Avoid triggering duckplyr's ALTREP row names

NEWS.md

@@ -1,5 +1,8 @@
 # dplyr (development version)
 
+* `dplyr_reconstruct()`'s default method has been rewritten to avoid
+  materializing duckplyr queries too early (#6947).
+
 * Updated the `storms` data to include 2022 data (#6937, @steveharoz).
 
 * Updated the `starwars` data to use a new API, because the old one is defunct.

R/generics.R

@@ -192,6 +192,7 @@ dplyr_col_modify.rowwise_df <- function(data, cols) {
 dplyr_reconstruct <- function(data, template) {
   # Strip attributes before dispatch to make it easier to implement
   # methods and prevent unexpected leaking of irrelevant attributes.
+  # This also enforces that `data` is a well-formed data frame.
   data <- dplyr_new_data_frame(data)
   return(dplyr_reconstruct_dispatch(data, template))
   UseMethod("dplyr_reconstruct", template)
@@ -202,12 +203,7 @@ dplyr_reconstruct_dispatch <- function(data, template) {
 
 #' @export
 dplyr_reconstruct.data.frame <- function(data, template) {
-  attrs <- attributes(template)
-  attrs$names <- names(data)
-  attrs$row.names <- .row_names_info(data, type = 0L)
-
-  attributes(data) <- attrs
-  data
+  .Call(ffi_dplyr_reconstruct, data, template)
 }
 
 #' @export

R/zzz.R

@@ -40,13 +40,16 @@
   setHook(packageEvent("plyr", "attach"), NULL, "replace")
 }
 
-import_vctrs <- function(name) {
-  import_from(name, "vctrs")
+import_vctrs <- function(name, optional = FALSE) {
+  import_from(name, "vctrs", optional = optional)
 }
-import_from <- function(name, package) {
+import_from <- function(name, package, optional = FALSE) {
   ns <- getNamespace(package)
 
   if (!exists(name, mode = "function", envir = ns, inherits = FALSE)) {
+    if (optional) {
+      return(NULL)
+    }
     abort(sprintf("No such '%s' function: `%s()`.", package, name))
   }
 

src/dplyr.h

@@ -96,6 +96,10 @@ inline bool obj_is_list(SEXP x) {
 
 }
 
+SEXP ffi_dplyr_reconstruct(SEXP data, SEXP template_);
+SEXP ffi_test_dplyr_attributes(SEXP x);
+SEXP ffi_test_dplyr_set_attributes(SEXP x, SEXP attributes);
+
 SEXP dplyr_expand_groups(SEXP old_groups, SEXP positions, SEXP s_nr);
 SEXP dplyr_cumall(SEXP x);
 SEXP dplyr_cumany(SEXP x);

src/init.cpp

@@ -95,6 +95,10 @@ SEXP dplyr_init_library(SEXP ns_dplyr, SEXP ns_vctrs, SEXP ns_rlang) {
 static const R_CallMethodDef CallEntries[] = {
   {"dplyr_init_library", (DL_FUNC)& dplyr_init_library, 3},
 
+  {"ffi_dplyr_reconstruct", (DL_FUNC)& ffi_dplyr_reconstruct, 2},
+  {"ffi_test_dplyr_attributes", (DL_FUNC)& ffi_test_dplyr_attributes, 1},
+  {"ffi_test_dplyr_set_attributes", (DL_FUNC)& ffi_test_dplyr_set_attributes, 2},
+
   {"dplyr_expand_groups", (DL_FUNC)& dplyr_expand_groups, 3},
   {"dplyr_cumall", (DL_FUNC)& dplyr_cumall, 1},
   {"dplyr_cumany", (DL_FUNC)& dplyr_cumany, 1},

