Prevent WaitForProperty() from using ~100% of CPU time on 32bit builds

Since 'struct timespec' members (time_t and long) are both 32bit on
32bit systems, and std::chrono::{seconds,nanoseconds}::rep are both
>32bit, timespec members assigned in DurationToTimeSpec() can have a
negative value, especially when WaitForProperty() is called with the
default timeout value which is std::chrono::milliseconds::max().

Regarding functionality, passing a negative value to
__system_property_wait() is okay because WaitForProperty() still
waits for the property value (so unit tests are passing), but while
WaitForProperty() does that, the function, to be more exact,
SystemProperties::Wait() in bionic/, consumes ~100% of CPU time. This
happens because SystemProperties::Wait() which implements
__system_property_wait() has a tight while-loop with a __futex_wait()
call, and the futex call immediately returns EINVAL when the timespec
passed in has a negative value.

With this CL, WaitForProperty() will never pass a negative timespec
to __system_property_wait(), and therefore the __futex_wait() call
in bionic works as expected without consuming too much CPU time even
on 32bit systems.

Bug: None
Test: libbase_test32 still passes
Test: strace no longer shows repeated EINVALs from __futex_wait
Change-Id: Id1834fac8cd2876b02dbe4479bf3d3eda2fa7da1

1 files changed, 2 insertions, 1 deletions

diff --git a/base/properties.cpp b/base/properties.cpp
index cde4d69e3..ca8e96fc4 100644
--- a/base/properties.cpp
+++ b/base/properties.cpp
@@ -23,6 +23,7 @@
 
 #include <algorithm>
 #include <chrono>
+#include <limits>
 #include <string>
 
 #include <android-base/parseint.h>
@@ -109,7 +110,7 @@ static void WaitForPropertyCallback(void* data_ptr, const char*, const char* val
 static void DurationToTimeSpec(timespec& ts, const std::chrono::milliseconds d) {
   auto s = std::chrono::duration_cast<std::chrono::seconds>(d);
   auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(d - s);
-  ts.tv_sec = s.count();
+  ts.tv_sec = std::min<std::chrono::seconds::rep>(s.count(), std::numeric_limits<time_t>::max());
   ts.tv_nsec = ns.count();
 }