Hook up the restructured processor to the inbox

Cargo.lock

@@ -518,6 +518,21 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "futures"
+version = "0.3.30"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "645c6916888f6cb6350d2550b80fb63e734897a8498abe35cfb732b6487804b0"
+dependencies = [
+ "futures-channel",
+ "futures-core",
+ "futures-executor",
+ "futures-io",
+ "futures-sink",
+ "futures-task",
+ "futures-util",
+]
+
 [[package]]
 name = "futures-channel"
 version = "0.3.30"
@@ -525,6 +540,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "eac8f7d7865dcb88bd4373ab671c8cf4508703796caa2b1985a9ca867b3fcb78"
 dependencies = [
  "futures-core",
+ "futures-sink",
 ]
 
 [[package]]
@@ -533,6 +549,34 @@ version = "0.3.30"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dfc6580bb841c5a68e9ef15c77ccc837b40a7504914d52e47b8b0e9bbda25a1d"
 
+[[package]]
+name = "futures-executor"
+version = "0.3.30"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a576fc72ae164fca6b9db127eaa9a9dda0d61316034f33a0a0d4eda41f02b01d"
+dependencies = [
+ "futures-core",
+ "futures-task",
+ "futures-util",
+]
+
+[[package]]
+name = "futures-io"
+version = "0.3.30"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a44623e20b9681a318efdd71c299b6b222ed6f231972bfe2f224ebad6311f0c1"
+
+[[package]]
+name = "futures-macro"
+version = "0.3.30"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "87750cf4b7a4c0625b1529e4c543c2182106e4dedc60a2a6455e00d212c489ac"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.60",
+]
+
 [[package]]
 name = "futures-sink"
 version = "0.3.30"
@@ -551,10 +595,16 @@ version = "0.3.30"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3d6401deb83407ab3da39eba7e33987a73c3df0c82b4bb5813ee871c19c41d48"
 dependencies = [
+ "futures-channel",
  "futures-core",
+ "futures-io",
+ "futures-macro",
+ "futures-sink",
  "futures-task",
+ "memchr",
  "pin-project-lite",
  "pin-utils",
+ "slab",
 ]
 
 [[package]]
@@ -1258,6 +1308,7 @@ dependencies = [
  "derive_more",
  "either",
  "fetch",
+ "futures",
  "serde_json",
  "store",
  "tracing",

bin/server/src/api.rs

@@ -214,7 +214,7 @@ const GET: &Method = &Method::GET;
 /// This function is where all requests to a protected endpoint have to go through. If the request
 /// was signed but does not target a protected endpoint, this function will fall back to the
 /// [`dispatch_public`] handler.
-#[tracing::instrument(level = "DEBUG", target = "router", skip_all)]
+// #[tracing::instrument(level = "DEBUG", target = "router", skip_all)]
 async fn dispatch_signed(
     cx: Context,
     verifier: &Verifier,
@@ -236,7 +236,7 @@ async fn dispatch_signed(
 /// Dispatch `req` to an unprotected endpoint. If the requested path does not exist, the
 /// function will return a 404 response. If the path *does* exist, but the signature is not
 /// valid, they will also get a 404.
-#[tracing::instrument(level = "DEBUG", target = "router", skip_all)]
+// #[tracing::instrument(level = "DEBUG", target = "router", skip_all)]
 async fn dispatch_public(
     cx: Context,
     verifier: &Verifier,

bin/server/src/api/ap.rs

@@ -1,11 +1,15 @@
 //! ActivityPub handlers.
 
+use std::sync::{Mutex, MutexGuard};
+
 use http_body_util::Full;
 use hyper::body::Bytes;
 use puppy::{
     actor::{get_signing_key, Actor},
-    fetch::object::Activity,
-    get_local_ap_object, Context, Error, Key,
+    fetch::{object::Activity, Client},
+    get_local_ap_object,
+    store::Transaction,
+    Context, Error, Key,
 };
 use serde_json::Value;
 use tracing::{info, instrument};
@@ -81,10 +85,38 @@ pub async fn inbox(
         "processing object '{id}'",
     };
 
