Make `Boot` a freestanding app type, rather than a view of `crate::app::App`'s internals.

In the course of working on web push, I determined that we probably need to make `App` generic over the web push client we're using, so that tests can use a dummy client while the real app uses a client created at startup and maintained over the life of the program's execution. The most direct implementation of that is to render App as `App<P>`, where the parameter is occupied by the specific web push client type in use. However, doing this requires refactoring at _every_ site that mentions `App`, including every handler, even though the vast majority of those sites will not be concerned with web push.

I reviewed a few options with @wlonk:

* Accept the type parameter and apply it everywhere, as the cost of supporting web push.
* Hard-code the use of a specific web push client.
* Insulate handlers &c from `App` via provider traits, mimicing what we do for repository provider traits today.
* Treat each app type as a freestanding state in its own right, so that only push-related components need to consider push clients (as far as is feasible).

This is a prototype towards that last point, using a simple app component (boot) as a testbed. `FromRef` allows handlers that take a `Boot` to be used in routes that provide an `App`, so this is a contained change. However, the structure of `FromRef` prevents `Boot` from carrying any lifetime narrower than `'static`, so it now holds clones of the state fields it acquires from App, instead of references. This is fine - that's just a database pool, and sqlx's pool type is designed to be shared via cloning. From <https://docs.rs/sqlx/latest/sqlx/struct.Pool.html>:

> Cloning Pool is cheap as it is simply a reference-counted handle to the inner pool state.

5 files changed, 35 insertions, 20 deletions

diff --git a/src/app.rs b/src/app.rs
index e61672f..e33ee39 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -1,3 +1,4 @@
+use axum::extract::FromRef;
 use sqlx::sqlite::SqlitePool;
 
 #[cfg(test)]
@@ -33,8 +34,8 @@ impl App {
 }
 
 impl App {
-    pub const fn boot(&self) -> Boot<'_> {
-        Boot::new(&self.db)
+    pub fn boot(&self) -> Boot {
+        Boot::new(self.db.clone())
     }
 
     pub const fn conversations(&self) -> Conversations<'_> {
@@ -70,3 +71,9 @@ impl App {
         Users::new(&self.db, &self.events)
     }
 }
+
+impl FromRef<App> for Boot {
+    fn from_ref(app: &App) -> Self {
+        app.boot()
+    }
+}
diff --git a/src/boot/app.rs b/src/boot/app.rs
index 0ed5d1b..840243e 100644
--- a/src/boot/app.rs
+++ b/src/boot/app.rs
@@ -10,12 +10,12 @@ use crate::{
     user::{self, repo::Provider as _},
 };
 
-pub struct Boot<'a> {
-    db: &'a SqlitePool,
+pub struct Boot {
+    db: SqlitePool,
 }
 
-impl<'a> Boot<'a> {
-    pub const fn new(db: &'a SqlitePool) -> Self {
+impl Boot {
+    pub const fn new(db: SqlitePool) -> Self {
         Self { db }
     }
 
diff --git a/src/boot/handlers/boot/mod.rs b/src/boot/handlers/boot/mod.rs
index 3e022b1..5ff7802 100644
--- a/src/boot/handlers/boot/mod.rs
+++ b/src/boot/handlers/boot/mod.rs
@@ -7,15 +7,18 @@ use axum::{
 use serde::Serialize;
 
 use crate::{
-    app::App, boot::Snapshot, error::Internal, event::Heartbeat, login::Login,
+    boot::{Snapshot, app::Boot},
+    error::Internal,
+    event::Heartbeat,
+    login::Login,
     token::extract::Identity,
 };
 
 #[cfg(test)]
 mod test;
 
-pub async fn handler(State(app): State<App>, identity: Identity) -> Result<Response, Internal> {
-    let snapshot = app.boot().snapshot().await?;
+pub async fn handler(State(boot): State<Boot>, identity: Identity) -> Result<Response, Internal> {
+    let snapshot = boot.snapshot().await?;
     let heartbeat = Heartbeat::TIMEOUT;
 
