bionic: add __system_property_area_serial()

Adds a new _internal_ function. Provide a global serial number to
support more efficient private caching algorithms. This allows
to skip re-running the __system_property_find() call on misses until
there is a global change in the properties. This call is a read
barrier, the property data to be read following this call will be
read sequentially and up to date.

Bug: 19544788
Change-Id: I58e6a92baa0f3e8e7b9ec79b10af6d56407dab48

libc/bionic/system_properties.cpp

@@ -598,6 +598,16 @@ int __system_property_area_init()
     return map_prop_area_rw();
 }
 
+unsigned int __system_property_area_serial()
+{
+    prop_area *pa = __system_property_area__;
+    if (!pa) {
+        return -1;
+    }
+    // Make sure this read fulfilled before __system_property_serial
+    return atomic_load_explicit(&(pa->serial), memory_order_acquire);
+}
+
 const prop_info *__system_property_find(const char *name)
 {
     if (__predict_false(compat_mode)) {

libc/include/sys/_system_properties.h

@@ -98,6 +98,24 @@ int __system_property_set_filename(const char *filename);
 */
 int __system_property_area_init();
 
+/* Read the global serial number of the system properties
+**
+** Called to predict if a series of cached __system_property_find
+** objects will have seen __system_property_serial values change.
+** But also aids the converse, as changes in the global serial can
+** also be used to predict if a failed __system_property_find
+** could in-turn now find an new object; thus preventing the
+** cycles of effort to poll __system_property_find.
+**
+** Called at the beginning of a cache cycle to signal _any_ possible
+** changes have occurred since last. If there is, check each individual
+** __system_property_serial to confirm dirty, or __system_property_find
+** to check if the property now exists.
+**
+** Returns the serial number on success, -1 on error.
+*/
+unsigned int __system_property_area_serial();
+
 /* Add a new system property.  Can only be done by a single
 ** process that has write access to the property area, and
 ** that process must handle sequencing to ensure the property