Basic minimal prototype

Use a postgres database as storage and start with sone basic
functionality.

src/main.rs

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+use anyhow::Result;
+use diesel::{
+    prelude::*,
+    r2d2::{ConnectionManager, Pool, PooledConnection},
+};
+use lettre::{Message, SmtpTransport, Transport};
+use std::{env, thread::park_timeout};
+use time::{Duration, OffsetDateTime};
+use tracing::{debug, info, trace};
+
+mod models;
+mod schema;
+
+use models::{NewReminder, Reminder};
+
+fn remind(
+    db: &mut PooledConnection<ConnectionManager<PgConnection>>,
+    mailer: &SmtpTransport,
+) -> Result<()> {
+    debug!("executing remind run");
+
+    for reminder in schema::reminders::dsl::reminders
+        .filter(schema::reminders::executed.is_null())
+        .order(schema::reminders::planned.asc())
+        .load::<Reminder>(db)?
+    {
+        trace!(?reminder, "checking reminder");
+        if reminder.planned <= OffsetDateTime::now_utc() {
+            let email = Message::builder()
+                .from("whakarite@localhost".parse()?)
+                .to(reminder.receiver.parse()?)
+                .subject(&reminder.title)
+                .body(reminder.message.to_string())?;
+
+            mailer.send(&email)?;
+
+            diesel::update(&reminder)
+                .set(schema::reminders::executed.eq(Some(OffsetDateTime::now_utc())))
+                .execute(db)?;
+
+            debug!("email sent to {}", reminder.receiver);
+        } else {
+            info!(
+                "parking reminder for {:?}",
+                reminder.planned - OffsetDateTime::now_utc(),
+            );
+
+            park_timeout(<std::time::Duration>::try_from(
+                reminder.planned - OffsetDateTime::now_utc(),
+            )?);
+        }
+        // Check for another remnd job if none dont loop forever
+    }
+
+    Ok(())
+}
+
+fn main() -> Result<()> {
+    tracing_subscriber::fmt::init();
+
+    info!(
+        "{} {} started",
+        env!("CARGO_BIN_NAME"),
+        env!("CARGO_PKG_VERSION")
+    );
+
+    let db_pool = Pool::builder().build(ConnectionManager::<PgConnection>::new(
+        "postgresql://whakarite:whakarite@localhost/whakarite",
+    ))?;
+
+    let test_reminder = NewReminder {
+        created: OffsetDateTime::now_utc(),
+        planned: (OffsetDateTime::now_utc() + Duration::MINUTE),
+        title: "Test title",
+        message: "Test message",
+        receiver: "finga@localhost",
+    };
+
+    diesel::insert_into(schema::reminders::table)
+        .values(&test_reminder)
+        .execute(&mut db_pool.get()?)?;
+
+    let mailer = SmtpTransport::unencrypted_localhost();
+
+    loop {
+        remind(&mut db_pool.get()?, &mailer)?;
+    }
+}

src/models.rs

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+use crate::schema::reminders;
+use diesel::prelude::*;
+use time::OffsetDateTime;
+
+#[derive(AsChangeset, Debug, Identifiable, Queryable)]
+pub struct Reminder {
+    id: i32,
+    created: OffsetDateTime,
+    pub planned: OffsetDateTime,
+    executed: Option<OffsetDateTime>,
+    pub title: String,
+    pub message: String,
+    pub receiver: String,
+}
+
+#[derive(Insertable)]
+#[diesel(table_name = reminders)]
+pub struct NewReminder<'a> {
+    pub created: OffsetDateTime,
+    pub planned: OffsetDateTime,
+    pub title: &'a str,
+    pub message: &'a str,
+    pub receiver: &'a str,
+}

src/schema.rs

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+// @generated automatically by Diesel CLI.
+
+diesel::table! {
+    reminders (id) {
+        id -> Int4,
+        created -> Timestamptz,
+        planned -> Timestamptz,
+        executed -> Nullable<Timestamptz>,
+        title -> Text,
+        message -> Text,
+        receiver -> Text,
+    }
+}