RFR: 4926314: Optimize Reader.read(CharBuffer) [v4]

[Peter Levart]

On Sat, 9 Jan 2021 23:06:22 GMT, Philippe Marschall <github.com+471021+marschall at openjdk.org> wrote:

>> Implement three optimiztations for Reader.read(CharBuffer)
>> 
>> * Add a code path for heap buffers in Reader#read to use the backing array instead of allocating a new one.
>> * Change the code path for direct buffers in Reader#read to limit the intermediate allocation to `TRANSFER_BUFFER_SIZE`.
>> * Implement `InputStreamReader#read(CharBuffer)` and delegate to `StreamDecoder`.
>> * Implement `StreamDecoder#read(CharBuffer)` and avoid buffer allocation.
>
> Philippe Marschall has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add unit tests
>   
>   - add unit test for Reader#read(CharBuffer)
>   - add unit test for InputStreamReader#reader(CharBuffer)
>   - test with both on-heap and off-heap buffers

src/java.base/share/classes/java/io/Reader.java line 207:

> 205:                         target.put(cbuf, 0, n);
> 206:                         nread += n;
> 207:                         remaining -= n;

Wouldn't there be a possibility for target.put(cbuf, 0, n) to throw BufferOverflowException ?
For example:
- there's room (remaining) for TRANSFER_BUFFER_SIZE + 1 characters in target
- cbuff is sized to TRANSFER_BUFFER_SIZE
- 1st iteration of do loop transfers TRANSFER_BUFFER_SIZE charasters (remaining == 1)
- 2nd iteration reads > 1 (up to TRANSFER_BUFFER_SIZE) characters
- target.put throws BufferOverflowException

You have to limit the amount read in each iteration to be Math.min(remaining, cbuf.length)

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PR: https://git.openjdk.java.net/jdk/pull/1915