Merged diffs

[varmar05]

Introduce merged diffs (aka. checkpoints) in various version ranges based on log 4 caching strategy.

PR contains:

• utility functions to calculate caching levels
• updated logic for gpkg restore function using checkpoints (backward restore procedure was dropped)
• new /v2/projects/.../raw endpoint for downloading diff files - diff is constructed on-the-fly if not found - anytime some lower order checkpoint is not present we use individual diffs as a fallback strategy

Deployment note: not a greenlet-safe endpoint as it might call geodiff library.
Clients note: in case of 422/502 from /raw/diff endpoint it does not make sense to continue as requested diff can not be created on server - instead client can try to use smaller project delta

[MarcelGeo]

Looks good.

Do we have any mechanism for maximum rank which will be created? To prevent huge diff files or applying such files.

[varmar05]

Do we have any mechanism for maximum rank which will be created? To prevent huge diff files or applying such files.

We can introduce something - it would be good to know our current limit though.
Also we only create checkpoints if they are requested from project delta endpoint - and this should ideally not span over 1000-10 000 versions. Maybe it would be better if clients had some upper limit and did multiple via smaller ranges instead

[coveralls]

Pull Request Test Coverage Report for Build 18220522665

Details

• 282 of 302 (93.38%) changed or added relevant lines in 14 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.002%) to 94.267%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server/mergin/sync/private_api_controller.py	5	6	83.33%
server/mergin/sync/utils.py	48	56	85.71%
server/mergin/sync/models.py	69	80	86.25%

Files with Coverage Reduction	New Missed Lines	%
server/mergin/sync/models.py	1	95.5%

Totals
Change from base Build 18186020786:	-0.002%
Covered Lines:	8106
Relevant Lines:	8599

---

💛 - Coveralls