     Ok(Response {
diff --git a/src/boot/handlers/boot/test.rs b/src/boot/handlers/boot/test.rs
index cb50442..a9891eb 100644
--- a/src/boot/handlers/boot/test.rs
+++ b/src/boot/handlers/boot/test.rs
@@ -8,7 +8,7 @@ async fn returns_identity() {
     let app = fixtures::scratch_app().await;
 
     let viewer = fixtures::identity::fictitious();
-    let response = super::handler(State(app), viewer.clone())
+    let response = super::handler(State(app.boot()), viewer.clone())
         .await
         .expect("boot always succeeds");
 
@@ -21,7 +21,7 @@ async fn includes_users() {
     let spectator = fixtures::user::create(&app, &fixtures::now()).await;
 
     let viewer = fixtures::identity::fictitious();
-    let response = super::handler(State(app), viewer)
+    let response = super::handler(State(app.boot()), viewer)
         .await
         .expect("boot always succeeds");
 
@@ -42,7 +42,7 @@ async fn includes_conversations() {
     let conversation = fixtures::conversation::create(&app, &fixtures::now()).await;
 
     let viewer = fixtures::identity::fictitious();
-    let response = super::handler(State(app), viewer)
+    let response = super::handler(State(app.boot()), viewer)
         .await
         .expect("boot always succeeds");
 
@@ -65,7 +65,7 @@ async fn includes_messages() {
     let message = fixtures::message::send(&app, &conversation, &sender, &fixtures::now()).await;
 
     let viewer = fixtures::identity::fictitious();
-    let response = super::handler(State(app), viewer)
+    let response = super::handler(State(app.boot()), viewer)
         .await
         .expect("boot always succeeds");
 
@@ -94,7 +94,7 @@ async fn includes_expired_messages() {
         .expect("expiry never fails");
 
     let viewer = fixtures::identity::fictitious();
-    let response = super::handler(State(app), viewer)
+    let response = super::handler(State(app.boot()), viewer)
         .await
         .expect("boot always succeeds");
 
@@ -136,7 +136,7 @@ async fn includes_deleted_messages() {
         .expect("deleting valid message succeeds");
 
     let viewer = fixtures::identity::fictitious();
-    let response = super::handler(State(app), viewer)
+    let response = super::handler(State(app.boot()), viewer)
         .await
         .expect("boot always succeeds");
 
@@ -175,7 +175,7 @@ async fn includes_expired_conversations() {
         .expect("expiry never fails");
 
     let viewer = fixtures::identity::fictitious();
-    let response = super::handler(State(app), viewer)
+    let response = super::handler(State(app.boot()), viewer)
         .await
         .expect("boot always succeeds");
 
@@ -214,7 +214,7 @@ async fn includes_deleted_conversations() {
         .expect("deleting a valid conversation succeeds");
 
     let viewer = fixtures::identity::fictitious();
-    let response = super::handler(State(app), viewer)
+    let response = super::handler(State(app.boot()), viewer)
         .await
         .expect("boot always succeeds");
 
diff --git a/src/test/fixtures/boot.rs b/src/test/fixtures/boot.rs
index 120726f..7421512 100644
--- a/src/test/fixtures/boot.rs
+++ b/src/test/fixtures/boot.rs
@@ -1,7 +1,12 @@
-use crate::{app::App, event::Sequence};
+use axum::extract::FromRef;
 
-pub async fn resume_point(app: &App) -> Sequence {
-    app.boot()
+use crate::{boot::app::Boot, event::Sequence};
+
+pub async fn resume_point<App>(app: &App) -> Sequence
+where
+    Boot: FromRef<App>,
+{
+    Boot::from_ref(app)
         .snapshot()
         .await
         .expect("boot always succeeds")