Revert of [GN] Don't rewrite files with the same contents

tools/gn/function_write_file.cc

Index: tools/gn/function_write_file.cc
diff --git a/tools/gn/function_write_file.cc b/tools/gn/function_write_file.cc
index feb1004c3f22d2ef668294e1d1c8d0d6ebf3133d..91599755a37e239ec3a4a155a8c46fb7459eda6b 100644
--- a/tools/gn/function_write_file.cc
+++ b/tools/gn/function_write_file.cc
@@ -18,6 +18,55 @@
 #include "tools/gn/scheduler.h"
 
 namespace functions {
+
+namespace {
+
+// On Windows, provide a custom implementation of base::WriteFile. Sometimes
+// the base version fails, especially on the bots. The guess is that Windows
+// Defender or other antivirus programs still have the file open (after
+// checking for the read) when the write happens immediately after. This
+// version opens with FILE_SHARE_READ (normally not what you want when
+// replacing the entire contents of the file) which lets us continue even if
+// another program has the file open for reading. See http://crbug.com/468437
+#if defined(OS_WIN)
+int DoWriteFile(const base::FilePath& filename, const char* data, int size) {
+  base::win::ScopedHandle file(::CreateFile(
+      filename.value().c_str(),
+      GENERIC_WRITE,
+      FILE_SHARE_READ,
+      NULL,
+      CREATE_ALWAYS,
+      0,
+      NULL));
+  if (!file.IsValid()) {
+    PLOG(ERROR) << "CreateFile failed for path "
+                  << base::UTF16ToUTF8(filename.value());
+    return -1;
+  }
+
+  DWORD written;
+  BOOL result = ::WriteFile(file.Get(), data, size, &written, NULL);
+  if (result && static_cast<int>(written) == size)
+    return written;
+
+  if (!result) {
+    // WriteFile failed.
+    PLOG(ERROR) << "writing file " << base::UTF16ToUTF8(filename.value())
+                << " failed";
+  } else {
+    // Didn't write all the bytes.
+    LOG(ERROR) << "wrote" << written << " bytes to "
+               << base::UTF16ToUTF8(filename.value()) << " expected " << size;
+  }
+  return -1;
+}
+#else
+int DoWriteFile(const base::FilePath& filename, const char* data, int size) {
+  return base::WriteFile(filename, data, size);
+}
+#endif
+
+}  // namespace
 
 const char kWriteFile[] = "write_file";
 const char kWriteFile_HelpShort[] =
@@ -89,14 +138,30 @@
   } else {
     contents << args[1].ToString(false);
   }
-
+  const std::string& new_contents = contents.str();
   base::FilePath file_path =
       scope->settings()->build_settings()->GetFullPath(source_file);
 
   // Make sure we're not replacing the same contents.
-  if (!WriteFileIfChanged(file_path, contents.str(), err))
-    *err = Err(function->function(), err->message(), err->help_text());
+  std::string existing_contents;
+  if (base::ReadFileToString(file_path, &existing_contents) &&
+      existing_contents == new_contents)
+    return Value();  // Nothing to do.
 
+  // Write file, creating the directory if necessary.
+  if (!base::CreateDirectory(file_path.DirName())) {
+    *err = Err(function->function(), "Unable to create directory.",
+               "I was using \"" + FilePathToUTF8(file_path.DirName()) + "\".");
+    return Value();
+  }
+
+  int int_size = static_cast<int>(new_contents.size());
+  if (DoWriteFile(file_path, new_contents.c_str(), int_size)
+      != int_size) {
+    *err = Err(function->function(), "Unable to write file.",
+               "I was writing \"" + FilePathToUTF8(file_path) + "\".");
+    return Value();
+  }
   return Value();
 }