Document every route, encrypted URLs and the config file search (closes #75)

README.md "Routes" lists every route pixa registers with its method,
purpose, what it needs and the status codes it answers with, read from
the handlers, and says q and fit are part of what is cached. A new
"Encrypted URLs" section covers logging in with the signing key, making
a URL on the generator page, how long it lasts, and the 410 once it has
expired. "Configuration" gives the order in which pixa looks for its
config file. config.example.yml now lists db_url and env and gives every
key's default. scripts/manual-test.sh is left to #97.

Model: opus-5-5

README.md

@@ -92,8 +92,61 @@ the metadata file stored beside it.
 
 ### Routes
 
+pixa answers these routes; any other path answers 404. A path in this list asked
+with a method the list does not give answers 405, except `/static/<file>`, which
+answers any method as it answers `GET`. A browser's CORS preflight request
+(`OPTIONS` with `Origin` and `Access-Control-Request-Method` headers) to any
+path under `/v1/` answers 200, in maintenance mode too.
+
+- `GET /` — the login page, or the URL generator page with a login session
+  (see Encrypted URLs). Needs: nothing. Answers: 200.
+- `POST /` — log in with the signing key typed into the login page. Needs: the
+  login page's form (below). Answers: 303 to `/` with a login session cookie
+  that lasts 30 days for the right key; 200 with the login page and an error for
+  a wrong key; 429 over the login limit (below).
+- `POST /generate` — make an encrypted URL from the generator page's form.
+  Needs: a login session and the generator page's form (below); without a login
+  session it answers 303 to `/`. Answers: 200 with the page showing the URL; 400
+  with the page naming a field that is not valid; 500 when the URL cannot be
+  made.
+- `GET /logout` — end the login session. Needs: nothing. Answers: 303 to `/`.
+- `GET` or `HEAD` `/v1/image/<host>/<path>/<size>.<format>` — an image, fetched,
+  resized and converted (below). Needs: a signature, unless the host is
+  allowlisted (see Source Hosts). Answers: 200; 304 when `If-None-Match` matches
+  the image's `ETag`; 400 for a URL or parameter that is not valid; 401 for a
+  missing or wrong signature, a missing `exp` or an `exp` in the past; 403 when
+  the upstream host, or a host it redirects to, is `localhost`, ends in
+  `.localhost` or `.local`, or has an address in a blocked network (see
+  `blocked_networks`); 502 when the upstream answered with an error status, and
+  for 5 minutes after that for the same source URL; 503 when pixa is busy or in
+  maintenance mode; 500 for any other failure.
+- `GET /v1/e/<token>/<name>` — an image through an encrypted URL (see Encrypted
+  URLs). Needs: nothing but the URL. Answers: 200; 400 for a token that does not
+  decrypt, or that asks for a size or fit that is not valid; 410 once it has
+  expired; 504 when the upstream has not sent its response headers within
+  `upstream_fetch_timeout`, but 500 when that time runs out while the image
+  itself is still arriving; 403, 502, 503 and 500 as for `/v1/image/`.
+- `GET /robots.txt` — asks every crawler to stay away (`Disallow: /`). Needs:
+  nothing. Answers: 200.
+- `GET /.well-known/healthcheck.json` — JSON with `status` (`ok`), `now`,
+  `uptime_seconds`, `uptime_human`, `version`, `appname` and
+  `maintenance_mode`. Needs: nothing. Answers: 200, always.
+- `GET /static/<file>` — the script the login and generator pages load. Needs:
+  nothing. Answers: 200, or 404 for a file that does not exist.
+- `GET /metrics` — Prometheus metrics (see Architecture). Needs: HTTP basic
+  authentication with `metrics.username` and `metrics.password`. Answers: 200;
+  401 without them; 404 when they are not set, as the route then does not exist.
+
+Both `POST` routes accept only a form that pixa's own page served: the page puts
+a token in the form and sets a cookie to match, and a request without both is
+refused with 403, so another site cannot submit the form from a visitor's
+browser. A form body over 1 MiB is refused with 413. The image routes answer the
+errors listed for them with JSON holding `error`, `status` and `timestamp`.
+
+An image URL has this form:
+
 ```
-/v1/image/<host>/<path>/<size>.<format>?sig=<signature>&exp=<expiration>
+/v1/image/<host>/<path>/<size>.<format>?sig=<signature>&exp=<expiration>&q=<quality>&fit=<fit>
 ```
 
