Implement android::base::WaitForProperty.

Also adapt libcutils to the bionic change that was necessary for this.

Bug: http://b/35201172
Test: ran tests
Change-Id: I72a98b70b03d23e958b46778b505fbd5c86c32ae

4 files changed, 65 insertions, 1 deletions

diff --git a/base/include/android-base/properties.h b/base/include/android-base/properties.h
index 4d7082a21..e275fa2a4 100644
--- a/base/include/android-base/properties.h
+++ b/base/include/android-base/properties.h
@@ -58,6 +58,9 @@ template <typename T> T GetUintProperty(const std::string& key,
 // tell you whether or not your call succeeded. A `false` return value definitely means failure.
 bool SetProperty(const std::string& key, const std::string& value);
 
+// Waits for the system property `key` to have the value `expected_value`, .
+void WaitForProperty(const std::string& key, const std::string& expected_value);
+
 } // namespace base
 } // namespace android
 
diff --git a/base/properties.cpp b/base/properties.cpp
index 37daf9a10..0f0f6382f 100644
--- a/base/properties.cpp
+++ b/base/properties.cpp
@@ -14,9 +14,12 @@
  * limitations under the License.
  */
 
+#define _REALLY_INCLUDE_SYS__SYSTEM_PROPERTIES_H_
+
 #include "android-base/properties.h"
 
 #include <sys/system_properties.h>
+#include <sys/_system_properties.h>
 
 #include <string>
 
@@ -78,5 +81,43 @@ bool SetProperty(const std::string& key, const std::string& value) {
   return (__system_property_set(key.c_str(), value.c_str()) == 0);
 }
 
+struct WaitForPropertyData {
+  bool done;
+  const std::string* expected_value;
+  unsigned last_read_serial;
+};
+
+static void WaitForPropertyCallback(void* data_ptr, const char*, const char* value, unsigned serial) {
+  WaitForPropertyData* data = reinterpret_cast<WaitForPropertyData*>(data_ptr);
+  if (*data->expected_value == value) {
+    data->done = true;
+  } else {
+    data->last_read_serial = serial;
+  }
+}
+
+void WaitForProperty(const std::string& key, const std::string& expected_value) {
+  // Find the property's prop_info*.
+  const prop_info* pi;
+  unsigned global_serial = 0;
+  while ((pi = __system_property_find(key.c_str())) == nullptr) {
+    // The property doesn't even exist yet.
+    // Wait for a global change and then look again.
+    global_serial = __system_property_wait_any(global_serial);
+  }
+
+  WaitForPropertyData data;
+  data.expected_value = &expected_value;
+  data.done = false;
+  while (true) {
+    // Check whether the property has the value we're looking for?
+    __system_property_read_callback(pi, WaitForPropertyCallback, &data);
+    if (data.done) return;
+
+    // It didn't so wait for it to change before checking again.
+    __system_property_wait(pi, data.last_read_serial);
+  }
+}
+
 }  // namespace base
 }  // namespace android
diff --git a/base/properties_test.cpp b/base/properties_test.cpp
index da89ec5a9..d8186be6e 100644
--- a/base/properties_test.cpp
+++ b/base/properties_test.cpp
@@ -18,7 +18,12 @@
 
 #include <gtest/gtest.h>
 
+#include <atomic>
+#include <chrono>
 #include <string>
+#include <thread>
+
+using namespace std::chrono_literals;
 
 TEST(properties, smoke) {
   android::base::SetProperty("debug.libbase.property_test", "hello");
@@ -119,3 +124,18 @@ TEST(properties, GetUintProperty_uint8_t) { CheckGetUintProperty<uint8_t>(); }
 TEST(properties, GetUintProperty_uint16_t) { CheckGetUintProperty<uint16_t>(); }
 TEST(properties, GetUintProperty_uint32_t) { CheckGetUintProperty<uint32_t>(); }
 TEST(properties, GetUintProperty_uint64_t) { CheckGetUintProperty<uint64_t>(); }
+
+TEST(properties, WaitForProperty) {
+  std::atomic<bool> flag{false};
+  std::thread thread([&]() {
+    std::this_thread::sleep_for(100ms);
+    android::base::SetProperty("debug.libbase.WaitForProperty_test", "a");
+    while (!flag) std::this_thread::yield();
+    android::base::SetProperty("debug.libbase.WaitForProperty_test", "b");
+  });
+
+  android::base::WaitForProperty("debug.libbase.WaitForProperty_test", "a");
+  flag = true;
+  android::base::WaitForProperty("debug.libbase.WaitForProperty_test", "b");
+  thread.join();
+}
diff --git a/libcutils/properties.cpp b/libcutils/properties.cpp
index 43ad5748e..d2645e640 100644
--- a/libcutils/properties.cpp
+++ b/libcutils/properties.cpp
@@ -129,7 +129,7 @@ struct callback_data {
     void* cookie;
 };
 
-static void trampoline(void* raw_data, const char* name, const char* value) {
+static void trampoline(void* raw_data, const char* name, const char* value, unsigned /*serial*/) {
     callback_data* data = reinterpret_cast<callback_data*>(raw_data);
     data->callback(name, value, data->cookie);
 }