src/reconstruct.cpp

@@ -0,0 +1,138 @@
+#include "dplyr.h"
+
+// Essentially, a C implementation of:
+//
+// ```
+// attributes <- attributes(template)
+// attributes$names <- names(data)
+// attributes$row.names <- .row_names_info(data, type = 0L)
+// attributes(data) <- attributes
+// ```
+//
+// The problem with that is that:
+// - `attributes()` ends up calling `Rf_getAttrib()`, which tries to check
+//   for internal `row.names` in `template` so they aren't leaked to the user.
+//   Unfortunately this materializes lazy ALTREP `row.names`, like those used
+//   by duckplyr.
+// - `attributes<-()` ends up calling `Rf_setAttrib()`, which tries to check
+//   if it can make efficient internal `row.names`.  Again, this materializes
+//   lazy ALTREP `row.names`, like those used by duckplyr.
+//
+// So we bypass that here by carefully manipulating the attribute pairlists.
+//
+// We expect that at this point, both `data` and `template_` are S3 data
+// frames, both of which have `names` and `row.names` attributes. If this isn't
+// true, we error.
+// - For `data`, we enforce this in `dplyr_reconstruct()`'s generic by calling
+//   `dplyr_new_data_frame()` (ideally no intermediate method invalidates this).
+// - For `template_`, we assume this since we got here through the S3 method
+//   `dplyr_reconstruct.data.frame()`, which dispatched off `template_`. A
+//   well-formed S3 data frame must have `names` and `row.names` attributes.
+//
+// https://github.com/tidyverse/dplyr/pull/6947
+// https://github.com/tidyverse/dplyr/issues/6525#issuecomment-1303619152
+// https://github.com/wch/r-source/blob/69b94f0c8ce9b2497f6d7a81922575f6c585b713/src/main/attrib.c#L176-L177
+// https://github.com/wch/r-source/blob/69b94f0c8ce9b2497f6d7a81922575f6c585b713/src/main/attrib.c#L57
+SEXP ffi_dplyr_reconstruct(SEXP data, SEXP template_) {
+  if (TYPEOF(data) != VECSXP) {
+    Rf_errorcall(R_NilValue, "Internal error: `data` must be a list.");
+  }
+  if (TYPEOF(template_) != VECSXP) {
+    Rf_errorcall(R_NilValue, "Internal error: `template` must be a list.");
+  }
+  if (!OBJECT(data)) {
+    Rf_errorcall(R_NilValue, "Internal error: `data` must be an object.");
+  }
+  if (!OBJECT(template_)) {
+    Rf_errorcall(R_NilValue, "Internal error: `template` must be an object.");
+  }
+
+  bool seen_names = false;
+  bool seen_row_names = false;
+
+  // Pull the `names` and `row.names` off `data`.
+  // These are the only 2 attributes from `data` that persist.
+  SEXP names = R_NilValue;
+  SEXP row_names = R_NilValue;
+
+  for (SEXP node = ATTRIB(data); node != R_NilValue; node = CDR(node)) {
+    SEXP tag = TAG(node);
+
+    if (tag == R_NamesSymbol) {
+      names = CAR(node);
+      MARK_NOT_MUTABLE(names);
+      seen_names = true;
+    }
+    if (tag == R_RowNamesSymbol) {
+      row_names = CAR(node);
+      MARK_NOT_MUTABLE(row_names);
+      seen_row_names = true;
+    }
+  }
+
+  if (!seen_names) {
+    Rf_errorcall(R_NilValue, "Internal error: `data` must have a `names` attribute.");
+  }
+  if (!seen_row_names) {
+    Rf_errorcall(R_NilValue, "Internal error: `data` must have a `row.names` attribute.");
+  }
+
+  seen_names = false;
+  seen_row_names = false;
+
+  // Now replace the `names` and `row.names` attributes in the `template_`
+  // attributes with the ones from `data`. This attribute set becomes the final
+  // one we set on `data`.
+  SEXP attributes = ATTRIB(template_);
+  attributes = PROTECT(Rf_shallow_duplicate(attributes));
+
+  for (SEXP node = attributes; node != R_NilValue; node = CDR(node)) {
+    SEXP tag = TAG(node);
+
+    if (tag == R_NamesSymbol) {
+      SETCAR(node, names);
+      seen_names = true;
+    }
+    if (tag == R_RowNamesSymbol) {
+      SETCAR(node, row_names);
+      seen_row_names = true;
+    }
+  }
+
+  if (!seen_names) {
+    Rf_errorcall(R_NilValue, "Internal error: `template` must have a `names` attribute.");
+  }
+  if (!seen_row_names) {
+    Rf_errorcall(R_NilValue, "Internal error: `template` must have a `row.names` attribute.");
+  }
+
+  // Make an ALTREP wrapper if possible, since the underlying data doesn't change.
+  // Won't actually make an ALTREP wrapper unless there are >64 columns
+  // (internally controlled by R).
+#if R_VERSION >= R_Version(3, 6, 0)
+  data = PROTECT(R_shallow_duplicate_attr(data));
+#else
+  data = PROTECT(Rf_shallow_duplicate(data));
+#endif
+
+  SET_ATTRIB(data, attributes);
+
+  UNPROTECT(2);
+  return data;
+}
+
+// Very unsafe wrappers needed for testing.
+// Bypass `Rf_getAttrib()` and `Rf_setAttrib()` calls to avoid forcing ALTREP
+// `row.names`.
+SEXP ffi_test_dplyr_attributes(SEXP x) {
+  return ATTRIB(x);
+}
+SEXP ffi_test_dplyr_set_attributes(SEXP x, SEXP attributes) {
+  if (TYPEOF(attributes) != LISTSXP) {
+    Rf_errorcall(R_NilValue, "`attributes` must be a pairlist.");
+  }
+  x = PROTECT(Rf_shallow_duplicate(x));
+  SET_ATTRIB(x, attributes);
+  UNPROTECT(1);
+  return x;
+}