 Images are only fetched from origins using TLS with valid certificates, unless
@@ -106,6 +159,11 @@ than once, is refused with 400.
 - `<format>`: one of `orig` (or `original`), `jpeg` (or `jpg`), `png`, `webp`,
   `avif`, `gif`
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)
+- `sig` and `exp`: the signature and its expiry, needed unless the host is
+  allowlisted (see Signature Specification)
+- `q` and `fit`: the output quality and how the image is fitted to `<size>`,
+  both optional (values under Signature Specification). Both are part of what
+  is cached, so each value of either is a separate cached image.
 
 An image is served with `Cache-Control: public, max-age=<seconds>, immutable`.
 When the URL has an expiry (an `exp`, or the TTL of an encrypted URL),
@@ -139,6 +197,38 @@ its own `X-Forwarded-For`, whether it connects directly or through the proxy,
 because its own address is trusted too. Setting `trusted_proxies` to only the
 address pixa sees for requests that come through the proxy closes this.
 
+### Encrypted URLs
+
+An encrypted URL is an image URL made on pixa's own web page by someone who
+knows the signing key. It works for any upstream host, allowlisted or not,
+without a signature, and whoever gets it can neither read the source URL from it
+nor change what it asks for.
+
+1. Open `/` in a browser and log in with the signing key (`signing_key`). The
+   login session lasts 30 days, or until `/logout`.
+2. On the generator page, give the source image's URL, the width and height, the
+   format, quality and fit, and how long the URL lasts, then submit the form
+   (`POST /generate`). Width and height both empty or `0` keep the original
+   size; if only one of them is empty or `0`, that side is scaled to keep the
+   image's proportions.
+3. The page shows the URL, `https://<host>/v1/e/<token>/img.<format>`, and when
+   it expires. `<host>` is the host the page was opened on, and the URL starts
+   with `http` instead while `debug` is on. The name after the token is ignored
+   and only gives the URL a file extension, `jpg` for `orig`.
+
+The token holds the source's host, path and query and the size, format,
+quality, fit and expiry, encrypted with a key derived from `signing_key`. The
+source URL's scheme is not kept: the image is fetched like any other (see
+Routes), and the blocked networks still apply.
+
+How long the URL lasts is chosen on the page, from 1 minute to 1 year, or
+never. The expiry is fixed in the token when the URL is made and cannot be
+changed or revoked afterwards. Until then the image is served with a `max-age`
+that ends at the expiry (see Routes); after it the URL answers 410
+`URL has expired`. A URL made to last forever stops working only when
+`signing_key` changes: changing it makes every encrypted URL already handed out
+answer 400, and ends every login session.
+
 ### Image Metadata
 
 pixa decodes and re-encodes every image it serves, and removes all metadata from
@@ -237,6 +327,14 @@ startup naming it, as an unknown config key does. The one other accepted
 name is `PIXA_CONFIG_PATH`, the config file's path (like `--config`). The
 variables set by the file's `env:` section are checked the same way.
 
+pixa reads at most one config file: the one given with `--config` (or `-c`),
+otherwise the one `PIXA_CONFIG_PATH` names, otherwise the first of these that
+exists: `/etc/pixa/config.yml`, `/etc/pixa/config.yaml`,
+`~/.config/pixa/config.yml`, `~/.config/pixa/config.yaml`, then `config.yml`
+and `config.yaml` in the working directory. A file that cannot be read or does
+not parse aborts startup, and so does a named file that does not exist; with no
+file, pixa uses the environment and the defaults.
+
 | Variable                             | Config key                      | Meaning                                                                      |
 | ------------------------------------ | ------------------------------- | ---------------------------------------------------------------------------- |
 | `PIXA_SIGNING_KEY`                   | `signing_key`                   | Required: secret for signed and encrypted URLs and login, 32+ characters     |