+    let tx = cx.store().start();
+    let key = get_signing_key(&tx, Actor { key: receiver }).unwrap();
+
+    struct InboxContext<'c> {
+        cx: &'c Context,
+        tx: Mutex<Transaction<'c>>,
+    }
+
+    impl<'a> puppy::systems::Context<'a> for InboxContext<'a> {
+        /// Access the transaction.
+        fn db<'y: 'x, 'x>(&'x self) -> MutexGuard<'x, Transaction<'y>>
+        where
+            'a: 'y,
+            'y: 'a,
+        {
+            self.tx.lock().unwrap()
+        }
+        /// Access the federation client.
+        fn client(&self) -> &Client {
+            self.cx.resolver()
+        }
+        /// Get the ActivityPub domain of this server.
+        fn domain(&self) -> &str {
+            &self.cx.config().ap_domain
+        }
+    }
+
     match Activity::from_json(json) {
         Ok(activity) => {
-            puppy::ingest(&cx, receiver, &activity).await.unwrap();
-            match puppy::interpret(&cx, activity) {
+            let context = InboxContext { tx: Mutex::new(tx), cx };
+            let r = puppy::systems::processor::process_incoming(&context, &key, activity).await;
+            match r {
                 Ok(_) => Ok(respond!(code: 202)),
                 Err(err) => fuck!(400: "error interpreting activity: {err}"),
             }

bin/server/src/main.rs

@@ -23,7 +23,7 @@ mod api;
 async fn main() {
     Registry::default()
         .with(filter_fn(|meta| !meta.target().starts_with("reqwest")))
-        .with(filter_fn(|meta| *meta.level() < Level::DEBUG))
+        //.with(filter_fn(|meta| *meta.level() < Level::DEBUG))
         .with(ForestLayer::default())
         .init();
     // TODO: load the config from a file or something.

lib/fetch/src/client.rs

@@ -38,13 +38,13 @@ impl Client {
     ///
     /// Note that in order for the request to be considered valid by most implementations, `key.owner`
     /// must equal `payload.actor`.
-    #[instrument(target = "fetch.delivery", skip_all, fields(activity = payload.id, url = inbox, key = key.id))]
-    pub async fn deliver(&self, key: &SigningKey, payload: &Activity, inbox: &str) {
+    #[instrument(target = "fetch.delivery", skip_all, fields(activity = payload.id, url = inbox.as_ref(), key = key.id))]
+    pub async fn deliver(&self, key: &SigningKey, payload: &Activity, inbox: impl AsRef<str>) {
         let system = Subsystem::Delivery;
 
         let body = serde_json::to_string(&payload.to_json_ld()).unwrap();
         let mut req = system
-            .new_request(inbox)
+            .new_request(inbox.as_ref())
             .unwrap()
             .method(Method::POST)
             .header("content-type", ACTIVITYPUB_TYPE)

lib/puppy/Cargo.toml

@@ -14,3 +14,4 @@ either = "*"
 derive_more = "*"
 serde_json = "*"
 tracing = "*"
+futures = "*"

lib/puppy/src/data.rs

@@ -90,6 +90,14 @@ pub struct FollowRequest {
     pub target: Key,
 }
 
+#[derive(Arrow, Debug, PartialEq, Eq, Clone, Copy)]
+pub struct Accept {
+    #[identity]
+    pub id: Key,
+    pub origin: Key,
+    pub target: Key,
+}
+
 /// The status of a [`FollowRequest`].
 ///
 /// Valid state transitions:
@@ -212,4 +220,5 @@ pub fn schema() -> Schema {
         .has::<AuthorOf>()
         .has::<Follows>()
         .has::<Create>()
+        .has::<Accept>()
 }

lib/puppy/src/lib.rs

@@ -2,7 +2,13 @@
 //! you should take a look at [`fetch`].
 
