Merge pull request 'feat(sidecar)!: protocol 4 — guild rosters, and a way to migrate the store (Teams cutover 2/6)' (#32) from edge into main

Reviewed-on: #32
Reviewed-by: Colby Whitlock <whitlocktech@gmail.com>

.gitea/workflows/pr-checks.yml

@@ -17,10 +17,12 @@
 #   so let this workflow run on one PR first. The `PR Checks / *` glob matches
 #   without needing the dropdown.
 #
-# Scope note: this gates PRs into `main` only. Feature work that lands on an
-# integration branch first (e.g. `edge`) is still caught on the branch's PR into
-# `main`. To gate that earlier hop too, add the branch to the `branches:` list
-# below — nothing else needs to change.
+# Scope note: `edge` is gated as well as `main`. Multi-phase work lands there
+# first, so gating only the `main` hop would run these checks for the first time
+# at the cutover — the one moment a red build is most expensive to discover. This
+# is the same call `RunicGateway/installer` made for the same reason, and it was
+# taken here after a nine-PR Android workstream landed on an ungated `edge` with
+# no CI at all. Adding a branch to the `branches:` list is the whole change.
 #
 # Runner: the same self-hosted `ubuntu-latest` runner release.yml uses. Rust is
 # not assumed to be preinstalled, so the toolchain step bootstraps it the same
@@ -31,7 +33,7 @@ name: PR Checks
 
 on:
   pull_request:
-    branches: [main]
+    branches: [main, edge]
 
 # A newer push to the same PR cancels the in-flight run.
 concurrency:

.gitea/workflows/release.yml

@@ -279,33 +279,43 @@ jobs:
           ls -l dist && echo "----" && cat dist/SHA256SUMS
 
       # ── RELEASE ENGINE: commit the bump, tag, push ───────────────────────
-      - name: Commit version bump and push tag
+      # Tag only — `main` is never pushed to.
+      #
+      # This step used to commit the version bump back to main first. Two things
+      # were wrong with that. It has never once executed: an EMPTY template
+      # expression written literally in a comment (the `$`+`{{ }}` token, which
+      # is why it is spelled out here) made the runner fail to build the script
+      # and skip the whole step silently, which is why sidecar/Cargo.toml still
+      # says 0.1.0 after six releases (the tags exist because the release API
+      # creates one when it publishes). And had it executed, it would have been
+      # declined — main is protected, and a release must not depend on a write
+      # to a protected branch.
+      #
+      # So the tag is the version, as it already is in servuo-plugins. The
+      # workflow still writes the real version into Cargo.toml before building,
+      # so a released binary self-reports correctly; what it no longer does is
+      # commit that edit back. The next version is computed from the newest tag,
+      # never from Cargo.toml, so nothing downstream depends on the file.
+      - name: Push the release tag
         if: ${{ steps.plan.outputs.release == 'true' }}
         env:
           REGISTRY_USER: ${{ secrets.REGISTRY_USER }}
           REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
         run: |
           set -euo pipefail
-          VERSION="${{ steps.plan.outputs.version }}"
           TAG="${{ steps.plan.outputs.tag }}"
           # Secrets can arrive with a trailing newline (depending on how they were
           # pasted); a stray CR/LF corrupts the remote URL ("credential url cannot
-          # be parsed"). Strip line breaks before building the URL. Passing them via
-          # env (not inline ${{ }}) also keeps a newline from breaking this script.
+          # be parsed"). Strip line breaks before building the URL. They are passed
+          # via env rather than interpolated into this script, so a newline cannot
+          # break it — do NOT write a template token literally in a comment here,
+          # or the runner will skip this step without failing the job.
           CI_USER="$(printf '%s' "${REGISTRY_USER}" | tr -d '\r\n')"
           CI_TOKEN="$(printf '%s' "${REGISTRY_TOKEN}" | tr -d '\r\n')"
           git config user.name  "uo-link-ci"
           git config user.email "ci@whitlocktech.com"
           git remote set-url origin \
             "https://${CI_USER}:${CI_TOKEN}@${GITEA_HOST}/${REPO}.git"
-
-          git add "${WORKDIR}/Cargo.toml" "${WORKDIR}/Cargo.lock"
-          if ! git diff --cached --quiet; then
-            git commit -m "chore(release): bump version to ${TAG} [skip ci]"
-            git push origin "HEAD:main"
-          else
-            echo "Version unchanged (first release) — no bump commit needed."
-          fi
           # The tag may already exist when finishing a run that died after tagging
           # (see the plan step). `git tag` on an existing name fails under
           # `set -e`; pushing an identical existing tag is a harmless no-op. A

README.md

@@ -12,11 +12,34 @@ ServUO plugin (C#, net48)  ──loopback TCP, newline-JSON──►  Rust sidec
 The shard never speaks WebSocket and exposes no port of its own — the sidecar is the only
 network-facing component, which is what keeps the game unreachable from the internet.
 
