add tests for isProcessForkedChild() and archSystemFile()

cover two 6.0.0 additions that previously had no automated tests:

- isProcessForkedChild(): FALSE in the parent, TRUE in parallel::mclapply
  workers (skipped on Windows, which has no fork())
- archSystemFile(): delegates to system.file() with no arch subdir, and
  composes the arch-specific subdirectory when .Platform$r_arch is set

Author: kevinushey

tests/test-arch-system-file.R

@@ -0,0 +1,37 @@
+# Unit tests for archSystemFile(), the arch-aware wrapper around system.file()
+# introduced when systemFile() was renamed (see #251, #252). It injects the
+# architecture-specific subdirectory (.Platform$r_arch) used on Windows, e.g.
+# archSystemFile("lib") -> 'lib/x64', while behaving like system.file()
+# elsewhere.
+
+RcppParallel:::test_init()
+
+arch <- .Platform$r_arch
+
+# with no arch subdirectory (the usual case off Windows), archSystemFile()
+# should be equivalent to a plain system.file() call
+if (!nzchar(arch)) {
+   assert(identical(
+      archSystemFile("include"),
+      system.file("include", package = "RcppParallel")
+   ))
+   assert(identical(
+      archSystemFile("include", "RcppParallel.h"),
+      system.file("include/RcppParallel.h", package = "RcppParallel")
+   ))
+}
+
+# the composed relative path must match what system.file() is handed: dir,
+# then the arch subdir when set, then the optional name -- joined with '/'
+expected <- function(dir, name = NULL) {
+   parts <- c(dir, if (nzchar(arch)) arch, name)
+   system.file(paste(parts, collapse = "/"), package = "RcppParallel")
+}
+
+assert(identical(archSystemFile("lib"), expected("lib")))
+assert(identical(archSystemFile("lib", "libtbb.so"), expected("lib", "libtbb.so")))
+
+# a real, shipped resource resolves to an existing path
+header <- archSystemFile("include", "RcppParallel.h")
+assert(nzchar(header))
+assert(file.exists(header))

tests/test-fork.R

@@ -0,0 +1,26 @@
+# Unit tests for isProcessForkedChild().
+#
+# The contract: it returns FALSE in the process where RcppParallel was loaded,
+# and TRUE in a fork() of that process (e.g. a parallel::mclapply worker). On
+# Windows there is no fork(), so it always returns FALSE.
+
+RcppParallel:::test_init()
+
+# in the parent process, we are never a forked child
+assert(isProcessForkedChild() == FALSE)
+
+# parallel::mclapply uses fork() on unix, so its workers must report TRUE.
+# on Windows there is no fork() (mclapply falls back to serial), so skip: the
+# workers there run in the parent process and would correctly report FALSE.
+if (!is_windows()) {
+
+   results <- parallel::mclapply(1:2, function(i) {
+      RcppParallel::isProcessForkedChild()
+   }, mc.cores = 2L)
+
+   assert(all(vapply(results, isTRUE, logical(1L))))
+
+}
+
+# forking must not disturb the parent's own view of itself
+assert(isProcessForkedChild() == FALSE)