TODO.md

@@ -29,6 +29,14 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 routes, encrypted URLs and config file documented (closes #75):
+  "Routes" in `README.md` lists every route with its method, purpose, what it
+  needs and the status codes it answers with, and says `q` and `fit` are part
+  of what is cached; "Encrypted URLs" covers logging in, making one on the
+  generator page, how long it lasts and the 410 once it has expired;
+  "Configuration" gives the order in which pixa looks for its config file;
+  `config.example.yml` lists `db_url` and `env` and gives every key's default;
+  `scripts/manual-test.sh` is left to #97.
 - 2026-10-04 upstream host semaphores and variant `.meta` files no longer
   outlive their use (closes #87): the fetcher counts the fetches holding or
   waiting for a slot of each upstream host's semaphore and removes the host's
@@ -427,7 +435,5 @@ P2: security: referer blacklist
     - integration tests for the image proxy flow
     - load tests to verify the 1k to 5k req/s target
 - P2: documentation
-    - configuration options
-    - API endpoints
     - deployment guide
     - example nginx or caddy reverse proxy config

config.example.yml

@@ -12,9 +12,13 @@
 # Durations are Go duration strings such as 30s or 2m and must be
 # positive; a bare number has no unit and aborts startup. Sizes are a
 # whole number of bytes.
+#
+# A key left out takes the default its comment gives.
 
-# Server settings
+# Port to listen on (default: 8080)
 port: 8080
+
+# Debug logging and plain-HTTP local development (default: false)
 debug: false
 
 # While true, the image routes (/v1/image/ and /v1/e/) answer every request
@@ -22,17 +26,24 @@ debug: false
 # reports maintenance_mode as true. It stays 200 because the image's Docker
 # HEALTHCHECK requests it: a 503 there would make the container unhealthy, and
 # upaas marks a deploy failed when its container is unhealthy.
+# (default: false)
 maintenance_mode: false
 
 # Data directory for SQLite database and cache files
+# (default: /var/lib/pixa)
 state_dir: ./data
 
+# SQLite database URL (default:
+# file:<state_dir>/state.sqlite3?_journal_mode=WAL). An empty value aborts
+# startup; leave the key out to use the default.
+# db_url: "file:./data/state.sqlite3?_journal_mode=WAL"
+
 # Image proxy settings
 # HMAC signing key for URL signatures (required, at least 32 characters)
 # Generate with: openssl rand -base64 32
 signing_key: "CHANGE_ME_generate_with_openssl_rand_base64_32"
 
-# Hosts that don't require signatures
+# Hosts that don't require signatures (default: none)
 # Use "." prefix for wildcard subdomain matching (e.g., ".example.com" matches "cdn.example.com")
 allowlist_hosts:
   - s3.sneak.cloud
@@ -45,7 +56,7 @@ allowlist_hosts:
 # SSRF protection. These are added to the always-enforced built-in ranges
 # (loopback, RFC 1918 private, link-local, CGNAT, benchmark, NAT64, and
 # similar), never replacing them. Each entry must be a valid CIDR in IPv4
-# or IPv6 form; an invalid entry aborts startup.
+# or IPv6 form; an invalid entry aborts startup. (default: none)
 # blocked_networks:
 #   - 100.64.0.0/10
 #   - 2001:db8::/32
@@ -72,6 +83,7 @@ allowlist_hosts:
 #   - 2001:db8::/32
 
 # Allow HTTP upstream (only for testing, always use HTTPS in production)
+# (default: false)
 allow_http: false
 
 # Maximum concurrent connections per upstream host (default: 20)
@@ -121,10 +133,16 @@ access_control_allow_origin: "*"
 # with a minimum of 500 MiB.
 # cache_max_bytes: 10737418240
 
-# Sentry error reporting (optional)
+# Sentry DSN for error reporting (default: empty, which turns it off)
 sentry_dsn: ""
 
-# Metrics endpoint authentication (optional)
+# Username and password for /metrics, set together (default: unset). Metrics
+# are measured and /metrics is served only when both are set.
 # metrics:
 #   username: "admin"
 #   password: "secret"
+
+# Environment variables set while this file loads, as described at the top
+# (default: none)
+# env:
+#   PIXA_DEBUG: "true"