 // Working with result types is such a bitch without these.
-#![feature(iterator_try_collect, try_blocks, once_cell_try, box_into_inner)]
+#![feature(
+    iterator_try_collect,
+    try_blocks,
+    once_cell_try,
+    box_into_inner,
+    type_changing_struct_update
+)]
 
 use std::hint::unreachable_unchecked;
 
@@ -336,7 +342,7 @@ pub async fn ingest(cx: &Context, auth: Key, activity: &Activity) -> Result<()>
         match object {
             Object::Activity(a) => interpret(&cx, a)?,
             Object::Actor(a) => cx.run(|tx| actor::create_remote(tx, a).map(void))?,
-            Object::Note(a) => post::create_post_from_note(cx, a).map(void)?,
+            Object::Note(a) => cx.run(|tx| post::create_post_from_note(tx, a).map(void))?,
             _ => todo!(),
         }
     }
@@ -345,3 +351,630 @@ pub async fn ingest(cx: &Context, auth: Key, activity: &Activity) -> Result<()>
 
 /// Discard the argument.
 fn void<T>(_: T) -> () {}
+
+pub mod systems {
+    //! Logic containment zone.
+
+    use std::sync::MutexGuard;
+
+    use fetch::Client;
+    use store::{Key, Transaction};
+    use tracing::warn;
+
+    /// Allows subsystems to interact with the [fetch] and [store] components.
+    pub trait Context<'a> {
+        /// Access the transaction.
+        fn db<'y: 'x, 'x>(&'x self) -> MutexGuard<'x, Transaction<'y>>
+        where
+            'a: 'y,
+            'y: 'a;
+        /// Access the federation client.
+        fn client(&self) -> &Client;
+        /// Get the ActivityPub domain of this server.
+        fn domain(&self) -> &str;
+        /// Format `key` as an object ID.
+        fn make_object_id(&self, key: Key) -> String {
+            format!("https://{}/o/{key}", self.domain())
+        }
+        /// Check whether we already know about an object.
+        fn is_known(&self, url: &str) -> bool {
+            use crate::data::Id;
+
+            match self.db().lookup(Id(url.to_string())) {
+                Ok(Some(_)) => true,
+                _ => false,
+            }
+        }
+        /// Check whether `url` refers to an object that is considered "local" to this server.
+        fn is_local(&self, url: &str) -> bool {
+            use crate::data::{Id, Object};
+
+            let Ok(Some(key)) = self.db().lookup(Id(url.to_string())) else {
+                return false;
+            };
+
+            match self.db().get_mixin(key) {
+                Ok(Some(Object { local, .. })) => local,
+                Ok(None) => false,
+                Err(err) => {
+                    warn!("error while trying to determine origin of {key}: {err}");
+                    false
+                }
+            }
+        }
+    }
+
+    pub mod notification {
+        //! Manages the delivery of notifications to local users.
+
+        use tracing::info;
+
+        use crate::Result;
+        use super::Context;
+
+        /// Something that can be sent to a local user as a (push) notification.
+        pub trait Notification {}
+
+        /// Get the notification where it needs to go.
+        pub fn dispatch<'x, C>(cx: &C, event: &impl Notification) -> Result<()>
+        where
+            C: Context<'x>,
+        {
+            info!("ding!");
+            Ok(())
+        }
+    }
+
+    pub mod delivery {
+        //! A mid-level system for delivering messages to remote servers.
+
+        use fetch::{
+            object::{Activity, Object},
+            signatures::SigningKey,
+        };
+        use futures::{stream::FuturesUnordered, Future, StreamExt as _};
+        use derive_more::Display;
+        use store::{arrow::Multi, Key};
+        use tracing::error;
+
+        use crate::{entities::Accept, Error, Result};
+
+        use super::Context;
+
+        /// Type tag for an activity.
+        #[derive(Clone, Copy, Display)]
+        pub enum Tag {
+            Follow,
+            Accept,
+            Reject,
+            Create,
+            Delete,
+            Bite,
+        }
+
+        /// Implemented for types that can represent an activity that can be delivered to a remote server.
+        pub trait Payload {
+            /// Construct an [`Activity`] for delivery.
+            fn prepare<'c, C>(self, cx: &C) -> Result<Activity>
+            where
+                C: Context<'c>;
+            /// Call the delivery subsystem to this payload everywhere it needs to go.
+            ///
+            /// A convenience method to call [`deliver`] as a method instead of a free-standing function.
+            fn deliver<'c, C>(self, cx: &C) -> impl Future<Output = Result<()>> + Send + Sync
+            where
+                Self: Sized + Send + Sync,
+                C: Context<'c> + Send + Sync,
+            {
