Add native extensions to the Dart shell, on the linux platform.

runtime/bin/extensions_linux.cc

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include <stdio.h>
+#include <dlfcn.h>
+
+#include "bin/file.h"

[Mads Ager (google), 2012/02/23 09:26:56]

Is anything from file used in this file?

+#include "include/dart_api.h"
+#include "platform/assert.h"
+#include "platform/globals.h"
+#include "bin/extensions.h"
+#include "bin/dartutils.h"
+
+// The current implementation allows names only, not paths, and

[Mads Ager (google), 2012/02/23 09:26:56]

I would get rid of the fixed-size buffer here and just use the computed length
and dynamic allocation.

+// PATH_MAX is too large (it may be a huge number).
+const int kBufferLength = 101;
+char library_path[kBufferLength];
+char library_init_function[kBufferLength];
+
+Dart_Handle Extensions::LoadExtension(const char* extension_url,
+                                      Dart_Handle parent_library) {
+  // TODO(whesse): Make loading extensions lazy, so dynamic library is loaded
+  // only when first native function is called.
+  ASSERT(DartUtils::IsDartExtensionSchemeURL(extension_url));
+  const char* library_name =
+      extension_url + strlen(DartUtils::kDartExtensionScheme);
+  if (strlen(library_name) > kBufferLength - 1 - strlen("./lib.so")) {
+    return Dart_Error("extension library name too long");
+  }
+  if (strchr(library_name, '/') != NULL) {
+    return Dart_Error("path components not allowed in extension library name");
+  }
+  snprintf(library_path, kBufferLength, "./lib%s.so", library_name);
+
+  void* lib_handle = dlopen(library_path, RTLD_LAZY);
+  if (!lib_handle) {
+    return Dart_Error("cannot find extension library");
+  }
+  snprintf(library_init_function, kBufferLength, "%s_Init", library_name);
+  typedef Dart_Handle (*InitFunctionType)(Dart_Handle import_map);
+  InitFunctionType fn = reinterpret_cast<InitFunctionType>(
+      dlsym(lib_handle, library_init_function));
+  char *error = dlerror();
+  if (error != NULL) {
+    return Dart_Error(error);
+  }
+  return (*fn)(parent_library);
+}