set call setalloccol for non-selfrefok tables when removing columns (#7500)

* add fix to set

* add assign and tests

* update tests

Author: ben-schwen

R/data.table.R

@@ -2848,6 +2848,14 @@ setcolorder = function(x, neworder=key(x), before=NULL, after=NULL, skip_absent=
 
 set = function(x,i=NULL,j,value)  # low overhead, loopable
 {
+  # If removing columns from a table that's not selfrefok, need to call setalloccol first, #7488
+  if ((is.null(value) || (is.list(value) && any(vapply_1b(value, is.null)))) && selfrefok(x, verbose=FALSE) < 1L) {
+    name = substitute(x)
+    setalloccol(x, verbose=FALSE)
+    if (is.name(name)) {
+      assign(as.character(name), x, parent.frame(), inherits=TRUE)
+    }
+  }
   .Call(Cassign,x,i,j,NULL,value)
   invisible(x)
 }

inst/tests/tests.Rraw

@@ -21898,7 +21898,14 @@ rm(DT, strings)
 
 # do remove columns in freshly unserialized data.tables, #7488
 DT = unserialize(serialize(as.data.table(mtcars), NULL))
-test(2351, DT[,carb:=NULL], as.data.table(mtcars)[,carb:=NULL])
+test(2351.1, DT[,carb:=NULL], as.data.table(mtcars)[,carb:=NULL])
+DT = unserialize(serialize(as.data.table(mtcars), NULL))
+test(2351.2, set(DT, j="carb", value=NULL), as.data.table(mtcars)[,carb:=NULL])
+DT = unserialize(serialize(as.data.table(mtcars), NULL))
+null_in_value <- NULL
+test(2351.3, "cyl" %notin% names(DT[, cyl := null_in_value]))
+DT = unserialize(serialize(as.data.table(mtcars), NULL))
+test(2351.4,  ncol(DT[, c("cyl", "mpg") := .(null_in_value, null_in_value)]), ncol(mtcars) - 2L)
 rm(DT)
 
 # rbindlist did not protect the temporary UTF-8 strings, #7452