Workaround removal of __system_property_get in Android NDK.

The 'L' release of Android will remove __system_property_get from the NDK.
Until we can replace all calls of __system_property_get, this CL adds a
temporary work-around for builds compiled against the 'L' API.  The workaround
involves creating a stub __system_property_get, which uses dlsym to dynamically
load the address of the real __system_property_get symbol in libc.so (which is
still available as a hidden symbol on the devices libc.so dispite having been
removed from the NDK).

BUG=392191,393903

Review URL: https://codereview.chromium.org/393923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284012 0039d316-1c4b-4281-b951-d872f2087c98

base/sys_info_android.cc

@@ -4,6 +4,7 @@
 
 #include "base/sys_info.h"
 
+#include <dlfcn.h>
 #include <sys/system_properties.h>
 
 #include "base/android/sys_utils.h"
@@ -14,6 +15,45 @@
 #include "base/strings/stringprintf.h"
 #include "base/sys_info_internal.h"
 
+// TODO(rmcilroy): Update API level when 'L' gets an official API level.
+#if (__ANDROID_API__ >= 9999 /* 'L' */)
+
+namespace {
+
+typedef int (SystemPropertyGetFunction)(const char*, char*);
+
+SystemPropertyGetFunction* DynamicallyLoadRealSystemPropertyGet() {
+  // libc.so should already be open, get a handle to it.
+  void* handle = dlopen("libc.so", RTLD_NOLOAD);
+  if (!handle) {
+    LOG(FATAL) << "Cannot dlopen libc.so: " << dlerror();
+  }
+  SystemPropertyGetFunction* real_system_property_get =
+      reinterpret_cast<SystemPropertyGetFunction*>(
+          dlsym(handle, "__system_property_get"));
+  if (!real_system_property_get) {
+    LOG(FATAL) << "Cannot resolve __system_property_get(): " << dlerror();
+  }
+  return real_system_property_get;
+}
+
+static base::LazyInstance<base::internal::LazySysInfoValue<
+    SystemPropertyGetFunction*, DynamicallyLoadRealSystemPropertyGet> >::Leaky
+    g_lazy_real_system_property_get = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+// Android 'L' removes __system_property_get from the NDK, however it is still
+// a hidden symbol in libc. Until we remove all calls of __system_property_get
+// from Chrome we work around this by defining a weak stub here, which uses
+// dlsym to but ensures that Chrome uses the real system
+// implementatation when loaded.  http://crbug.com/392191.
+int __system_property_get(const char* name, char* value) {
+  return g_lazy_real_system_property_get.Get().value()(name, value);
+}
+
+#endif
+
 namespace {
 
 // Default version of Android to fall back to when actual version numbers