keep the legacy observe definition out of the bundled tbb on windows

The bundled oneTBB carries a local (non-upstream) definition of
tbb::internal::task_scheduler_observer_v3::observe in observer_proxy.cpp,
alongside the declarations in task_scheduler_observer.h. Once the bundled
copy is actually built on Windows, libtbb12.a and tbb-compat.o both define
it and the stub fails to link:

  libtbb12.a(observer_proxy.cpp.obj): multiple definition of
    'tbb::internal::task_scheduler_observer_v3::observe(bool)'

The stub has to own that definition on Windows, because R CMD SHLIB builds
the DLL's export list from the objects it compiles -- a definition sitting
in the archive would never make it into tmp.def. So leave it out of the
library there; other platforms keep it, where it serves binaries built
against RcppParallel 5.x.

Also stop trusting R CMD SHLIB's exit code: it reported success through the
link failure above, so the install completed and shipped a package with no
tbb.dll -- precisely the failure mode that packages linking '-ltbb' only
hit at load time. Check for the artifact, and for the copy, instead.

Author: kevinushey

src/install.libs.R

@@ -152,7 +152,15 @@ buildTbbStub <- function(tbbDest) {
 
    }
 
-   file.copy("tbb-compat/tbb.dll", file.path(tbbDest, "tbb.dll"))
+   # 'R CMD SHLIB' has been seen to report success on Windows even when the
+   # link failed, which would otherwise leave us shipping a package with no
+   # stub at all -- and packages linking '-ltbb' only discover that when they
+   # fail to load. Check for the artifact rather than trusting the exit code.
+   if (!file.exists("tbb-compat/tbb.dll"))
+      stop("tbb.dll stub was not produced")
+
+   if (!file.copy("tbb-compat/tbb.dll", file.path(tbbDest, "tbb.dll")))
+      stop("couldn't copy tbb.dll stub to '", tbbDest, "'")
 
 }
 

src/tbb/src/tbb/observer_proxy.cpp

@@ -318,6 +318,13 @@ void __TBB_EXPORTED_FUNC observe(d1::task_scheduler_observer &tso, bool enable)
 } // namespace detail
 } // namespace tbb
 
+// Not on Windows: there the old ABI is published by the tbb.dll stub built
+// from src/tbb-compat/tbb-compat.cpp, which has to define this itself in
+// order for R CMD SHLIB to pick it up when generating the export list.
+// Defining it here as well makes the stub fail to link with "multiple
+// definition of tbb::internal::task_scheduler_observer_v3::observe".
+#ifndef _WIN32
+
 namespace tbb {
 namespace internal {
 
@@ -327,4 +334,6 @@ void __TBB_EXPORTED_FUNC task_scheduler_observer_v3::observe( bool enable ) {
 }
 
 } // namespace internal
-} // namespace tbb
+} // namespace tbb
+
+#endif /* _WIN32 */