remove bare asserts for segment out-of-range errors in mutable retrieve

[daira]

Traceback (most recent call last):
  File "/home/davidsarah/tahoe/git/trunk/src/allmydata/mutable/filenode.py", line 990, in <lambda>
    self._serializer.addCallback(lambda ignore: cb(*args, **kwargs))
  File "/home/davidsarah/tahoe/git/trunk/src/allmydata/mutable/filenode.py", line 980, in _read
    d = r.download(consumer, offset, size)
  File "/home/davidsarah/tahoe/git/trunk/src/allmydata/mutable/retrieve.py", line 244, in download
    self._setup_encoding_parameters()
  File "/home/davidsarah/tahoe/git/trunk/src/allmydata/mutable/retrieve.py", line 411, in _setup_encoding_parameters
    assert start < self._num_segments
exceptions.AssertionError: 

There are at least two bare asserts, one for the start segment and one for the end segment.

[daira]

The exception raised should be as documented in interfaces.py, per #2461.

[daira]

The current error behaviour is confusing; it appears that you can read one byte past the end of the file, only if the file is not a multiple of k bytes (i.e. you can read the dummy bytes that are added to make the size of the input to encoding a multiple of k bytes). This should probably be made stricter.

[daira]

Replying to daira:

The current error behaviour is confusing; it appears that you can read one byte past the end of the file, only if the file is not a multiple of k bytes (i.e. at this API layer, you can read the dummy bytes that are added to make the size of the input to encoding a multiple of k bytes).

See ​https://github.com/tahoe-lafs/tahoe-lafs/commit/e437bfd64bf4fb6838036fde9aa05f987f911320#diff-8d5163da85905f8ddd5ce6a535206479R3403, which reads one byte past the end of the file, and passes. (The same thing happens when reading 101 bytes of a 100-byte SDMF file.) The explanation above may not be correct, though; it's just a guess.

[Brian Warner <warner@…>]

In 9076db137ac5fa5c1efd63b26970803cbc59a654/trunk:

Remove some bare asserts in retrieve.py (there are still quite a few left). refs #2462

Signed-off-by: Daira Hopwood <daira@…>

[Brian Warner <warner@…>]

In 29ab496bd75c8f9c1891f7a97ff9e682c79277e4/trunk:

mutable/retrieve.py: remove all bare assert()s

Replace them with _assert() or precondition(). Closes ticket:2462.

[daira]

Reviewed, +1.