Use os_log instead of syslog on Apple platforms (#13487)

Migrates to using os_log for app logging on iOS 13 and above, macOS
10.11 and above. On older platform, fall back to syslog(), which is what
we used previously.

fml/logging.cc

@@ -11,7 +11,8 @@
 
 #if defined(OS_ANDROID)
 #include <android/log.h>
-#elif defined(OS_IOS)
+#elif defined(OS_MACOSX)
+#include <os/log.h>
 #include <syslog.h>
 #endif
 
@@ -81,8 +82,37 @@ LogMessage::~LogMessage() {
       break;
   }
   __android_log_write(priority, "flutter", stream_.str().c_str());
-#elif defined(OS_IOS)
-  syslog(LOG_ALERT, "%s", stream_.str().c_str());
+#elif defined(OS_MACOSX)
+  const char* chars = stream_.str().c_str();
+  // Unified logging (os_log) became available in iOS 9.0 and syslog stopped
+  // working in iOS 13.0. idevicesyslog made device syslog available on the
+  // connected host, but there is no known API to view device unified logging on
+  // the host. Flutter tool will continue to observe syslog on devices older
+  // than iOS 13.0 since it provides more logging context, particularly for
+  // application crashes.
+  if (__builtin_available(iOS 13.0, macOS 10.11, *)) {
+    os_log_t engine_log = os_log_create("io.flutter", "engine");
+    switch (severity_) {
+      // TODO(flutter/flutter#45931): LogSeverity LOG_INFO and LOG_WARNING
+      // collide with syslog log level macros.
+      case 0 /* LOG_INFO */:
+        os_log_debug(engine_log, "%s", chars);
+        break;
+      case 1 /* LOG_WARNING */:
+      case LOG_ERROR:
+        os_log_error(engine_log, "%s", chars);
+        break;
+      case LOG_FATAL:
+        os_log_fault(engine_log, "%s", chars);
+        break;
+      default:
+        os_log(engine_log, "%s", chars);
+        break;
+    }
+  } else {
+    syslog(LOG_ALERT, "%s", chars);
+  }
+
 #else
   std::cerr << stream_.str();
   std::cerr.flush();

lib/ui/dart_runtime_hooks.cc

@@ -30,7 +30,8 @@
 
 #if defined(OS_ANDROID)
 #include <android/log.h>
-#elif defined(OS_IOS)
+#elif defined(OS_MACOSX)
+#include <os/log.h>
 extern "C" {
 // Cannot import the syslog.h header directly because of macro collision.
 extern void syslog(int, const char*, ...);
@@ -199,19 +200,29 @@ void Logger_PrintString(Dart_NativeArguments args) {
     // Write to the logcat on Android.
     __android_log_print(ANDROID_LOG_INFO, logger_prefix.c_str(), "%.*s",
                         (int)length, chars);
-#elif defined(OS_IOS)
-    // Write to syslog on iOS.
-    //
+#elif defined(OS_MACOSX)
     // TODO(cbracken): replace with dedicated communication channel and bypass
     // iOS logging APIs altogether.
-    syslog(1 /* LOG_ALERT */, "%.*s", (int)length, chars);
+    //
+    // Unified logging (os_log) became available in iOS 9.0 and syslog stopped
+    // working in iOS 13.0. idevicesyslog made device syslog available on the
+    // connected host, but there is no known API to view device unified logging
+    // on the host. Flutter tool will continue to observe syslog on devices
+    // older than iOS 13.0 since it provides more logging context, particularly
+    // for application crashes.
+    if (__builtin_available(iOS 13.0, macOS 10.11, *)) {
+      os_log_t dart_log = os_log_create("io.flutter", "dart");
+      os_log(dart_log, "%.*s", static_cast<int>(length), chars);
+    } else {
+      syslog(1 /* LOG_ALERT */, "%.*s", @(length).intValue, chars);
+    }
 #else
     std::cout << log_string << std::endl;
 #endif
   }
 
   if (dart::bin::ShouldCaptureStdout()) {
-    // For now we report print output on the Stdout stream.
+    // Report print output on the Stdout stream.
     uint8_t newline[] = {'\n'};
     Dart_ServiceSendDataEvent("Stdout", "WriteEvent",
                               reinterpret_cast<const uint8_t*>(chars), length);