net/http: FileServer: Content-Length is not set when Content-Encoding was set about go

[seankhliao]

I believe the comment preceding the line sufficiently explains why it isn't set.

[paralin]

@seankhliao It does not sufficiently cover it. The comment even says within that this is a hacky fix.

I am raising a situation where this needs to be set and there should be some kind of way to force that code branch to not be taken.

[paralin]

CC @neild who introduced this check & the comment: fdc21f3

[Jorropo]

I don't find the comment above this if to be clear.

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@paralin your code is incorrect, Range gives offsets in the plaintext, not compressed archive, however because you give the already compressed archive to net/http's ranging code, it will interpret them as offsets into the archive and answer with invalid truncated brotli streams no valid client would know to do anything with.

Your best bet is to not use http.FS and use a custom handler instead,
short as an if check on Accept-Encoding, then grab the compressed archive, set Content-Length to the compressed size, write status and finally io.Copy.
If you also want to support Range, then you can use the same thing as above, but also send Accept-Ranges: bytes and then add one more if which instead call http.ServeFile with the plaintext if Range header is set.