[Issue 13878] New: Appending to an array block with modified flags loses flag info

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13878

          Issue ID: 13878
           Summary: Appending to an array block with modified flags loses
                    flag info
           Product: D
           Version: D2
          Hardware: x86
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: druntime
          Assignee: schveiguy at yahoo.com
          Reporter: schveiguy at yahoo.com

This happens because the block info is cached, but when you set the flags for
that block, the flags do not get back to the cache.

We can half-solve the problem because the block info cache only uses base,
size, and the APPENDABLE flag. The other flags are just bonus storage, but can
deviate from the block info in the GC.

For example, if you create a block like so:

ubyte[] b = new ubyte[1000];

Then cast it to a void []: 

void[] v = cast(void[]) b;
GC.setAttr(v.ptr, GC.BlkAttr.APPENDABLE) // remove NO_SCAN

Now, if we append to v, once it reallocates, it will use the cached bits, which
contain the "NO_SCAN" bit, and remove the change we made. If v now has pointers
in it, those may be collected prematurely.

We need to provide a way for the runtime to copy the flags from one block to
another, and use that instead of the cache bits. Otherwise, we cannot support
setting flags on an array. It shouldn't be too bad performance-wise, because at
that point, we are already reallocating anyway.

I say half-solve, because removing the APPENDABLE bit will not clear it out of
the cache, and we may make incorrect assumptions about the block from it's
cached APPENDABLE bit. I would caution anyone, however, to not ever clear the
APPENDABLE bit.

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