Fix fd leaks in getContents error paths

[Copilot]

getContents opens storage.File handles via getSliceChunkFile but fails to close them on several error paths, leading to fd exhaustion under load.

• Close f when checkChunkSize fails instead of falling through with an open handle
• Close chunkFile when checkChunkSize fails before returning error
• Close chunkFile when getUpstreamReader fails before returning error

if f != nil {
    if err := checkChunkSize(c, f, idx); err == nil {
        return f, 1, nil
    }
    _ = f.Close() // was missing
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>getSliceChunkFile in function getContents maybe leak fds when return err</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes getSliceChunkFile in function getContents maybe leak fds when return err #44

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[Copilot]

Pull request overview

This PR addresses file-descriptor leaks in the caching middleware by ensuring storage.File handles opened by getSliceChunkFile are closed on additional error paths within getContents, preventing fd exhaustion under load.

Changes:

• Close the initially opened chunk file when checkChunkSize fails and the function continues.
• Close chunkFile before returning when checkChunkSize fails in the “anchor chunk” path.
• Close chunkFile before returning when getUpstreamReader returns an error.

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[Copilot]

checkChunkSize should be called with the chunk index that corresponds to the file being validated. Here chunkFile is opened for availableChunks[index], but idx (the requested starting chunk) is passed instead. This can incorrectly fail validation (notably when the anchor chunk is the last, smaller chunk) and trigger an unnecessary discard. Pass availableChunks[index] to checkChunkSize here.

[Copilot]

The error return from getSliceChunkFile is ignored here (chunkFile, _ := ...). getSliceChunkFile can return non-NotExist errors (including EMFILE via isTooManyFiles), and ignoring it will silently fall back to an upstream fill instead of failing fast. Handle and return the error (similar to the earlier f, err1 := ... branch).

Suggested change

	chunkFile, _ := getSliceChunkFile(c, availableChunks[index])
	chunkFile, err2 := getSliceChunkFile(c, availableChunks[index])
	if err2 != nil {
	return nil, 0, err2
	}