents;
       if (!base::ReadFileToString(to_write, &original_contents)) {
         Err(Location(), std::string("Couldn't read \"") +
                             to_write.AsUTF8Unsafe() +
                             std::string("\" for comparison.")).PrintToStdout();
         return 1;
       }
+      if (dry_run)
+        return original_contents == output_string ? 0 : 2;
       if (original_contents != output_string) {
         if (base::WriteFile(to_write,
                             output_string.data(),
                             static_cast<int>(output_string.size())) == -1) {
           Err(Location(),
               std::string("Failed to write formatted output back to \"") +
                   to_write.AsUTF8Unsafe() + std::string("\".")).PrintToStdout();
           return 1;
         }
         printf("Wrote formatted to '%s'.\n", to_write.AsUTF8Unsafe().c_str());
       }
     } else {
       printf("%s", output_string.c_str());
     }
   }
 
   return 0;
 }
 
 }  // namespace commands