It’s not
possible to state exactly when a managed object will be collected. The garbage collector schedules itself
based on various heuristics. Even
if a garbage collection occurs, it may only collect the younger generations of
the heap. And the JIT has some
freedom to lengthen or shorten the lifetime of instances, based on how it
generates code and reports liveness.
class C
{
IntPtr
_handle;
Static void OperateOnHandle(IntPtr
h) { ... }
void m()
{
OperateOnHandle(_handle);
...
}
...
}
class Other
{
void work()
{
if (something)
{
C aC = new
C();
aC.m();
... // most guess
here
} else
{
...
}
}
}
So we
can’t say how long ‘aC’ might live in the above code. The JIT might report the reference until
Other.work() completes. It might
inline Other.work() into some other method, and report aC even longer. Even if you add “aC = null;” after your
usage of it, the JIT is free to consider this assignment to be dead code and
eliminate it. Regardless of when
the JIT stops reporting the reference, the GC might not get around to collecting
it for some time.
It’s
more interesting to worry about the earliest point that aC could be
collected. If you are like most
people, you’ll guess that the soonest aC becomes eligible for collection is at
the closing brace of Other.work()’s “if” clause, where I’ve added the
comment. In fact, braces don’t
exist in the IL. They are a
syntactic contract between you and your language compiler. Other.work() is free to stop reporting
aC as soon as it has initiated the call to aC.m().
Another
common guess is that the soonest aC could be collected is when C.m() stops
executing. Or perhaps after the
call to C.OperateOnHandle().
Actually, aC could become eligible for collection before C.m() even calls
C.OperateOnHandle(). Once we’ve
extracted _handle from ‘this’, there are no further uses of this object. In other words, ‘this’ can be collected
even while you are executing an instance method on that object.
Why
should you care? Well, for the
example above, you don’t care. The
GC’s reachability will ensure that objects won’t be collected until we are
finished with them. But what if
class C has a Finalize() method which closes _handle? When we call C.OperateOnHandle(), we now
have a race between the application and the GC / Finalizer. Eventually, that’s a race we’re going to
lose.
One way
to fix this race is to add a call to GC.KeepAlive(this) right after the call to
OperateOnHandle(). This indicates
that we need the JIT to keep reporting ‘this’ to the GC until we get to that
point in the execution. KeepAlive
is just a light-weight method call that is opaque to the JIT. So the JIT cannot inline the call and
recognize that the call has no real side effects and hence could be
eliminated.
The
reason you need to add this call is that you have really broken the
encapsulation of the _handle resource.
The lifetime of the enclosing object and the required lifetime of the
_handle are separated when you extract the value from the object’s
field.
It’s bad
enough that you must use GC.KeepAlive() to tie those two lifetimes back together
in your encapsulation. It would be
disastrous if you required the clients of your class to be responsible for
calling KeepAlive. Public fields on
classes are a bad idea for many reasons.
As we’ve seen, when they expose a resource that is subject to
finalization, they are an exceptionally bad idea.
(You may
wonder why we don’t just extend all lifetimes to the end of methods. This has a terrible impact on code
quality, particularly on X86 where we are cursed with limited registers. And a change like that doesn’t really
fix the problem. It’s still
possible for you to return the _handle, place it in a static field, or otherwise
cause its lifetime to escape the lifetime of the enclosing object).
There’s
another wrinkle to this issue. So
far we’ve seen how the Finalizer thread and the application can race when the
resource can be separated from its enclosing object. The same sort of thing can happen when
you expose IDisposable on your class.
Now a multi-threaded application can simultaneously use the resource on
one thread and imperatively call Dispose on another thread. GC.KeepAlive isn’t going to solve this
problem, since you’ve provided a public API to disassociate the lifetime of the
resource from the lifetime of the enclosing object.
This is
more than application issue. It can
also be used to mount security attacks.
If malicious code can open a file to an uninteresting part of the
filesystem, it could simultaneously Read and Dispose that file object on two
different threads. In a server
environment, it’s possible that some other component is opening a file to a
sensitive part of the filesystem.
Eventually, the malicious code could exploit the race condition to read
the other component’s file. This is
a handle-recycling attack.
We’ve
taken care to prevent this situation in our frameworks. When we use a resource in a PInvoke to
the operating system (like reading from a file handle), we place a reference
count on the resource. If malicious
or poorly-timed code calls Dispose, this simply removes the reference count that
was created when the resource was acquired. The result is that all current uses of
the resource will be drained, the resource will then be safely disposed, and
subsequent attempts to use the resource will be failed gracefully.
For now,
you should consider similar approaches if you are encapsulating sensitive
resources like this, which are subject to recycling.
But of
course this is all far too messy.
It runs counter to the goals for our new managed platform to force
developers to worry about this sort of thing. In the future, we hope to provide some
convenient mechanisms which will allow you to build safe, efficient resource
managers. These mechanisms address
most of the issues noted above, and some other issues related to reliability and
performance. As usual, I can’t
really talk about them yet.
