iOS: Cache the result of IsRunningOnIOS10OrLater()

base/ios/ios_util.mm

Index: base/ios/ios_util.mm
diff --git a/base/ios/ios_util.mm b/base/ios/ios_util.mm
index 196d5ef90ae4d81b894c4c23b8d2b87b6e878696..6938410b998a9bd3880612693a592c8268834d7d 100644
--- a/base/ios/ios_util.mm
+++ b/base/ios/ios_util.mm
@@ -21,6 +21,9 @@ const int32_t* OSVersionAsArray() {
   return digits;
 }
 
+// Caches the result of IsRunningOnIOS10OrLater().

[rohitrao (ping after 24h), 2017/06/14 01:10:13]

We'll be adding IsRunningOnIOS11OrLater() shortly, so it might be worth either
adding this now or considering how we would add it later.  For example, the
ternary operation on line 37 wouldn't work.

+enum { kUnknown, kIos9orEarlier, kIos10orLater } g_ios_version = kUnknown;

[Eugene But (OOO till 7-30), 2017/06/14 00:59:48]

Should we cache the result of OSVersionAsArray instead? Otherwise the issue may
show up again later.

[rohitrao (ping after 24h), 2017/06/14 01:10:13]

Can you clarify what issue might "show up again later"?  Is the problem that
some code calls IsRunningOnOrLater() directly?

[Eugene But (OOO till 7-30), 2017/06/14 01:31:53]

Sorry, I meant IsRunningOnIOS11(12/13/etc..)OrLater may become a bottleneck in
the future in some other place. So if improving performance for
IsRunningOnOrLater is an option, then we should probably do that.

+
 std::string* g_icudtl_path_override = nullptr;
 
 }  // namespace
@@ -29,7 +32,11 @@ namespace base {
 namespace ios {
 
 bool IsRunningOnIOS10OrLater() {
-  return IsRunningOnOrLater(10, 0, 0);
+  if (g_ios_version == kUnknown) {

[Eugene But (OOO till 7-30), 2017/06/14 00:59:48]

This makes the function non-thread safe, which you can fix by using
dispatch_once API.

+    g_ios_version =
+        IsRunningOnOrLater(10, 0, 0) ? kIos10orLater : kIos9orEarlier;
+  }
+  return g_ios_version == kIos10orLater;
 }
 
 bool IsRunningOnOrLater(int32_t major, int32_t minor, int32_t bug_fix) {