Make later.h compatible with old versions of later DLL

DESCRIPTION

@@ -1,7 +1,7 @@
 Package: later
 Type: Package
 Title: Utilities for Scheduling Functions to Execute Later with Event Loops
-Version: 0.8.0.9003
+Version: 0.8.0.9004
 Authors@R: c(
     person("Joe", "Cheng", role = c("aut", "cre"), email = "joe@rstudio.com"),
     person(family = "RStudio", role = "cph"),

inst/include/later.h

@@ -62,7 +62,37 @@ inline void later(void (*func)(void*), void* data, double secs, int loop) {
 }
 
 inline void later(void (*func)(void*), void* data, double secs) {
-  later(func, data, secs, GLOBAL_LOOP);
+  typedef void (*elnfun)(void (*func)(void*), void*, double);
+  static elnfun eln = NULL;
+  if (!eln) {
+    // Initialize if necessary
+    if (func) {
+      // We're not initialized but someone's trying to actually schedule
+      // some code to be executed!
+      REprintf(
+        "Warning: later::execLaterNative called in uninitialized state. "
+        "If you're using <later.h>, please switch to <later_api.h>.\n"
+      );
+    }
+    eln = (elnfun)R_GetCCallable("later", "execLaterNative");
+  }
+
+  // We didn't want to execute anything, just initialize
+  if (!func) {
+    return;
+  }
+
+  eln(func, data, secs);
+
+
+  // Note 2019-09-11: The above code in this function is here just in case a
+  // package built with this version of later.h is run with an older version
+  // of the later DLL which does not have the execLaterNative2 function. In
+  // the next release of later, after we are confident that users have
+  // installed the newer later DLL which has execLaterNative2, it should be
+  // safe to replace the code in this function with just this:
+  //
+  // later(func, data, secs, GLOBAL_LOOP);
 }