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
diff --git a/libc/include/sys/_system_properties.h b/libc/include/sys/_system_properties.h
index ffa6d2e..fa98d11 100644
--- a/libc/include/sys/_system_properties.h
+++ b/libc/include/sys/_system_properties.h
@@ -30,6 +30,7 @@
#define _INCLUDE_SYS__SYSTEM_PROPERTIES_H
#include <sys/cdefs.h>
+#include <stdint.h>
#ifndef _REALLY_INCLUDE_SYS__SYSTEM_PROPERTIES_H_
#error you should #include <sys/system_properties.h> instead
@@ -91,7 +92,7 @@
**
** Returns the serial number on success, -1 on error.
*/
-unsigned int __system_property_area_serial();
+uint32_t __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
@@ -118,12 +119,22 @@
**
** Returns the serial number on success, -1 on error.
*/
-unsigned int __system_property_serial(const prop_info *pi);
+uint32_t __system_property_serial(const prop_info* pi);
-/* Wait for any system property to be updated. Caller must pass
-** in 0 the first time, and the previous return value on each
-** successive call. */
-unsigned int __system_property_wait_any(unsigned int serial);
+/*
+ * Waits for any system property to be updated past `old_serial`.
+ * If you don't know the current global serial number, use 0.
+ * Returns the new global serial number.
+ */
+uint32_t __system_property_wait_any(uint32_t old_serial);
+
+/*
+ * Waits for the specific system property identified by `pi` to be updated past `old_serial`.
+ * If you don't know the current serial, use 0.
+ * Returns the serial number for `pi` that caused the wake.
+ */
+uint32_t __system_property_wait(const prop_info* pi, uint32_t old_serial)
+ __INTRODUCED_IN_FUTURE;
/* Initialize the system properties area in read only mode.
* Should be done by all processes that need to read system