HELP: Add raster and 'RasterLayer' to the list of object that may hav…

…e external pointers [#453] [ci skip]

vignettes/future-4-non-exportable-objects.md.rsp

@@ -74,6 +74,7 @@ _If you identify other cases, please consider [reporting](https://github.com/Hen
   **keras**       | keras.engine.sequential.Sequential (`externalptr`)
   **magick**      | magick-image (`externalptr`)
   **ncdf4**       | ncdf4 (custom reference; _non-detectable_)
+  **raster**      | RasterLayer (`externalptr`; _not all_)
   **Rcpp**        | NativeSymbol (`externalptr`)
   **reticulate**  | python.builtin.function (`externalptr`), python.builtin.module (`externalptr`)
   **rJava**       | jclassName (`externalptr`)
@@ -142,7 +143,7 @@ the future expression
 
 #### Package: magic
 
-[magick](https://cran.r-project.org/package=magick) provides an R-level API for ImageMagick to work with images.  When working with this API, the images are represented internally as external pointers of class 'magick_image' that cannot be be exported to another R process, e.g.
+The [magick](https://cran.r-project.org/package=magick) package provides an R-level API for ImageMagick to work with images.  When working with this API, the images are represented internally as external pointers of class 'magick_image' that cannot be be exported to another R process, e.g.
 ```r
 library(future)
 plan(multisession)
@@ -167,6 +168,30 @@ we'll see that this is caught even before attempting to run this in parallel;
 ```
 
 
+#### Package: raster
+
+The [raster](https://cran.r-project.org/package=raster) package provides methods for working with spatial data, which are held in 'RasterLayer' objects.  Not all but some of these objects use an external pointer.  For example,
+
+```r
+library(future)
+plan(multisession)
+
+library(raster)
+r <- raster(system.file("external/test.grd", package = "raster"))
+tf <- tempfile(fileext = ".grd")
+s <- writeStart(r, filename = tf,  overwrite = TRUE)
+
+f <- future({
+  print(dim(r))
+  print(dim(s))
+})
+Error: Detected a non-exportable reference ('externalptr') in one of the
+globals ('s' of class 'RasterLayer') used in the future expression
+```
+
+Note that it is only the RasterLayer object `s` that carries an external pointer and cannot be passed on to an external worker.  In constract, RasterLayer object `r` does not have this problem and would be fine to pass on to a worker.
+
+
 #### Package: Rcpp
 
 Another example is [Rcpp](https://cran.r-project.org/package=Rcpp) , which allow us to easily create R functions that are implemented in C++, e.g.