Add __system_property_wait and return the serial in __system_property_read_callback.

In order to implement android::base::WaitForProperty well, we need a way to
wait not for *any* property to change (__system_property_wait_any), but to
specifically wait for the property represented by a given `prop_info` to
change.

The android::base::WaitForProperty implementation, like attempts to cache
system properties in the past, also needs a way to keep serials and values
in sync, but the existing functions don't provide a cheap way to get a
consistent snapshot. Change the __system_property_read_callback callback's
type to include the serial corresponding to the given value.

Add a test, slightly clean up some of the existing tests (and name them to
include the names of the functions they're testing, in our usual style).

Bug: http://b/35201172
Test: ran tests
Change-Id: Ibc8ebe2e88eef1e333a1bd3dd7f68135f1ba7fb5

1 files changed, 10 insertions, 8 deletions

diff --git a/tests/system_properties_test2.cpp b/tests/system_properties_test2.cpp
index 056096001..e6e7ef25c 100644
--- a/tests/system_properties_test2.cpp
+++ b/tests/system_properties_test2.cpp
@@ -90,20 +90,22 @@ TEST(properties, smoke) {
     ASSERT_TRUE(pi != nullptr);
 
     std::string expected_name = property_name;
-    __system_property_read_callback(pi, [](void* cookie, const char *name, const char *value) {
-      const std::string* expected_name = static_cast<const std::string*>(cookie);
-      ASSERT_EQ(*expected_name, name);
-      ASSERT_STREQ("value1-1", value);
+    __system_property_read_callback(pi,
+      [](void* cookie, const char* name, const char* value, unsigned /*serial*/) {
+        const std::string* expected_name = static_cast<const std::string*>(cookie);
+        ASSERT_EQ(*expected_name, name);
+        ASSERT_STREQ("value1-1", value);
     }, &expected_name);
 
     pi = __system_property_find(long_property_name.c_str());
     ASSERT_TRUE(pi != nullptr);
 
     expected_name = long_property_name;
-    __system_property_read_callback(pi, [](void* cookie, const char *name, const char *value) {
-      const std::string* expected_name = static_cast<const std::string*>(cookie);
-      ASSERT_EQ(*expected_name, name);
-      ASSERT_STREQ("value2", value);
+    __system_property_read_callback(pi,
+      [](void* cookie, const char* name, const char* value, unsigned /*serial*/) {
+        const std::string* expected_name = static_cast<const std::string*>(cookie);
+        ASSERT_EQ(*expected_name, name);
+        ASSERT_STREQ("value2", value);
     }, &expected_name);
 
     // Check that read() for long names still works but returns truncated version of the name