+                deliver(cx, self)
+            }
+        }
+
+        impl Payload for Accept {
+            fn prepare<'c, C>(self, cx: &C) -> Result<Activity>
+            where
+                C: Context<'c>,
+            {
+                let (actor, object, tag) = get_activity_data(cx, self.into())?;
+                Ok(Activity {
+                    id: cx.make_object_id(self.into()),
+                    actor,
+                    object: Box::new(Object::Id { id: object }),
+                    kind: tag.to_string(),
+                })
+            }
+        }
+
+        /// Deliver the activity-like payload to all its intended recipients.
+        pub async fn deliver<'c, C>(cx: &C, payload: impl Payload) -> Result<()>
+        where
+            C: Context<'c>,
+        {
+            let activity = payload.prepare(cx)?;
+
+            if !cx.is_local(&activity.id) {
+                error!("delivery of non-local activity!!");
+                return Ok(());
+            }
+
+            let signing_key = get_keypair(cx, &activity.actor)?;
+            get_targets(cx, &activity)?
+                .into_iter()
+                .map(|inbox| cx.client().deliver(&signing_key, &activity, inbox))
+                .collect::<FuturesUnordered<_>>()
+                .collect::<()>()
+                .await;
+
+            Ok(())
+        }
+
+        /// Calculate the list of inboxes to send the activity to.
+        fn get_targets<'c, C>(
+            cx: &C,
+            activity: &Activity,
+        ) -> Result<impl IntoIterator<Item = String>>
+        where
+            C: Context<'c>,
+        {
+            use crate::data::{Channel, Id};
+
+            let get_inbox = |id: &str| -> Result<Option<String>> {
+                let Some(key) = cx.db().lookup(Id(id.to_owned()))? else {
+                    return Ok(None);
+                };
+                cx.db()
+                    .get_mixin(key)
+                    .map(|o| o.map(|Channel { inbox }| inbox))
+                    .map_err(Error::Store)
+            };
+
+            match *activity.object {
+                Object::Id { ref id } => get_inbox(&id),
+                Object::Activity(ref a) => get_inbox(&a.actor),
+                Object::Actor(ref a) => Ok(Some(a.inbox.clone())),
+                Object::Note(ref a) => get_inbox(&a.author),
+            }
+        }
+
+        /// Get a signing key for the given actor id.
+        ///
+        /// Will fail if the actor isn't local.
+        fn get_keypair<'c, C>(cx: &C, actor_id: &str) -> Result<SigningKey>
+        where
+            C: Context<'c>,
+        {
+            use crate::{
+                actor::{get_signing_key, Actor},
+                data::Id,
+            };
+            let Some(actor_key) = cx.db().lookup(Id(actor_id.to_string()))? else {
+                panic!("could not get db key for {actor_id}");
+            };
+            get_signing_key(&*cx.db(), Actor { key: actor_key }).map_err(Error::Store)
+        }
+
+        /// Get actor id, the object id, and the type of the activity.
+        fn get_activity_data<'c, C>(cx: &C, key: Key) -> Result<(String, String, Tag)>
+        where
+            C: Context<'c>,
+        {
+            // TODO: instead of panicking, return a normal error.
+            use crate::data::{Id, Object, ActivityKind, ObjectKind};
+
+            let Some(Object { kind, .. }) = cx.db().get_mixin(key)? else {
+                panic!("activity must be an object")
+            };
+            let Some(Multi { origin, target, .. }) = cx.db().get_arrow_raw(key)? else {
+                panic!("expected activity to have multi-arrow repr")
+            };
+            let Some(Id(object)) = cx.db().get_alias(target)? else {
+                panic!("expected object of activity to have an id")
+            };
+            let Some(Id(actor)) = cx.db().get_alias(origin)? else {
+                panic!("expected author of activity to have an id")
+            };
+
+            let tag = match kind {
+                ObjectKind::Activity(tag) => match tag {
+                    ActivityKind::Create => Tag::Create,
+                    ActivityKind::Follow => Tag::Follow,
+                    ActivityKind::Accept => Tag::Accept,
+                    ActivityKind::Reject => Tag::Reject,
+                    ActivityKind::Bite => Tag::Bite,
+                },
+                _ => panic!("invalid kind for activity"),
+            };
+
+            Ok((actor, object, tag))
+        }
+    }
+
+    pub mod ingestion {
+        //! Processing remote objects to make them part of our local graph.
+    }
+
+    pub mod processor {
+        //! The structure and interpretation of activity graphs.
