Range support

[Federico Schwindt]

Hi,

phk asked me to summarise what's wrong with our range support.

- We don't support multiple ranges (e.g. Range: bytes=0-500,5000-).
This is regardless of streaming.

- If streaming is enabled, the object is not in the cache, we have a range
request and it doesn't specify the last-byte-pos (e.g. bytes=F-), it is a
suffix range (e.g. bytes=-L) or a full range (e.g. bytes=F-L) we might
return the wrong range based on the so-far fetched object.

Using #1643 as base for this example, the following cases will be wrong:

bytes=0- will return Content-Range: 0-65535/65536
bytes=65536- will return 416
bytes=32768-131072 will return Content-Range: 32768-65535/65536
bytes=131072-262144 will return 416
bytes=-131072 will return Content-Range: 0-65535/65536

We can send * if we don't know the Content-Length but we need to ensure
that we have the minimum required to fulfill the range request.

As it stands today, for ranges where first-byte-pos is present but not
last-byte-pos (first 2 examples above), turning streaming off would be
equivalent as we don't support range requests to the backend.

For the remaining 3 examples we need to get up to last-byte-pos in case 3
and 4 and use * if C-L is unknown. For the last case we'll need to fetch
the whole object.

Unless I'm missing something that's all.

Salute.
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