Ivan Krylov
2024-Apr-05 12:49 UTC
[Rd] Bug in out-of-bounds assignment of list object to expression() vector
On Fri, 5 Apr 2024 08:15:20 -0400 June Choe <jchoe001 at gmail.com> wrote:> When assigning a list to an out of bounds index (ex: the next, n+1 > index), it errors the same but now changes the values of the vector > to NULL: > > ``` > x <- expression(a,b,c) > x[[4]] <- list() # Error > x > #> expression(NULL, NULL, NULL) > ``` > > Curiously, this behavior disappears if a prior attempt is made at > assigning to the same index, using a different incompatible object > that does not share this bug (like a function)Here's how the problem happens: 1. The call lands in src/main/subassign.c, do_subassign2_dflt(). 2. do_subassign2_dflt() calls SubassignTypeFix() to prepare the operand for the assignment. 3. Since the assignment is "stretching", SubassignTypeFix() calls EnlargeVector() to provide the space for the assignment. The bug relies on `x` not being IS_GROWABLE(), which may explain why a plain x[[4]] <- list() sometimes doesn't fail. The future assignment result `x` is now expression(a, b, c, NULL), and the old `x` set to expression(NULL, NULL, NULL) by SET_VECTOR_ELT(newx, i, VECTOR_ELT(x, i)); CLEAR_VECTOR_ELT(x, i); during EnlargeVector(). 4. But then the assignment fails, raising the error back in do_subassign2_dflt(), because the assignment kind is invalid: there is no way to put data.frames into an expression vector. The new resized `x` is lost, and the old overwritten `x` stays there. Not sure what the right way to fix this is. It's desirable to avoid shallow_duplicate(x) for the overwriting assignments, but then the sub-assignment must either succeed or leave the operand untouched. Is there a way to perform the type check before overwriting the operand? -- Best regards, Ivan
iuke-tier@ey m@iii@g oii uiow@@edu
2024-Apr-05 14:12 UTC
[Rd] [External] Re: Bug in out-of-bounds assignment of list object to expression() vector
On Fri, 5 Apr 2024, Ivan Krylov via R-devel wrote:> On Fri, 5 Apr 2024 08:15:20 -0400 > June Choe <jchoe001 at gmail.com> wrote: > >> When assigning a list to an out of bounds index (ex: the next, n+1 >> index), it errors the same but now changes the values of the vector >> to NULL: >> >> ``` >> x <- expression(a,b,c) >> x[[4]] <- list() # Error >> x >> #> expression(NULL, NULL, NULL) >> ``` >> >> Curiously, this behavior disappears if a prior attempt is made at >> assigning to the same index, using a different incompatible object >> that does not share this bug (like a function) > > Here's how the problem happens: > > 1. The call lands in src/main/subassign.c, do_subassign2_dflt(). > > 2. do_subassign2_dflt() calls SubassignTypeFix() to prepare the operand > for the assignment. > > 3. Since the assignment is "stretching", SubassignTypeFix() calls > EnlargeVector() to provide the space for the assignment. > > The bug relies on `x` not being IS_GROWABLE(), which may explain > why a plain x[[4]] <- list() sometimes doesn't fail. > > The future assignment result `x` is now expression(a, b, c, NULL), and > the old `x` set to expression(NULL, NULL, NULL) by SET_VECTOR_ELT(newx, > i, VECTOR_ELT(x, i)); CLEAR_VECTOR_ELT(x, i); during EnlargeVector(). > > 4. But then the assignment fails, raising the error back in > do_subassign2_dflt(), because the assignment kind is invalid: there is > no way to put data.frames into an expression vector. The new resized > `x` is lost, and the old overwritten `x` stays there. > > Not sure what the right way to fix this is. It's desirable to avoid > shallow_duplicate(x) for the overwriting assignments, but then the > sub-assignment must either succeed or leave the operand untouched. > Is there a way to perform the type check before overwriting the operand?Yes. There are two places where there are some checks, one early and the other late. The early one is explicitly letting this one through and shouldn't. So a one line change would address this particular problem. But it would be a good idea to review why we the late checks are needed at all and maybe change that. I'll look into it. Best, luke -- Luke Tierney Ralph E. Wareham Professor of Mathematical Sciences University of Iowa Phone: 319-335-3386 Department of Statistics and Fax: 319-335-3017 Actuarial Science 241 Schaeffer Hall email: luke-tierney at uiowa.edu Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu