[sword-devel] curl library and download termination

[Eeli Kaikkonen]

Currently the install manager ftp transport curl implementation doesn't
correctly terminate the file transport. It blocks until the ongoing file
download has been completed. It's of course very bad for usability -
think of a module with large images and a slow connection. The whole
user interface could be stalled for minutes and the only way to cancel
downloading would be to terminate the whole program.

I think this could be fixed quite easily. Curl supports termination with
CURLOPT_PROGRESSFUNCTION which is set in sword curlftpt.cpp. The sword
implementation of this callbackfunction returns always 0, but if it
would return !=0, the curl download would terminate (within about 1
second). The sword class already has the term member data which is set
by the instmgr, but it's not used in the transport implementation.

curl_easy_perform returns an error code which is is set to a certain
value. I'm not sure if this should be taken into consideration. But
there shouldn't be anything else to change.

BibleTime has also a more serious issue with the curlftpt
implementation.  When I earlier looked into curl docs I thought that the
"easy" functions are not thread safe. However, it looks like I was
wrong, but sword implemenation calls the library in a way which is not
thread safe.

The curl doc about curl_easy_init:

"If you did not already call curl_global_init(3), curl_easy_init(3) does
it automatically. This may be lethal in multi-threaded cases, since
curl_global_init(3) is not thread-safe, and it may result in resource
problems because there is no corresponding cleanup.
You are strongly advised to not allow this automatic behaviour, by
calling curl_global_init(3) yourself properly."

It shouldn't be too hard to replace per-object easy_init with static
global_init.

This thread safety problem with the download termination problem have
caused crashes in BibleTime. I'm planning to implement our own transport
with the Qt library, but if these problems are fixed, we might go on
with the default sword implementation.

  Yours,
	Eeli Kaikkonen (Mr.), Oulu, Finland
	e-mail: eekaikko at mailx.studentx.oulux.fix (with no x)