Refuse invalid settings at start, health check follows PORT (closes #31)

PORT (1 to 65535) and XDG_DATA_HOME (an absolute path) are now checked in
the config package, which is built before the database opens, so a bad
value stops the start with an error and a non-zero exit instead of leaving
the daemon running without HTTP or putting the database somewhere else.
entrypoint.sh refuses a MALLOC_ARENA_MAX that is not a positive whole
number, since glibc ignores a bad one silently. The HEALTHCHECK probes the
port PORT names. The README corrects the Linux state directory, lists
XDG_DATA_HOME, and adds a "Running under upaas" section.

Model: opus-5-5

README.md

@@ -166,14 +166,21 @@ Configuration is handled via environment variables and OS-specific paths:
 
 | Variable | Default | Description |
 |----------|----------|-------------|
-| `PORT` | `8080` | HTTP server port |
+| `PORT` | `8080` | HTTP server port, a whole number from 1 to 65535 |
 | `DEBUG` | (empty) | Set to `routewatch` for debug logging |
+| `XDG_DATA_HOME` | `/var/lib` (in the Docker image) | Base of the state directory; must be an absolute path |
 | `GOMEMLIMIT` | `1536MiB` (in the Docker image) | Go soft memory limit; see Memory |
-| `MALLOC_ARENA_MAX` | `2` (in the Docker image) | glibc malloc arena cap; see Memory |
+| `MALLOC_ARENA_MAX` | `2` (in the Docker image) | glibc malloc arena cap, a positive whole number; see Memory |
+
+A variable that is set to an invalid value stops the start with an error and a
+non-zero exit. An empty variable counts as unset.
 
 State directory (database location):
 - macOS: `~/Library/Application Support/routewatch/`
-- Linux: `/var/lib/routewatch/` or `~/.local/share/routewatch/`
+- Linux: `/var/lib/berlin.sneak.app.routewatch/` when running as root,
+  otherwise `$XDG_DATA_HOME/berlin.sneak.app.routewatch/` (with `XDG_DATA_HOME`
+  unset, `~/.local/share/berlin.sneak.app.routewatch/`). In the Docker image
+  this is `/var/lib/berlin.sneak.app.routewatch/`.
 
 ## Memory
 
@@ -220,6 +227,24 @@ What happens at each limit:
 With `DEBUG=routewatch` the daemon logs a `System stats` line every 60 seconds
 with the goroutine count and Go memory figures.
 
+## Running under upaas
+
+What the [upaas](https://git.eeqj.de/sneak/upaas) app needs:
+
+- Container port: `8080`.
+- Volume: one, at container path `/var/lib/berlin.sneak.app.routewatch`. upaas
+  does not create the host directory, so create it before the first deploy. The
+  entrypoint takes ownership of it, so a root-owned directory works.
+- Environment: nothing is required. Leave `XDG_DATA_HOME`, `GOMEMLIMIT` and
+  `MALLOC_ARENA_MAX` at the image's values. `DEBUG=routewatch` is optional and
+  adds the `System stats` memory line to the log.
+- Memory Limit: `5g`, the 5 GiB limit from Memory above. upaas sets no swap
+  limit, so on a host with swap Docker allows the same amount of swap again.
+- Health check: the image's `HEALTHCHECK` requests
+  `/.well-known/healthcheck.json` on the container port. upaas reads the
+  container's health 60 seconds after a deploy and fails the deploy unless it
+  is `healthy`.
+
 ## Development
 
 ```bash