Split the server into three crates (#56)

This will make it easier to build variations on the server, or embed it
into larger projects.

server/src/api/mod.rs

@@ -0,0 +1,59 @@
+use actix_web::{error, http::StatusCode, web, HttpRequest, Result, Scope};
+use taskchampion_sync_server_core::{ClientId, ServerConfig, Storage};
+
+mod add_snapshot;
+mod add_version;
+mod get_child_version;
+mod get_snapshot;
+
+/// The content-type for history segments (opaque blobs of bytes)
+pub(crate) const HISTORY_SEGMENT_CONTENT_TYPE: &str =
+    "application/vnd.taskchampion.history-segment";
+
+/// The content-type for snapshots (opaque blobs of bytes)
+pub(crate) const SNAPSHOT_CONTENT_TYPE: &str = "application/vnd.taskchampion.snapshot";
+
+/// The header name for version ID
+pub(crate) const VERSION_ID_HEADER: &str = "X-Version-Id";
+
+/// The header name for client id
+pub(crate) const CLIENT_ID_HEADER: &str = "X-Client-Id";
+
+/// The header name for parent version ID
+pub(crate) const PARENT_VERSION_ID_HEADER: &str = "X-Parent-Version-Id";
+
+/// The header name for parent version ID
+pub(crate) const SNAPSHOT_REQUEST_HEADER: &str = "X-Snapshot-Request";
+
+/// The type containing a reference to the persistent state for the server
+pub(crate) struct ServerState {
+    pub(crate) storage: Box<dyn Storage>,
+    pub(crate) config: ServerConfig,
+}
+
+pub(crate) fn api_scope() -> Scope {
+    web::scope("")
+        .service(get_child_version::service)
+        .service(add_version::service)
+        .service(get_snapshot::service)
+        .service(add_snapshot::service)
+}
+
+/// Convert a failure::Error to an Actix ISE
+fn failure_to_ise(err: anyhow::Error) -> impl actix_web::ResponseError {
+    error::InternalError::new(err, StatusCode::INTERNAL_SERVER_ERROR)
+}
+
+/// Get the client id
+fn client_id_header(req: &HttpRequest) -> Result<ClientId> {
+    fn badrequest() -> error::Error {
+        error::ErrorBadRequest("bad x-client-id")
+    }
+    if let Some(client_id_hdr) = req.headers().get(CLIENT_ID_HEADER) {
+        let client_id = client_id_hdr.to_str().map_err(|_| badrequest())?;
+        let client_id = ClientId::parse_str(client_id).map_err(|_| badrequest())?;
+        Ok(client_id)
+    } else {
+        Err(badrequest())
+    }
+}