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digitalmars.D.bugs - [Issue 2632] New: Setting length on invalid arrays causes assertion failure with a debug runtime

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2632

           Summary: Setting length on invalid arrays causes assertion
                    failure with a debug runtime
           Product: D
           Version: 1.039
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: major
          Priority: P2
         Component: Phobos
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: matti.niemenmaa+dbugzilla iki.fi


Both of the below test cases work unless compiled against a debug runtime.

Test case 1:

void main() {
        int[] a = (cast(int*)null)[0..1];
        a.length = 1;
        assert (a.length == 1);
        assert (a[0] == 0);
}

Test case 2:

union Union {
        int i;
        ubyte[] a;
}
void main() {
        Union u;
        u.i = 10;
        u.a.length = 1;
        assert (u.a.length == 1);
        assert (u.a[0] == 1);
}

The problematic assertion is assert(!p.length || p.data). It occurs in three
functions:

- _d_delarray
- _d_arraysetlengthT
- _d_arraysetlengthiT

The second one of the above is triggered by the given test cases. It'd be easy
to come up with ones that trigger the other two as well.

This behaviour doesn't seem to be specified, so I'm not completely sure whether
this should be fixed or not. I have two arguments in favour of removing the
assertions:

1. They're only sanity checks: they don't actually matter for behaviour. Code
which trips the asserts on a debug runtime runs fine on a release runtime.
Nothing actually expects the assertions to succeed.

2. Code which relies on such behaviour actually exists: test case 2 was reduced
from tango.text.Regex.

In Phobos, the functions are in phobos/internal/gc/gc.d; in Tango, they can be
found in tango/lib/compiler/{dmd,gdc}/lifetime.d; in LDC, they're in
runtime/internal/lifetime.d. This issue affects all of them.


--
Jan 29 2009
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2632


matti.niemenmaa+dbugzilla iki.fi changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|Phobos                      |www.digitalmars.com
           Keywords|                            |spec





-------
I'm moving this to www.digitalmars.com and adding the 'spec' keyword since it
seems that there's absolutely nothing written down about this possibility. I
believe this needs to be clarified. If the answer indicates that Phobos should
change, I'll file a separate bug for that.

The basic issue is, what operations, if any, are allowed on dynamic arrays
which are in an invalid state, i.e. have a nonzero length but a null pointer?
(I suppose the converse—nonnull pointer but zero length—is fine.)
"Implementation defined" is a fine answer as long as it's actually specified.

Associative arrays don't have this problem since their ABI says that they're
just a pointer to an implementation defined type.


--
Feb 08 2009
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2632


Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla digitalmars.com
         Resolution|                            |INVALID



22:11:06 PST ---
I think both cases are clearly undefined behavior. The first uses a cast to
bypass checks in order to create an array of elements that don't exist, the
second uses a union to achieve the same.

I don't think the spec needs to be changed to reflect that using unions to
manipulate things under the hood, and using forcible casts, mean you're on your
own.

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