trying to upload a delta if delta service is disabled fails

[Daniel Manrique]

If upload deltas are disabled on the snap store servers, snapcraft fails like this when trying to upload a delta (easy way to ensure it's a delta upload: push two revisions of the same snap, one after the other).

After pushing, an attempt will be made to release to 'edge'
Preparing to push '/home/ubuntu/minisnap/hello-roadmr-1_2018-07-09-02_amd64.snap' to the store.
Found cached source snap /home/ubuntu/.cache/snapcraft/projects/hello-roadmr-1/snap_hashes/amd64/81d67619708155032388f69f382e0975d7c9dbdce6ec25590c1e6faf844b155ca307966c9c1af8e6b29ebfe44dcc0c68.
Generating xdelta3 delta for hello-roadmr-1_2018-07-09-02_amd64.snap.
Pushing delta /home/ubuntu/minisnap/hello-roadmr-1_2018-07-09-02_amd64.snap.xdelta3.
Pushing hello-roadmr-1_2018-07-09-02_amd64.snap.xdelta3 [==========================================] 100%
The Snap Store encountered an error while processing your request: not implemented (code 501).
The operational status of the Snap Store can be checked at https://status.snapcraft.io/

> snapcraft --version
snapcraft, version 2.42.1+18.4
> lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04 LTS
Release: 18.04
Codename: bionic

this regressed in 2.42.1, because with 2.42 things work well, snapcraft gets the same 501 code from the store (which means deltas are not available/not implemented) and falls back to a full-snap upload (with a nice error saying delta support is disabled):

Pushing hello-roadmr-1_2018-07-09-02_amd64.snap
After pushing, an attempt will be made to release to 'edge'
Preparing to push '/home/ubuntu/minisnap/hello-roadmr-1_2018-07-09-02_amd64.snap' to the store.
Found cached source snap /home/ubuntu/.cache/snapcraft/projects/hello-roadmr-1/snap_hashes/amd64/81d67619708155032388f69f382e0975d7c9dbdce6ec25590c1e6faf844b155ca307966c9c1af8e6b29ebfe44dcc0c68.
Generating xdelta3 delta for hello-roadmr-1_2018-07-09-02_amd64.snap.
Pushing delta /home/ubuntu/minisnap/hello-roadmr-1_2018-07-09-02_amd64.snap.xdelta3.
Pushing hello-roadmr-1_2018-07-09-02_amd64.snap.xdelta3 [==========================================] 100%
Unable to push delta to store: The delta upload support is currently disabled.
Falling back to pushing full snap...
Pushing hello-roadmr-1_2018-07-09-02_amd64.snap [==================================================] 100%
Processing...|
Ready to release!
Revision 51 of 'hello-roadmr-1' created.
Track Arch Channel Version Revision Expires at
latest amd64 stable 2018-06-08-01 47
                 candidate ^ ^
                 beta ^ ^
                 edge 2018-07-09-02 51
                 stable/mybranch 2018-06-08-01 47 2018-07-15T21:44:58.667374

We think the commit that regressed here was:

https://github.com/snapcore/snapcraft/commit/c6500ca0ed3cf4fe564a15adfb0a46ff0a3a787f

storeapi: handle 5xx error codes for all store endpoints (#2116)
Currently, the snapcraft CLI handles a 500 response when releasing a
snap. Make this more robust, and nicely handle 5xx codes for all
endpoints.

As reference, the response when deltas are disabled is that 501 HTTP status code with this payload:
{"error_list": [{"message": "The delta upload support is currently disabled.", "code": "feature-disabled"}]}

[Kyle Fazzari (kyrofa)]

Thanks Daniel, looks like we're missing a test case for that scenario.

[Kyle Fazzari (kyrofa)]

Just learned at Montreal that this is an issue making store development quite difficult, since in development deltas are disabled. Upping priority as a result. Proposed fix is here: https://github.com/snapcore/snapcraft/pull/2181 .

[Maximiliano Bertacchini (maxiberta)]

On the Snap Store side, if upload deltas are disabled it should not return a 5xx status.

[Maximiliano Bertacchini (maxiberta)]

The Snap Store API currently returns 501 if snap upload support is disabled, and snapcraft correctly falls back to pushing the full snap. Being a server-side condition the 501 status makes sense, in contrast to a 4xx status that would confusingly indicate a client side error.

$ snapcraft push htop-snap_2.2.0-18-gb7b4200_amd64.snap
...
Unable to push delta to store: The delta upload support is currently disabled.
Falling back to pushing full snap...
...

$ snapcraft version
snapcraft, version 3.0.1