When deleting an item on phone and locally, the next sync fails with
ERROR messages about "object not found". This has several reasons:
- libsynthesis super data store attempts to read items
which may or may not exist (triggers ERROR message)
- it checks for 404 but Evolution backends only return a generic
database error (causes sync to fail)
It turned out that ReadItem and DeleteItem are expected to return a
404 status when the requested item does not exist. This patch documents
that (only in the TrackingSyncSource, though), adds tests and fixes
EDS, WebDAV, file and sqlite backends accordingly.
This patch also suppresses the 404 error logging inside DeleteItem(),
while still returning that error code to the Synthesis engine. Not
logging that particular situation is consistent with the previous
SyncEvolution behavior of silently returning successfully when there
wasn't anything to delete.
In addition, more recent libsynthesis versions also no longer do
a ReadItem() call to test for existence. That would still trigger
a spurious (albeit now harmless) ERROR message.
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