arrow

ARROW-15471: [R] ExtensionType support in R

This PR implements extension type support and registration in the R bindings (as has been possible in the Python bindings for some time). The details still need to be worked out, but we at least have a working pattern:

``` r
library(arrow, warn.conflicts = FALSE)
library(R6)

SomeExtensionTypeSubclass <- R6Class(
  "SomeExtensionTypeSubclass", inherit = arrow:::ExtensionType,
  public = list(
    some_custom_method = function() {
      private$some_custom_field
    },

    .Deserialize = function(storage_type, extension_name, extension_metadata) {
      private$some_custom_field <- head(extension_metadata, 5)
    }
  ),
  private = list(
    some_custom_field = NULL
  )
)

SomeExtensionArraySubclass <- R6Class(
  "SomeExtensionArraySubclass", inherit = arrow:::ExtensionArray,
  public = list(
    some_custom_method = function() {
      self$type$some_custom_method()
    }
  )
)

type <- arrow:::MakeExtensionType(
  int32(),
  "some_extension_subclass",
  charToRaw("some custom metadata"),
  type_class = SomeExtensionTypeSubclass,
  array_class = SomeExtensionArraySubclass
)

arrow:::RegisterExtensionType(type)

# survives the C API round trip
ptr_type <- arrow:::allocate_arrow_schema()
type$export_to_c(ptr_type)
type2 <- arrow:::DataType$import_from_c(ptr_type)

type2
#> SomeExtensionTypeSubclass
#> SomeExtensionTypeSubclass <some custom metadata>
type2$some_custom_method()
#> [1] 73 6f 6d 65 20

(array <- type$WrapArray(Array$create(1:10)))
#> SomeExtensionArraySubclass
#> <SomeExtensionTypeSubclass <some custom metadata>>
#> [
#>   1,
#>   2,
#>   3,
#>   4,
#>   5,
#>   6,
#>   7,
#>   8,
#>   9,
#>   10
#> ]
array$some_custom_method()
#> [1] 73 6f 6d 65 20

ptr_array <- arrow:::allocate_arrow_array()
array$export_to_c(ptr_array, ptr_type)
(array2 <- Array$import_from_c(ptr_array, ptr_type))
#> SomeExtensionArraySubclass
#> <SomeExtensionTypeSubclass <some custom metadata>>
#> [
#>   1,
#>   2,
#>   3,
#>   4,
#>   5,
#>   6,
#>   7,
#>   8,
#>   9,
#>   10
#> ]

arrow:::delete_arrow_schema(ptr_type)
arrow:::delete_arrow_array(ptr_array)
```

<sup>Created on 2022-02-18 by the [reprex package](https://reprex.tidyverse.org) (v2.0.1)</sup>

Closes #12467 from paleolimbot/r-extension-type

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>