Add initial prototype for the GN meta-buildsystem.

tools/gn/function_read_file.cc

Index: tools/gn/function_read_file.cc
diff --git a/tools/gn/function_read_file.cc b/tools/gn/function_read_file.cc
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+++ b/tools/gn/function_read_file.cc
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+// Copyright (c) 2013 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 "base/file_util.h"
+#include "tools/gn/err.h"
+#include "tools/gn/filesystem_utils.h"
+#include "tools/gn/functions.h"
+#include "tools/gn/input_conversion.h"
+#include "tools/gn/input_file.h"
+#include "tools/gn/scheduler.h"
+
+// TODO(brettw) consider removing this. I originally wrote it for making the
+// WebKit bindings but misundersood what was required, and didn't need to
+// use this. This seems to have a high potential for misuse.
+
+/*
+read_file: Read a file into a variable.
+
+  read_file(filename, how_to_read)
+
+  Whitespace will be trimmed from the end of the file. Throws an error if the
+  file can not be opened.
+
+Arguments:
+
+  filename:
+      Filename to read, relative to the build file.
+
+  input_conversion:
+      Controls how the file is read and parsed. See "help input_conversion".
+
+Example:
+
+  lines = read_file("foo.txt", "list lines")
+*/
+Value ExecuteReadFile(Scope* scope,
+                      const FunctionCallNode* function,
+                      const std::vector<Value>& args,
+                      Err* err) {
+  if (args.size() != 2) {
+    *err = Err(function->function(), "Wrong number of args to read_file",
+               "I expected two arguments.");
+    return Value();
+  }
+  if (!args[0].VerifyTypeIs(Value::STRING, err))
+    return Value();
+
+  // Compute the file name.
+  const SourceDir& cur_dir = SourceDirForFunctionCall(function);
+  SourceFile source_file = cur_dir.ResolveRelativeFile(args[0].string_value());
+  base::FilePath file_path =
+      scope->settings()->build_settings()->GetFullPath(source_file);
+
+  // Ensure that everything is recomputed if the read file changes.
+  g_scheduler->AddGenDependency(source_file);
+
+  // Read contents.
+  std::string file_contents;
+  if (!file_util::ReadFileToString(file_path, &file_contents)) {
+    *err = Err(args[0], "Could not read file.",
+               "I resolved this to \"" + FilePathToUTF8(file_path) + "\".");
+    return Value();
+  }
+
+  return ConvertInputToValue(file_contents, function, args[1], err);
+}