feat: read proxy origin from QubesDB when available; otherwise environment

[cfm]

Status

Work in progress

Description

On top of #1718, adds the qubesdb feature to the v2 proxy to read the QubesDB key /vm-config/SD_PROXY_ORIGIN.

Test Plan

TK, but roughly:

# By environment:
dev$ export SD_PROXY_ORIGIN=foobar
dev$ cargo run

# By QubesDB:
dom0$ qubesdb-write -d dev -c write /vm-config/SD_PROXY_ORIGIN
dev$ unset SD_PROXY_ORIGIN
dev$ cargo run --features qubesdb

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, the AppArmor profile may need to be updated. Please check as applicable:

• I have updated the AppArmor profile
• No update to the AppArmor profile is required for these changes
• I don't know and would appreciate guidance

If these changes modify the database schema, you should include a database migration. Please check as applicable:

• I have written a migration and upgraded a test database based on main and confirmed that the migration is self-contained and applies cleanly
• I have written a migration but have not upgraded a test database based on main and would like the reviewer to do so
• I need help writing a database migration
• No database schema changes are needed

[legoktm]

I like where this is going! It worked correctly when I set the qubesdb value, but when I first tried to run it with nothing set, it SIGSEV'd (instead of gracefully erroring):

$ cargo run --features qubesdb
    Finished dev [unoptimized + debuginfo] target(s) in 0.05s
     Running `/home/user/.cargo/target/debug/securedrop-proxy`
fish: Job 1, 'cargo run --features qubesdb' terminated by signal SIGSEGV (Address boundary error)

We may also want to move all the config-related code into a separate file to keep the qubesdb/env/etc. related code separate from the HTTP-over-JSON stuff.

[cfm]

Thanks for your feedback, @legoktm! I think I've addressed all of it, including factoring out the config module, as of a8fb9cf. Let me know whether you'd like to review again here or have me append to proxy-rusting.

[legoktm]

I realized that CI is failing because the build.rs script/bindgen is unconditionally trying to link against qubesdb, regardless of whether we enabled the feature. And then I pointed out two other things inline.

Overall I think this is ready to rebase into the main proxy-rusting branch. We do need to figure out a solution for integration testing though, which I'm guessing requires integrating into the new Qubes CI stuff...

[cfm]

Okay, I think we've got it! Thanks for iterating on this with me, @legoktm. Pushing to proxy-rusting and closing here.