RFR (M) 8234510: Remove file seeking requirement for writing a heap dump

[Schmelter, Ralf]

Hi Thomas,

thanks for the feedback.

> In DumpWriter, _current_entry_left and _entry_ended seem only to be needed for
> asserting. Please enclose their definition in DEBUG_ONLY, and initialize them in the ctor.

Good catch. I made them debug only.

> (not your patch): since DumperSupport::dump_class_and_array_classes(Klass*)
> should assert that Klass* is an InstanceKlass; or, even better, use InstanceKlass*
> as parameter.

A former version of the patch had a lot of the Klass* types replaced by InstanceKlass* where appropriate. I removed those changes ultimately because they had not much to with making the heap dump streamable. But a later patch could change this.

> DumpWriter::start_dump_entry(): It took me a while to understand how the
> segment size is updated if the entry is huge, since by the time we finish the
> entry the segment header will already be flushed out. The answer is, I think,
> that this is not needed since we only write one record so the initial size we wrote
> into the segment header is still valid.

Correct.

> Proposed comment change:
>
> -// Will be fixed up later if we can add more entries.
> +// Seed segment size with size of its first record. Should we add more records later,
>    we will update the segment size (see >finish_dump_segment())

I’ve changed the comment to make it more clear.

Best regards,
Ralf
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