8329829: HttpClient: Add a BodyPublishers.ofFileChannel method

[vy]

Adds a new ofFileChannel(FileChannel channel, long offset, long length) method to java.net.HttpRequest.BodyPublishers to provide an HttpClient publisher to upload a certain region of a file. The new publisher does not modify the state of the passed FileChannel, streams the file channel bytes as it publishes (i.e., avoids reading the entire file into the memory), and can be leveraged to implement sliced uploads. As noted in the Javadoc:

The file channel will not be closed upon completion. The caller is
expected to manage the life cycle of the channel, and close it
appropriately when not needed anymore.

Implementation notes

• FileChannel is preferred over {Readable,Seekable}ByteChannel, since the latter does not provide a positional read without modifying the state of the FileChannel, which is necessary to use a single FileChannel instance to implement sliced uploads.
• ofFileChannel(FileChannel,long,long) is preferred over ofPath(Path,long,long) to avoid overloading the maximum file descriptor limit of the platform.

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Progress

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Issue

• JDK-8329829: HttpClient: Add a BodyPublishers.ofFileChannel method (Enhancement - P4)

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• 02: Full - Incremental (3af61c5f)
• 01: Full - Incremental (0eda56f1)
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