DIP77

Title:	Fix Unsafe RC Pass By Ref
DIP:	77
Version:	1
Status:	Draft
Created:	2015-04-08
Last Modified:	2015-04-8
Author:	Walter Bright
Links:	NG Discussion for DIP25

Definitions

Reference Counted Object (RCO):

An object is assumed to be reference counted if it has a postblit and a destructor, and does not have an opAssign marked @system.

Payload:

A value returned by ref by a function that has an RCO passed by 'return ref'.

Problem

A couple of problems have been reported with D's current support for reference counting.

1. Problem 1

    struct T {
        void doSomething();
    }
    struct S {
        RCArray!T array;
    }
    void main() {
        auto s = S(RCArray!T([T()])); // s.array's refcount is now 1
        foo(s, s.array[0]);           // pass by ref
    }
    void foo(ref S s, ref T t) {
        s.array = RCArray!T([]);      // drop the old s.array
        t.doSomething();              // oops, t is gone
    }
   

2. Problem 2

   void main()
   {
       auto arr = RCArray!int([0]);
       foo(arr, arr[0]);
   }
   
   void foo(ref RCArray!int arr, ref int val)
   {
       {
   	auto copy = arr; //arr's (and copy's) reference counts are both 2
   	arr = RCArray!int([]); // There is another owner, so arr 
   			       // forgets about the old payload
       } // Last owner of the array ('copy') gets destroyed and happily
         // frees the payload.
       val = 3; // Oops.
   }
   

The problem stems from a function taking a reference to a payload and a reference to the RCO that contains the payload, then manipulating the RCO into freeing the payload before the function returns (i.e. the payload reference is still active).

Solution Summary

Wrap the call to the function with an increment/decrement of the RCO's ref count, ensuring that its payload will remain valid for the duration of the function call.

More accurately, given a reference counted object rc that has a payload passed by ref to function foo():

T foo(ref Payload payload);
...
RC rc;
foo(rc.payload);

rewrite foo(rc) as:

auto tmp = rc;
foo(rc.payload);

The initialization of tmp will cause the reference count to be incremented, and will guarantee the lifetime of rc's payload will be longer than the call to foo(). The lifetime of tmp will be the same as that of a temporary with a destructor.

Optimizations

The point of passing an RC object payload by ref is to avoid the increment and decrement, especially since the decrement will have to be wrapped in an exception handler. Thus, when the compiler can statically prove that the RC object will not be reassigned in foo(), it does not need to generate the tmp assignment. Whether such optimizations occur or not is implementation defined, more opportunities for tmp elision will present themselves as compilers improve.

Reassignment can occur if foo() has access to a mutable reference to rc.

The most obvious case where the tmp copy can be elided is for:

T foo(const ref Payload payload) pure;

The pure means that no other mutable references to rc can come from globals. The const insures that no mutable ref to rc can come from payload (a cyclical data structure).

If Payload can be statically determined to not contain any mutable references to RC, then the elision can occur for:

T foo(ref Payload payload) pure;

Const references to RC do not prevent elision:

T foo(const ref RC rc, ref Payload payload) pure;

If RC has no transitive reference to Payload, then this does not prevent elision:

T foo(ref RC rc, ref Payload payload) pure;

Nested Functions

Nested functions can have uplevel references to variables that are not parameters. These need to be accounted for in looking for mutable references to rc.

Tracking of Local RC Objects

Static analysis of uses of local RC objects can be used to prove that no mutable references to them can exist in a called function.

Inference

In functions where the source exists and attributes are inferred, the function body can also be analyzed for attempts to reassign rc.

Solutions In Other Languages

C++

Don't write code that way.

Cons: No mechanical checking for memory corruption errors.

Objective C

Uses allocation pools, which have the effect of keeping a reference count above 1 so the payload does not get prematurely deleted.

Cons: A lot has been written about such pools, but they remain complex and confusing to use properly. Not obvious how to mechanically check that they are used correctly.

Rust

Statically prevent more than one mutable reference to an object. Regard access to mutable globals as unsafe.

Cons: Many useful idioms are precluded, high barrier to learn how to write code that passes the borrow checker.