I've figured this out, thanks to Robert Newson looking at a TCP dump Pieter van
der Eems sent him. It turns out to be an issue with CouchDB that I already knew
about but had forgotten would bite in this particular circumstance.
Specifically, CouchDB isn't associating the MIME bodies with the attachments
correctly; it gets them mixed up. As a result it gets confused about the
lengths and blows up.
The issue is with CouchDB's multipart support, specifically the way in which it
matches MIME bodies to attachment names. The IMHO correct way to do this would
be to look at the filename in the Content-Disposition header, and this is in
fact what TouchDB generates:
Content-Disposition: attachment; filename="20120808-092628.png"
But CouchDB ignores this header. Instead it assumes that the order in which the
MIME bodies appear matches the order in which the attachment objects appear in
the _attachments object.
The problem with this is that JSON objects (dictionaries) are _not_ ordered
collections. I know that Erlang's implementation of them (as linked lists of
key/value pairs) happens to be ordered, and I think some JavaScript
implementations have the side effect of preserving order; but in many languages
these are implemented as hash tables and genuinely unordered.
So when TouchDB serializes the NSDictionary object representing the
attachments, it has _no idea_ in what order the JSON encoder will write the
keys. This means it can't comply with CouchDB's ordering requirement because it
doesn't know what order in which to write out the attachments. I believe I am
going to have to work around this by using a custom JSON encoder that I can
make write out dictionary entries in a known (sorted?) order.
I've filed this as COUCHDB-1521. As I said, I can work around it, but I really
think this should be fixed as it's a hurdle for interoperability.
(Ironically I ran into the flip side of this issue last year and filed a bug on
it (COUCHDB-1368): when _receiving_ a multipart body from CouchDB, it's
difficult to match attachments with their MIME bodies because CouchDB doesn't
put any headers into the MIME bodies to indicate filenames; the only clue is
the ordering of the entries in the _attachments dictionary, and that ordering
is lost when Cocoa's JSON parser converts it into an NSDictionary object.)
—Jens
