GH-42220: [R] handle vctrs_rcrd extension type in metadata cleaning (#…

…42226)

### Rationale for this change

See #42220. Extension type metadata serialization works slightly differently from the general R metadata path: it uses a vctrs::ptype object, which is 0-length with attributes. The sanitization needs to happen on `attributes()`, not the object being serialized. 

The error was caused because `vctrs_rcrd` has a special `[<-` method, so iterating over it and assigning back in the sanitized version (even if it were the right thing to do) failed.

### What changes are included in this PR?

Sanitize the `attributes()` of vctrs ptypes.

### Are these changes tested?

yes

### Are there any user-facing changes?

other than the bug being fixed, no
* GitHub Issue: #42220

Authored-by: Neal Richardson <[email protected]>
Signed-off-by: Nic Crane <[email protected]>

r/R/extension.R

@@ -429,7 +429,8 @@ VctrsExtensionType <- R6Class("VctrsExtensionType",
       paste0(capture.output(print(self$ptype())), collapse = "\n")
     },
     deserialize_instance = function() {
-      private$.ptype <- safe_r_metadata(safe_unserialize(self$extension_metadata()))
+      private$.ptype <- safe_unserialize(self$extension_metadata())
+      attributes(private$.ptype) <- safe_r_metadata(attributes(private$.ptype))
     },
     ExtensionEquals = function(other) {
       inherits(other, "VctrsExtensionType") && identical(self$ptype(), other$ptype())
@@ -510,11 +511,13 @@ vctrs_extension_array <- function(x, ptype = vctrs::vec_ptype(x),
 vctrs_extension_type <- function(x,
                                  storage_type = infer_type(vctrs::vec_data(x))) {
   ptype <- vctrs::vec_ptype(x)
+  # Make sure there are no unsupported objects buried in there
+  attributes(ptype) <- safe_r_metadata(attributes(ptype))
 
   new_extension_type(
     storage_type = storage_type,
     extension_name = "arrow.r.vctrs",
-    extension_metadata = serialize(ptype, NULL),
+    extension_metadata = serialize(ptype, NULL, ascii = TRUE),
     type_class = VctrsExtensionType
   )
 }

r/tests/testthat/test-extension.R

@@ -343,3 +343,11 @@ test_that("Dataset/arrow_dplyr_query can roundtrip extension types", {
 
   expect_identical(unclass(roundtripped$extension), roundtripped$letter)
 })
+
+test_that("Handling vctrs_rcrd type", {
+  df <- data.frame(
+    x = vctrs::new_rcrd(fields = list(special = 1:3), class = "special")
+  )
+  tab <- arrow_table(df)
+  expect_identical(as.data.frame(tab), df)
+})