+        //!
+        //! This is where the magic happens.
+        //!
+        //! This module contains the code that assembles and executes a graph of activities. This is a higher
+        //! level system, that depends on the following sibling systems:
+        //!
+        //! - [`following`][super::following]
+        //! - [`delivery`][super::delivery]
+        //!
+        //! Its main purpose is to integrate all the above system in the context of receiving an activity in
+        //! an inbox, and the subsequent processing that needs to happen. In order to *correctly* interpret an
+        //! activity, we need to know about the other nodes that it references.
+
+        use fetch::{
+            object::{Activity, Actor, Note, Object},
+            signatures::SigningKey,
+            FetchError,
+        };
+        use store::Key;
+        use tracing::{debug, info, trace, warn};
+
+        use crate::{data, Result};
+
+        use super::{
+            delivery::{self, Tag},
+            following, Context,
+        };
+
+        /// Process an incoming activity.
+        #[tracing::instrument(target = "puppy.processor", skip_all, fields(activity = root.id))]
+        pub async fn process_incoming<'c, C>(
+            cx: &C,
+            on_behalf_of: &SigningKey,
+            root: Activity,
+        ) -> Result<(Key, Tag)>
+        where
+            C: Context<'c>,
+        {
+            if cx.is_local(&root.id) || cx.is_known(&root.id) {
+                panic!("could not process activity, it already exists");
+            }
+
+            // Fetch all transitive dependencies of `root`, returned in the order that they need to be processed.
+            let (actors, notes, activities) =
+                fetch_dependencies(cx, &root, &on_behalf_of, 3).await?;
+
+            trace!("storing actors");
+            for actor in actors {
+                store_actor(cx, actor)?;
+            }
+
+            trace!("storing notes");
+            for note in notes {
+                store_note(cx, note)?;
+            }
+
+            // Process the activities that have to execute before `activity`. These dependencies are returned
+            // in the exact order that they need to be applied. `fetch_dependencies` only returns stuff that is
+            // missing from the graph.
+            trace!("applying dependency activities");
+            for dep in activities {
+                let (key, tag) = apply_activity(cx, dep).await?;
+                debug!(
+                    dependency_of = root.id,
+                    kind = tag.to_string(),
+                    "processed activity {key}"
+                );
+            }
+
+            // Now that all activities that this depends on have been executed, we can execute the original
+            // one.
+            trace!("executing target activity");
+            apply_activity(cx, root).await
+        }
+
+        /// Execute the activity.
+        ///
+        /// # Panics
+        ///
+        /// This function assumes that everything the activity references by ActivityPub ID is already present in the context.
+        /// If this is not the case, it will panic.
+        ///
+        /// Specifically,
+        #[tracing::instrument(level = "TRACE", target = "puppy.processor", skip_all, fields(activity = activity.id, tag = activity.kind))]
+        async fn apply_activity<'c, C>(cx: &C, activity: Activity) -> Result<(Key, Tag)>
+        where
+            C: Context<'c>,
+        {
+            use crate::data::{Id, Create, Object, Accept};
+            // Get a key and error out if it does not exist
+            let get_key = |url: &str| -> Key {
+                cx.db()
+                    .lookup(data::Id(url.to_string()))
+                    .expect("database should be operable")
+                    .expect("url should already have been inserted")
+            };
+
+            let tag = tagof(&activity);
+            let key = match tag {
+                Tag::Follow => {
+                    let requester = get_key(&activity.actor);
+                    let target = get_key(activity.object.id());
+
+                    let req = following::create_follow_request(cx, requester, target)?;
+
+                    // For now, automatically accept follow requests.
+                    if cx.is_local(activity.object.id()) {
+                        debug!("auto-accepting follow request for local actor {target}",);
+                        let accept = following::accept_follow_request(cx, req)?;
+                        // TODO: Add object components to accept activity before sending
+                        if !cx.is_local(&activity.actor) {
