bug#22333: 24.5; EWW downloads invalid compressed tar-files

["Iñigo Serna" <inigoserna@gmail.com>]

Hi and thanks for your time,

Andreas Schwab writes:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>> Could it be that the saved tar file is already uncompressed?
>
> I'd rather guess it's compressed twice.

I think I've discovered the problem with the downloaded files:

0. As downloaded (remember it's saved from an emacs buffer) is not a
valid .tar.gz file
1. but it is a valid gzipped file
2. the uncompressed file contains some HTTP headers at
the start of the file.
3. After manually removing these lines and changing extension (to add
the '.gz' again), file is correctly recognized as a gzipped tar-file.

[0] /home/inigo/Downloads ⚡ tar xvfz filename.tar.gz 
    tar: This does not look like a tar archive
    tar: Skipping to next header
    tar: Exiting with failure status due to previous errors

[1] /home/inigo/Downloads ⚡ gzip -d filename.tar.gz 
    /home/inigo/Downloads ⚡ file filename.tar 
    filename.tar: data

[2] /home/inigo/Downloads ⚡ head -21 filename.tar 
    HTTP/1.1 200 OK
    x-amz-replication-status: COMPLETED
    Last-Modified: Sun, 08 Nov 2015 13:11:05 GMT
    ETag: "9c13c5fafcb1aecd43f51fa9b0278000"
    Content-Type: application/octet-stream
    Server: AmazonS3
    Via: 1.1 varnish
    Fastly-Debug-Digest: 37a3779f444d206796099a174bb873c853bb8f1b9a13cf06f29108f907a9d50b
    Cache-Control: max-age=31557600, public
    Content-Length: 87341
    Accept-Ranges: bytes
    Date: Sat, 09 Jan 2016 10:16:29 GMT
    Via: 1.1 varnish
    Age: 135058
    Connection: keep-alive
    X-Served-By: cache-sea1920-SEA, cache-fra1247-FRA
    X-Cache: HIT, HIT
    X-Cache-Hits: 2, 1
    X-Timer: S1452334589.874408,VS0,VE1
    
    *I?V.�dist/lfm-3.0.tar��[w۸�0���.�d���H�$;�>:����N�+��c;}9n/

[3] [...remove HTTP headers from first lines of file...]
    /home/inigo/Downloads ⚡ file filename.tar 
    filename.tar: gzip compressed data, was "dist/lfm-3.0.tar", last modified: Sun Nov  8 14:07:54 2015, max compression
    /home/inigo/Downloads ⚡ mv filename.tar filename.tar.gz
    /home/inigo/Downloads ⚡ file filename.tar.gz 
    filename.tar.gz: gzip compressed data, was "dist/lfm-3.0.tar", last modified: Sun Nov  8 14:07:54 2015, max compression
    /home/inigo/Downloads ⚡ tar xvfz filename.tar.gz 
    lfm-3.0/
    [...]
    

So I think the error comes with those extra spurious headers added to
the start of the buffer when downloading the file, which mess buffer
saving operation.

The bug can be easily reproduced with code like:

    (let ((url "https://pypi.python.org/packages/source/l/lfm/lfm-3.0.1.tar.gz"))
      (url-retrieve url 'test-cb (list url)))
    
    (defun test-cb (status url)
      (message "Downloaded: %s" url)
      ;; (sleep-for 60)
      (write-file "~/a.tgz")
      (message "Saved: %s" url))
    

`eww-download-callback` (called by `url-retrieve`) should remove those headers 
after download is finished and before buffer is saved to disk.

A simple fix could be to add (excuse my poor elisp skills):

    (goto-char (point-min))
    (search-forward-regexp "^$")
    (forward-line)
    (delete-region (point-min) (point))

before '(write-file file)' in `eww-download-callback`.

... and this almost works, but the downloaded file is compressed twice
now as Andreas stated (ie, tar.gz is gzipped again at file saving).

Thus it looks there is an additional issue when saving gzipped tar-files
from a buffer.

Any idea on how to solve this?


Thanks,
Iñigo Serna