Add a /version API endpoint

* Introduce the `VersionInfo` struct, build from the vergen environment
  variables
* Add the `version` handler to construct and return the version info
* Update the README

README.md

@@ -82,7 +82,7 @@ This also uses `Rocket.toml` from the current working directory as configuration
 You can alternatively pass a set of environment variables instead. See
 `docker-compose.yml` for a list.
 
-## API endpoint
+## Status API Endpoint
 
 The `/` API endpoint provides the current statistical data of your solar panels
 once it has successfully logged into the cloud service using your credentials.
@@ -92,9 +92,9 @@ There is no path and no query parameters, just:
 GET /
 ```
 
-### Response
+### Status API Response
 
-A response uses the JSON format and typically looks like this:
+The response uses the JSON format and typically looks like this:
 
 ```json
 {"current_w":23.0,"total_kwh":6159.0,"last_updated":1661194620}
@@ -104,7 +104,7 @@ This contains the current production power (`current_w`) in Watt,
 the total of produced energy since installation (`total_kwh`) in kilowatt-hour
 and the (UNIX) timestamp that indicates when the information was last updated.
 
-### Error response
+### (Status) API Error Response
 
 If the API endpoint is accessed before any statistical data has been retrieved,
 or if any other request than `GET /` is made, an error response is returned
@@ -114,6 +114,26 @@ that looks like this:
 {"error":"No status found (yet)"}
 ```
 
+## Version API Endpoint
+
+The `/version` endpoint provides information of the current version and build
+of the service.  This can be used to check if it needs to be updated.
+Again, there is no path and no query parameters, just:
+
+```http
+GET /version
+```
+
+### Version API Response
+
+The response uses the JSON format and typically looks like this:
+
+```json
+{"version":"0.2.1","timestamp":"2023-01-29T14:10:24.971748027Z","git_sha":"5cbc3a04","git_timestamp":"2023-01-16T20:18:20Z"}
+```
+
+(Build and git information may be out of date.)
+
 ## Integration in Home Assistant
 
 To integrate the Solar Grabber service in your [Home Assistant](https://www.home-assistant.io/)

src/lib.rs

@@ -79,6 +79,38 @@ async fn status() -> Result<Json<Status>, Json<Error>> {
         .ok_or_else(|| Json(Error::from("No status found (yet)")))
 }
 
+/// The version information as JSON response.
+#[derive(Debug, Serialize)]
+#[serde(crate = "rocket::serde")]
+struct VersionInfo {
+    /// The version of the build.
+    version: String,
+    /// The timestamp of the build.
+    timestamp: String,
+    /// The (most recent) git SHA used for the build.
+    git_sha: String,
+    /// The timestamp of the last git commit used for the build.
+    git_timestamp: String,
+}
+
+impl VersionInfo {
+    /// Retrieves the version information from the environment variables.
+    fn new() -> Self {
+        Self {
+            version: String::from(env!("VERGEN_BUILD_SEMVER")),
+            timestamp: String::from(env!("VERGEN_BUILD_TIMESTAMP")),
+            git_sha: String::from(&env!("VERGEN_GIT_SHA")[0..8]),
+            git_timestamp: String::from(env!("VERGEN_GIT_COMMIT_TIMESTAMP")),
+        }
+    }
+}
+
+/// Returns the version information.
+#[get("/version", format = "application/json")]
+async fn version() -> Result<Json<VersionInfo>, Json<Error>> {
+    Ok(Json(VersionInfo::new()))
+}
+
 /// Default catcher for any unsuppored request
 #[catch(default)]
 fn unsupported(status: rocket::http::Status, _request: &Request<'_>) -> Json<Error> {
@@ -92,7 +124,7 @@ fn unsupported(status: rocket::http::Status, _request: &Request<'_>) -> Json<Err
 /// Creates a Rocket and attaches the config parsing and update loop as fairings.
 pub fn setup() -> Rocket<Build> {
     rocket::build()
-        .mount("/", routes![status])
+        .mount("/", routes![status, version])
         .register("/", catchers![unsupported])
         .attach(AdHoc::config::<Config>())
         .attach(AdHoc::on_liftoff("Updater", |rocket| {