Closes #970. rare alloccol error/segfault fix.

README.md

@@ -123,6 +123,8 @@
 
   33. List columns can be assigned to columns of `factor` type by reference. Closes [#936](https://github.com/Rdatatable/data.table/issues/936). Thanks to @richierocks for the minimal example.
 
+  34. After setting the `datatable.alloccol` option, creating a data.table with more than the set `truelength` resulted in error or segfault. This is now fixed. Closes [#970](https://github.com/Rdatatable/data.table/issues/970). Thanks to @caneff for the nice minimal example.
+
 #### NOTES
 
   1. Clearer explanation of what `duplicated()` does (borrowed from base). Thanks to @matthieugomez for pointing out. Closes [#872](https://github.com/Rdatatable/data.table/issues/872).

inst/tests/tests.Rraw

@@ -5912,6 +5912,14 @@ test(1479, rbindlist(replicate(4,rbindlist(replicate(47, NULL),
 DT <- data.table(x = factor(c("a", "b c", "d e f")))
 test(1480, DT[, x := strsplit(as.character(x), " ")], data.table(x=list("a", letters[2:3], letters[4:6])))
 
+# #970, over-allocation issue
+a=data.frame(matrix(1,ncol=101))
+options(datatable.alloccol=100)
+ans1 = data.table(a)
+options(datatable.alloccol=101)
+ans2 = data.table(a)
+test(1481, ans1, ans2)
+
 ##########################
 
 

src/assign.c

@@ -170,7 +170,8 @@ SEXP alloccol(SEXP dt, R_len_t n, Rboolean verbose)
     // So, careful to use length() on names, not LENGTH(). 
     if (length(names)!=l) error("Internal error: length of names (%d) is not length of dt (%d)",length(names),l);
     if (!selfrefok(dt,verbose))
-        return shallow(dt,R_NilValue,n);  // e.g. test 848 and 851 in R > 3.0.2  
+        return shallow(dt,R_NilValue,(n>l) ? n : l);  // e.g. test 848 and 851 in R > 3.0.2 
+        // added (n>l) ? ... for #970, see test 1481.
     // TO DO:  test realloc names if selfrefnamesok (users can setattr(x,"name") themselves for example.
     // if (TRUELENGTH(getAttrib(dt,R_NamesSymbol))!=tl)
     //    error("Internal error: tl of dt passes checks, but tl of names (%d) != tl of dt (%d)", tl, TRUELENGTH(getAttrib(dt,R_NamesSymbol)));