+                            debug!("delivering to remote actor");
+                            delivery::deliver(cx, accept).await?;
+                        }
+                    }
+
+                    req.into()
+                }
+                Tag::Accept => {
+                    let req_id = get_key(activity.object.id());
+                    following::accept_follow_request(cx, req_id.into())?.into()
+                }
+                Tag::Reject => {
+                    let req_id = get_key(activity.object.id());
+                    following::reject_follow_request(cx, req_id.into())?.into()
+                }
+                Tag::Create => {
+                    let id = Key::gen();
+                    let actor = get_key(&activity.actor);
+                    let object = get_key(activity.object.id());
+                    cx.db().add_alias(id, Id(cx.make_object_id(id)))?;
+                    cx.db().create(Create { id, actor, object })?;
+                    id.into()
+                }
+                Tag::Delete => todo!(),
+                Tag::Bite => todo!(),
+            };
+
+            Ok((key, tag))
+        }
+
+        /// Fetch all transitive dependencies of `root` using the given `auth` as the signing authority.
+        ///
+        /// The `budget` parameter specifies the maximum number of recursive calls.
+        #[tracing::instrument(target = "puppy.processor", skip_all, fields(budget = budget, target = root.id))]
+        async fn fetch_dependencies<'c, C>(
+            cx: &C,
+            root: &Activity,
+            auth: &SigningKey,
+            budget: usize,
+        ) -> Result<(Vec<Actor>, Vec<Note>, Vec<Activity>)>
+        where
+            C: Context<'c>,
+        {
+            let mut actors = Vec::new();
+            let mut notes = Vec::new();
+            let mut activities = Vec::new();
+
+            for url in [root.actor.as_str(), root.object.id()] {
+                if cx.is_known(url) {
+                    debug!(parent = root.id, url, "already known, skipping");
+                    continue;
+                } else {
+                    debug!(parent = root.id, url, budget, "fetching dependency");
+                }
+
+                let json = cx.client().resolve(auth, url).await?;
+                let object = Object::from_json(json).map_err(FetchError::BadJson)?;
+
+                match object {
+                    Object::Id { id } => {
+                        warn!(parent = root.id, url, "could not fetch {id}");
+                    }
+                    Object::Activity(a) if budget == 0 => {
+                        info!(parent = root.id, url, "exceeded budget, skipping");
+                        activities.push(a);
+                        continue;
+                    }
+                    Object::Activity(a) => {
+                        debug!(parent = root.id, url, "fetching dependencies");
+                        // BUG: this won't hit cache because none of the stuff is in there yet.
+                        let (x, y, z) =
+                            Box::pin(fetch_dependencies(cx, &a, auth, budget - 1)).await?;
+                        trace!(
+                            parent = root.id,
+                            url,
+                            "fetch completed, total: {}",
+                            x.len() + y.len() + z.len()
+                        );
+                        actors.extend(x);
+                        notes.extend(y);
+                        activities.extend(z);
+                        activities.push(a);
+                    }
+                    Object::Actor(a) => actors.push(a),
+                    Object::Note(a) => notes.push(a),
+                }
+            }
+
+            Ok((actors, notes, activities))
+        }
+
+        fn store_actor<'c, C>(cx: &C, actor: Actor) -> Result<()>
+        where
+            C: Context<'c>,
+        {
+            if !cx.is_known(&actor.id) {
+                crate::actor::create_remote(&*cx.db(), actor)?;
+            } else {
+                debug!("actor {} is already known", actor.id);
+            }
+            Ok(())
+        }
+
+        fn store_note<'c, C>(cx: &C, note: Note) -> Result<()>
+        where
+            C: Context<'c>,
+        {
+            if !cx.is_known(&note.id) {
+                crate::post::create_post_from_note(&*cx.db(), note)?;
+            } else {
+                debug!("note {} is already known", note.id);
+            }
+            Ok(())
+        }
+
+        /// Get the tag of the activity.
+        fn tagof(a: &Activity) -> Tag {
+            match a.kind.as_str() {
+                "Create" => Tag::Create,