tests/testthat/helper-lazy.R

@@ -0,0 +1,22 @@
+skip_if_no_lazy_character <- function() {
+  skip_if(getRversion() <= "3.5.0")
+
+  new_lazy_character <- import_vctrs("new_lazy_character", optional = TRUE)
+  lazy_character_is_materialized <- import_vctrs("lazy_character_is_materialized", optional = TRUE)
+
+  if (is.null(new_lazy_character) || is.null(lazy_character_is_materialized)) {
+    skip("Lazy character helpers from vctrs are not available.")
+  }
+
+  invisible()
+}
+
+new_lazy_character <- function(fn) {
+  f <- import_vctrs("new_lazy_character")
+  f(fn)
+}
+
+lazy_character_is_materialized <- function(x) {
+  f <- import_vctrs("lazy_character_is_materialized")
+  f(x)
+}

tests/testthat/test-generics.R

@@ -137,3 +137,54 @@ test_that("dplyr_reconstruct() strips attributes before dispatch", {
   dplyr_reconstruct(df, df)
   expect_identical(out, data.frame(x = 1, row.names = "a"))
 })
+
+test_that("`dplyr_reconstruct()` retains attribute ordering of `template`", {
+  df <- vctrs::data_frame(x = 1)
+  expect_identical(
+    attributes(dplyr_reconstruct(df, df)),
+    attributes(df)
+  )
+})
+
+test_that("`dplyr_reconstruct()` doesn't modify the original `data` in place", {
+  data <- new_data_frame(list(x = 1), foo = "bar")
+  template <- vctrs::data_frame(x = 1)
+
+  out <- dplyr_reconstruct(data, template)
+
+  expect_null(attr(out, "foo"))
+  expect_identical(attr(data, "foo"), "bar")
+})
+
+test_that("`dplyr_reconstruct()`, which gets and sets attributes, doesn't touch `row.names` (#6525)", {
+  skip_if_no_lazy_character()
+
+  dplyr_attributes <- function(x) {
+    .Call(ffi_test_dplyr_attributes, x)
+  }
+  dplyr_set_attributes <- function(x, attributes) {
+    .Call(ffi_test_dplyr_set_attributes, x, attributes)
+  }
+
+  df <- vctrs::data_frame(x = 1)
+
+  attributes <- attributes(df)
+  attributes$row.names <- new_lazy_character(function() "a")
+  attributes <- as.pairlist(attributes)
+
+  df_with_lazy_row_names <- dplyr_set_attributes(df, attributes)
+
+  # Ensure `data` row names aren't materialized
+  x <- dplyr_reconstruct(df_with_lazy_row_names, df)
+  attributes <- dplyr_attributes(df_with_lazy_row_names)
+  expect_false(lazy_character_is_materialized(attributes$row.names))
+
+  # `data` row names should also propagate into the result unmaterialized
+  attributes <- dplyr_attributes(x)
+  expect_false(lazy_character_is_materialized(attributes$row.names))
+
+  # Ensure `template` row names aren't materialized
+  x <- dplyr_reconstruct(df, df_with_lazy_row_names)
+  attributes <- dplyr_attributes(df_with_lazy_row_names)
+  expect_false(lazy_character_is_materialized(attributes$row.names))
+})