Handle get0() in setDT() to preserve .internal.selfref (#7101)

* added get0

* added test and the news

* tests

* don't reference internal attribute

* Update tests.Rraw

* removed test

* simplify

---------

Co-authored-by: Michael Chirico <[email protected]>

Author: venom1204

NEWS.md

@@ -198,6 +198,8 @@ rowwiseDT(
 
 20. Fixed a memory issue causing segfaults in `forder`, [#6797](https://github.com/Rdatatable/data.table/issues/6797). Thanks @dkutner for the report and @MichaelChirico for the fix.
 
+21. `setDT(get0('var'))` now correctly modifies `var` by reference, consistent with the long-standing behavior of `setDT(get('var'))`, [#6864](https://github.com/Rdatatable/data.table/issues/6864). Thanks to @rikivillalba for the report and @venom1204 for the fix.
+
 ### NOTES
 
 1. There is a new vignette on joins! See `vignette("datatable-joins")`. Thanks to Angel Feliz for authoring it! Feedback welcome. This vignette has been highly requested since 2017: [#2181](https://github.com/Rdatatable/data.table/issues/2181).

R/data.table.R

@@ -2982,7 +2982,7 @@ setDT = function(x, keep.rownames=FALSE, key=NULL, check.names=FALSE) {
   } else if (.is_simple_extraction(name)) {
     # common case is call from 'lapply()'
     .reassign_extracted_table(name, x)
-  } else if (name %iscall% "get") { # #6725
+  } else if (name %iscall% c("get", "get0")) { # #6725
     # edit 'get(nm, env)' call to be 'assign(nm, x, envir=env)'
     name = match.call(get, name)
     name[[1L]] = quote(assign)

inst/tests/tests.Rraw

@@ -21183,3 +21183,12 @@ test(2317.6, DT1[DF1, on='a', .(d = x.a + i.d)]$d, 5)
 test(2317.7, DT1[DF2, on='a', e := i.e]$e, 5)
 test(2317.8, DT1[DF2, on='a', e2 := x.a + i.e]$e2, 6)
 test(2317.9, DT1[DF2, on='a', .(e = x.a + i.e)]$e, 6)
+
+#6864
+dt_get = data.frame(a = 1:3, b = letters[1:3])
+setDT(get("dt_get"))
+test(2319.1, !is.null(attr(dt_get, ".internal.selfref")))
+
+dt_get0 = data.frame(a = 1:3, b = letters[1:3])
+setDT(get0("dt_get0"))
+test(2319.2, !is.null(attr(dt_get0, ".internal.selfref")))