RFR: 8149330: Capacity of StringBuilder should not get close to Integer.MAX_VALUE unless necessary

[Ivan Gerasimov]

Hello!

When the capacity of a StringBuilder needs to be increased (either due 
to append() or due to explicit call to ensureCapacity()), the new 
capacity is calculated as "twice the old capacity, plus 2", rounded down 
to Integer.MAX_VALUE:
http://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html#ensureCapacity-int-
Because of that, StringBuilder throws OOM early, even though there may 
be room to grow.

The proposed solution is to reserve a few bytes at the top of the range 
and only try to allocate them if absolutely required.

The regression test is @ignored by default, as it is too greedy for memory.

Would you please help review the fix?

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8149330
WEBREV: http://cr.openjdk.java.net/~igerasim/8149330/00/webrev/

Sincerely yours,
Ivan