1 files changed, 87 insertions, 0 deletions
diff --git a/libc/private/CachedProperty.h b/libc/private/CachedProperty.h
new file mode 100644
index 000000000..0a41abf80
--- /dev/null
+++ b/libc/private/CachedProperty.h
@@ -0,0 +1,87 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <string.h>
+
+#define _REALLY_INCLUDE_SYS__SYSTEM_PROPERTIES_H_
+#include <sys/_system_properties.h>
+
+#include "private/bionic_lock.h"
+
+class CachedProperty {
+ public:
+  CachedProperty(const char* property_name)
+    : property_name_(property_name),
+      prop_info_(nullptr),
+      cached_area_serial_(0),
+      cached_property_serial_(0) {
+    cached_value_[0] = '\0';
+  }
+
+  const char* Get() {
+    lock_.lock();
+
+    // Do we have a `struct prop_info` yet?
+    if (prop_info_ == nullptr) {
+      // `__system_property_find` is expensive, so only retry if a property
+      // has been created since last time we checked.
+      uint32_t property_area_serial = __system_property_area_serial();
+      if (property_area_serial != cached_area_serial_) {
+        prop_info_ = __system_property_find(property_name_);
+        cached_area_serial_ = property_area_serial;
+      }
+    }
+
+    if (prop_info_ != nullptr) {
+      // Only bother re-reading the property if it's actually changed since last time.
+      uint32_t property_serial = __system_property_serial(prop_info_);
+      if (property_serial != cached_property_serial_) {
+        __system_property_read_callback(prop_info_, &CachedProperty::Callback, this);
+      }
+    }
+
+    lock_.unlock();
+    return cached_value_;
+  }
+
+ private:
+  Lock lock_;
+  const char* property_name_;
+  const prop_info* prop_info_;
+  uint32_t cached_area_serial_;
+  uint32_t cached_property_serial_;
+  char cached_value_[PROP_VALUE_MAX];
+
+  static void Callback(void* data, const char*, const char* value, uint32_t serial) {
+    CachedProperty* instance = reinterpret_cast<CachedProperty*>(data);
+    instance->cached_property_serial_ = serial;
+    strcpy(instance->cached_value_, value);
+  }
+};