RFR: 8274112: (fc) Tune FileChannel.transferTo()

[Brian Burkhalter]

On Wed, 22 Sep 2021 13:29:19 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Please consider this patch which would improve the performance of
>> `FileChannel.transferTo()` on macOS and Windows.
>
> src/java.base/share/classes/sun/nio/ch/FileChannelImpl.java line 559:
> 
>> 557:         int thisFDVal = IOUtil.fdVal(fd);
>> 558:         int targetFDVal = IOUtil.fdVal(targetFD);
>> 559:         if (thisFDVal != -1 && targetFDVal != -1 && thisFDVal == targetFDVal)
> 
> How can the fd values be -1 here, we must have an issue somewhere else if this happens.

On Windows this can happen.

> src/java.base/windows/native/libnio/ch/FileChannelImpl.c line 177:
> 
>> 175:         if (WriteFile(dst, &buf, nr, &nw, NULL) == 0) {
>> 176:             JNU_ThrowIOExceptionWithLastError(env, "WriteFile failed");
>> 177:             return IOS_THROWN;
> 
> If ReadFile or WriteFile fails then I assume we should return tw when > 0.

In the macOS code if `read()` or `write()` fails then an exception is thrown. I suppose the two read-write versions should be consistent one way or the other.

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PR: https://git.openjdk.java.net/jdk/pull/5623