RFR: JDK-8260694: (fc) put clear message in FileChannel.transferFrom/transferTo that it will not return -1

[Hamlin Li]

We'd better put more detailed messages in FileChannel.transferFrom/transferTo that it will not return -1, as we met several cases where developers of framework/program assume that it will return -1 on some error conditions, e.g. EOF of src Channel of transferFrom.

please check more details in JDK-8260486, and discussion in https://mail.openjdk.java.net/pipermail/nio-dev/2021-January/008094.html

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Commit messages:
 - JDK-8260694: (fc) put clear message in FileChannel.transferFrom/transferTo that it will not return -1

Changes: https://git.openjdk.java.net/jdk/pull/2329/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2329&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260694
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2329.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2329/head:pull/2329

PR: https://git.openjdk.java.net/jdk/pull/2329