chore(prod): region-layers mount reuses ${DATA_DIR}, so the region defs page serves the data dir #204

Reported by ITblacksheep in Discord: the region definitions page is not working on prod. He suspected a missing /app prefix in the compose mount. It is not that — the container-side path is correct.

Root cause

/opt/corescope/docker-compose.yml line 17 reuses ${DATA_DIR} instead of its own variable:

- ${DATA_DIR:-./data}:/app/data
- ${DATA_DIR:-./frontend/region-layers}:/app/public/region-layers:ro

:-default only applies when the variable is unset. /opt/corescope/.env contains exactly DATA_DIR=./data, so the ./frontend/region-layers default never fires and both lines resolve to the same host directory.

Verified on the live container, not inferred

[box: deadbeef, 2026-10-16]

docker inspect corescope --format '{{range .Mounts}}...'
  /opt/corescope/data -> /app/data                    (rw)
  /opt/corescope/data -> /app/public/region-layers    (ro)

docker exec corescope ls /app/public/region-layers
  caddy  config.json  config.json.bak-20260727-031511  ...

The container is serving the data directory at that path.

The precise user-visible failure

[repo: public/region-def.js:4] the page fetches region-layers/layers.json.

So the fetch succeeds and returns the SPA's index.html. The page then fails parsing JSON. That is why it breaks rather than erroring cleanly.

The data is fine

/opt/corescope/frontend/region-layers/ on the host holds layers.json plus aoh, chm, cvg, day, fwa, oki, states, us-midwest, us-southeast geojson. Nothing is missing or misplaced. Only the mount source is wrong.

Ruled out

• Stale mount from an older compose. Compose last modified 2026-10-16 17:08:14 UTC; container created 2026-10-16 20:37:30 UTC. The container is 3.5h newer, so the live mounts reflect the current file.
• An override file. No docker-compose.override.yml. Only .bak copies, which compose does not read.
• Wrong container-side path. The server runs -public /app/public, so /app/public/region-layers is correct.

The fix

Give line 17 its own source. Hardcoding rather than adding a second variable:

- ./frontend/region-layers:/app/public/region-layers:ro

DATA_DIR is a variable because the data location is genuinely environment-specific. The region layers are repo-relative content that ships with the app, so a variable adds indirection with no benefit — and indirection is what caused this. A hardcoded relative path resolves against the compose file's directory and has no failure mode.

⚠ The larger finding: this file is not under version control

[verified: 2026-10-16]

• /opt/corescope is not a git clone
• no repo compose file mentions region-layers — grep -l region-layers docker-compose*.yml returns nothing across all five
• deploy-prod.yml never deploys a compose file; it references /opt/corescope/docker-compose.yml as a one-time prereq and then runs docker compose up -d against whatever is already on the box
• the only history is six .bak files with epoch-stamped names

So prod's compose is load-bearing configuration with no version control, no review, and no CI that could catch a defect in it. This bug shipped and nothing could have stopped it. This is the same shape as the config-contract hazard in docs/UPSTREAM.md checklist item 1: the only artifact that disagrees with the code is the config on the box, and CI has never seen it.

Filed separately as a gap; this issue is the fix only. ITblacksheep's own Discord remark — "I need to switch our docker compose to ansible" — is the same conclusion from the other direction.

Cost of applying

A bind-mount change requires recreating the container. Prod's cold start measured ~260s to /api/healthz ready, so roughly four minutes unavailable. A manual docker compose up -d has no auto-rollback — that logic lives only in deploy-prod.yml. Backup taken before the edit.

Verification

• docker compose config parses before applying
• docker inspect shows /opt/corescope/frontend/region-layers -> /app/public/region-layers
• docker exec corescope ls /app/public/region-layers lists the geojson and layers.json
• GET /region-layers/layers.json returns JSON, not HTML
• container healthy, no exited: corescope- in logs, counts still climbing

Done when

The region definitions page loads on prod and /region-layers/layers.json serves the real manifest.

Refs #187