+                "Delete" => Tag::Delete,
+                "Follow" => Tag::Follow,
+                "Accept" => Tag::Accept,
+                "Reject" => Tag::Reject,
+                "Bite" => Tag::Bite,
+                _ => todo!(),
+            }
+        }
+    }
+
+    pub mod timelines {
+        //! Manages the rendering of timelines.
+    }
+
+    pub mod following {
+        //! Follow requests and management thereof.
+
+        use store::Key;
+
+        use super::Context;
+        use crate::{
+            entities::{Accept, FollowRequest, Reject, Undo},
+            Result,
+        };
+
+        /// Create a follow request.
+        pub fn create_follow_request<'c, C>(
+            cx: &C,
+            follower: Key,
+            target: Key,
+        ) -> Result<FollowRequest>
+        where
+            C: Context<'c>,
+        {
+            Ok(Key::gen().into())
+        }
+
+        /// Cancel a follow request.
+        ///
+        /// If the follow request was already accepted, the follow request's target is unfollowed by the actor. Otherwise,
+        /// the follow request is withdrawn.
+        ///
+        /// This creates a new [`Undo`] entry in the database to which data may be attached.
+        pub fn cancel_follow_request<'c, C>(cx: &C, req: FollowRequest) -> Result<Undo>
+        where
+            C: Context<'c>,
+        {
+            Ok(Key::gen().into())
+        }
+
+        /// Apply the changes related to accepting a follow request to the social graph and create a new node representing
+        /// the event.
+        pub fn accept_follow_request<'c, C>(cx: &C, req: FollowRequest) -> Result<Accept>
+        where
+            C: Context<'c>,
+        {
+            Ok(Key::gen().into())
+        }
+
+        /// Apply the changes related to rejecting a follow request to the social graph and create a new node representing
+        /// the event.
+        pub fn reject_follow_request<'c, C>(cx: &C, req: FollowRequest) -> Result<Reject>
+        where
+            C: Context<'c>,
+        {
+            Ok(Key::gen().into())
+        }
+    }
+
+    pub mod reverse {
+        //! Undoing operations.
+        //!
+        //! This module defines the behavior of the [`Undo`] activity.
+
+        use crate::entities::{FollowRequest, Undo};
+
+        use super::{following, Context};
+
+        /// Describes objects which have a "revert" operation defined (that is, they can be the target of an [`Undo`] activity).
+        pub trait Reversible {
+            /// Undo `self` and generate a corresponding [`Undo`] object recording this fact.
+            fn revert<'c, C>(&self, cx: &C) -> crate::Result<Undo>
+            where
+                C: Context<'c>;
+        }
+
+        impl Reversible for FollowRequest {
+            /// Withdraw a follow request if it wasn't yet accepted, or unfollow someone.
+            fn revert<'c, C>(&self, cx: &C) -> crate::Result<Undo>
+            where
+                C: Context<'c>,
+            {
+                following::cancel_follow_request(cx, *self)
+            }
+        }
+    }
+}
+
+pub mod entities {
+    //! Virtual data types composed from [components] and operated on by [systems].
+    //!
+    //! [components]: crate::components
+    //! [systems]: crate::systems
+
+    use store::Key;
+    use derive_more::{From, Into, Display};
+
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct Poster(Key);
+
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct Server(Key);
+
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct Post(Key);
+
+    /// Represents a `Bite` activity.
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct Bite(Key);
+
+    /// Represents an `Undo` activity.
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct Undo(Key);
+
+    /// Represents an `Accept` activity.
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct Accept(Key);
+
+    /// Represents a `Reject` activity.
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct Reject(Key);
+
+    /// Represents a `Create` activity.
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct Create(Key);
+
+    /// Represents a `Follow` activity.
+    ///
+    /// Also see the [`following`][crate::systems::following] module, which defines the logic for following, follow requests
+    /// and other related stuff.
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct FollowRequest(Key);
+
+    #[derive(Clone, Copy, Debug, From, Into, Display)]
+    pub struct PublicKey(Key);
+
+    /// A key newtype that represents an object.
+    pub trait Entity: Into<Key> + From<Key> + Copy {}
+
+    impl<T> Entity for T where T: Into<Key> + From<Key> + Copy {}
+}
+
+pub mod components {}

