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

[Alan Bateman]

On Wed, 22 Sep 2021 01:25:37 GMT, Brian Burkhalter <bpb 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.

src/java.base/share/classes/sun/nio/ch/IOStatus.java line 42:

> 40:     @Native public static final int THROWN = -5;           // Exception thrown in JNI code
> 41:     @Native public static final int UNSUPPORTED_CASE = -6; // This case not supported
> 42:     @Native public static final int UNSUPPORTED_SUBCASE = -7; // This subcase not supported

I think it's way too confusing to introduce an "unsupported subclass", I think we need to find a better solution for this.

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.

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