lib/puppy/src/post.rs

@@ -152,29 +152,27 @@ pub fn create_local_post(
 }
 
 /// Assumes all objects referenced already exist.
-#[tracing::instrument(skip(cx))]
-pub fn create_post_from_note(cx: &Context, note: Note) -> crate::Result<Post> {
-    cx.run(|tx| {
-        let Some(author) = tx.lookup(Id(note.author))? else {
-            panic!("needed author to already exist")
-        };
+#[tracing::instrument(skip(tx))]
+pub fn create_post_from_note(tx: &Transaction<'_>, note: Note) -> crate::Result<Post> {
+    let Some(author) = tx.lookup(Id(note.author))? else {
+        panic!("needed author to already exist")
+    };
 
-        let key = Key::gen();
+    let key = Key::gen();
 
-        tx.add_alias(key, Id(note.id.clone()))?;
-        tx.create(AuthorOf { object: key, author })?;
-        tx.add_mixin(key, Content {
-            content: note.content,
-            warning: note.summary,
-        })?;
-        tx.add_mixin(key, data::Object {
-            kind: ObjectKind::Notelike(note.kind),
-            id: Id(note.id),
-            local: false,
-        })?;
+    tx.add_alias(key, Id(note.id.clone()))?;
+    tx.create(AuthorOf { object: key, author })?;
+    tx.add_mixin(key, Content {
+        content: note.content,
+        warning: note.summary,
+    })?;
+    tx.add_mixin(key, data::Object {
+        kind: ObjectKind::Notelike(note.kind),
+        id: Id(note.id),
+        local: false,
+    })?;
 
-        Ok(Post { key })
-    })
+    Ok(Post { key })
 }
 
 #[tracing::instrument(skip(cx))]

lib/store/src/arrow.rs

@@ -222,11 +222,15 @@ impl Transaction<'_> {
     where
         A: Arrow<Kind = Multi>,
     {
+        Ok(self.get_arrow_raw(key)?.map(A::from))
+    }
+    /// Construct the arrow from its identifier.
+    pub fn get_arrow_raw(&self, key: Key) -> Result<Option<Multi>> {
         let arrow = self
             .open(crate::types::MULTIEDGE_HEADERS)
             .get(key)?
             .map(|v| Key::split(v.as_ref()))
-            .map(|(origin, target)| A::from(Multi { origin, target, identity: key }));
+            .map(|(origin, target)| Multi { origin, target, identity: key });
         Ok(arrow)
     }
 }

lib/store/src/key.rs

@@ -3,13 +3,14 @@ use std::{
     str::FromStr,
 };
 
+use bincode::{Decode, Encode};
 use chrono::{DateTime, Utc};
 use ulid::Ulid;
 
 use crate::StoreError;
 
 /// A unique identifier for vertices in the database.
-#[derive(Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[derive(Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash, Encode, Decode)]
 pub struct Key(pub(crate) [u8; 16]);
 
 impl Key {

lib/store/src/lib.rs

@@ -68,7 +68,7 @@ pub struct Batch {
 }
 
 impl Store {
-    /// Run a [transaction][Transaction].
+    /// Run a [transaction][Transaction] and ensure that it is either committed or rolled back.
     ///
     /// In a transaction, either all writes succeed, or the transaction is aborted and the changes are not
     /// recorded. Changes made inside a transaction can be read from within that transaction before they are
@@ -85,15 +85,20 @@ impl Store {
             store: &self,
         };
         let r = f(&tx);
-        if let Err(e) = if r.is_err() {
-            tx.inner.rollback()
+        if r.is_err() {
+            tx.cancel()?;
         } else {
-            tx.inner.commit()
-        } {
-            return Err(E::from(StoreError::Internal(e)));
+            tx.commit()?;
         }
         r
     }
+    /// Begin a transaction.
+    pub fn start(&self) -> Transaction<'_> {
+        Transaction {
+            inner: self.inner.transaction(),
+            store: &self,
+        }
+    }
     /// Apply a batch of changes atomically.
     pub fn apply(&self, batch: Batch) -> Result<()> {
         self.inner.write(batch.inner.into_inner())?;
@@ -132,6 +137,17 @@ impl Store {
     }
 }
 
+impl<'db> Transaction<'db> {
+    /// Complete the transaction successfully.
+    pub fn commit(self) -> Result<(), StoreError> {
+        self.inner.commit().map_err(StoreError::from)
+    }
+    /// Cancel the transaction.
+    pub fn cancel(self) -> Result<(), StoreError> {
+        self.inner.rollback().map_err(StoreError::from)
+    }
+}
+
 /// A shorthand for committing a [`Transaction`] (because I think `Ok(())` is ugly).
 pub const OK: Result<()> = Ok(());