+## Running a shard? Don't build this
+
+The [**Runic Gateway installer**](https://gitea.whitlocktech.com/RunicGateway/installer) installs
+this sidecar for you — the released binary, its config, a hardened service account and the service
+registration — alongside the shard plugin, in one run, on Linux or Windows:
+
+```bash
+sudo ./runicgateway-installer-linux-x86_64 install
+```
+
+It ends by printing the base URL, WebSocket URL, protocol version and auth token to paste into
+**Admin → Shard** on your site. Guide:
+[installer/INSTALL.md](https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/installer/INSTALL.md).
+
+Installing it yourself is supported too — the release binaries on this repo's
+[releases page](https://gitea.whitlocktech.com/RunicGateway/link/releases) are the same ones the
+installer fetches, and
+[INSTALL.md Appendix A3–A4](https://gitea.whitlocktech.com/RunicGateway/docs/src/branch/main/installer/INSTALL.md#a3-install-the-sidecar)
+covers placing the binary and registering the service by hand.
+
+Everything below this line is for **developing on the sidecar**.
+
 ## Related repos
 
 | Repo | What |
 |------|------|
 | **this** — `RunicGateway/link` | The Rust sidecar (`sidecar/`). |
+| [RunicGateway/installer](https://gitea.whitlocktech.com/RunicGateway/installer) | The **installer** — deploys this sidecar and the plugin onto a shard host. The supported way to set one up. |
 | [RunicGateway/servuo-plugins](https://gitea.whitlocktech.com/RunicGateway/servuo-plugins) | The **C# ServUO plugin** — the shard side of the bridge (`overlay/`, `patches/`, `deploy.ps1`, test scaffolding). |
 | [RunicGateway/docs](https://gitea.whitlocktech.com/RunicGateway/docs) | All project documentation — design docs, protocol spec, integration guide, research. |
 
@@ -28,9 +51,10 @@ network-facing component, which is what keeps the game unreachable from the inte
 | `.gitea/workflows/pr-checks.yml` | Gates every PR into `main` on `cargo fmt --check`, `cargo clippy -D warnings`, and `cargo test`. |
 | `.gitea/workflows/release.yml` | Builds + releases the sidecar binary (Linux + Windows) on every merge to `main`. |
 
-## Build & run
+## Build & run (development)
 
-The sidecar is a standard cargo crate:
+Building from source is for working *on* the sidecar; a deployment gets its binary from a release,
+via the installer or by hand. The sidecar is a standard cargo crate:
 
 ```bash
 cd sidecar
@@ -39,7 +63,7 @@ cp sidecar.toml.example sidecar.toml   # then edit
 cargo run --release
 ```
 
-Deploying it rather than developing on it: `--config <PATH>` names the config file (as does
+Deploying it by hand rather than developing on it: `--config <PATH>` names the config file (as does
 `$UOLINK_CONFIG`), and `--print-config` prints the resolved settings — **including the auth token
 the website needs** — as JSON, provisioning the config file on first run. That is the supported way
 to read the token back; it is not meant to be scraped from the log.

sidecar/src/app.rs

@@ -14,11 +14,13 @@
 //! - `shutdown` is whatever "stop" means on this host: Ctrl-C and `SIGTERM` on Unix, the SCM's
 //!   `Stop` control on Windows.
 
+use std::collections::HashMap;
 use std::future::Future;
 use std::sync::atomic::AtomicI64;
 use std::sync::Arc;
 use std::time::Instant;
 
+use serde_json::Value;
 use tokio::sync::{broadcast, mpsc};
 use tracing::info;
 
@@ -88,6 +90,9 @@ where
     let last_event_ts = last_event.clone();
     let replay_handle = handle.clone(); // re-push external news to the shard on (re)connect
     let mut total: u64 = 0;
+    // Partly-received guild rosters, keyed by guild id. Lives in the event-loop task, so it needs
+    // no lock and dies with the loop. See `accumulate_roster`.
+    let mut roster_parts: HashMap<i64, RosterParts> = HashMap::new();
     tokio::spawn(async move {
         while let Some(ev) = event_rx.recv().await {
             // Any line from the shard — including pong heartbeats — is a sign of life.
@@ -161,6 +166,43 @@ where
                             }
                         }
                     }
+                    // Guild roster (Protocol 4): the member list that `guild.update`'s counts cannot
+                    // express. It writes a *different column* of the same row, so it never races
+                    // guild.update. `guild.leave` deliberately has no arm here — the sidecar
+                    // forwards it (persisted and broadcast below, like any event) and the board's
+                    // roster self-corrects on the next `guild.roster`, which the shard re-emits
+                    // whenever the member set changes. Keeping the delta out of the board is what
+                    // keeps the sidecar a forwarder rather than a thing that maintains state.
+                    //
+                    // A roster over the shard's per-line cap arrives as several frames, so it is
+                    // reassembled before it is stored — see `accumulate_roster` for why that happens
+                    // here rather than by appending to the column.
+                    "guild.roster" => {
+                        if let Some(id) = ev.value.get("id").and_then(|v| v.as_i64()) {
+                            let seq = ev.value.get("seq").and_then(|v| v.as_i64()).unwrap_or(0);
+                            let more = ev
+                                .value
+                                .get("more")
+                                .and_then(|v| v.as_bool())
+                                .unwrap_or(false);
+                            let members = ev
+                                .value
+                                .get("members")
+                                .and_then(|m| m.as_array())
+                                .cloned()
+                                .unwrap_or_default();
+
+                            if let Some(complete) =
+                                accumulate_roster(&mut roster_parts, id, seq, more, members)
+                            {
+                                let json = serde_json::Value::Array(complete).to_string();
+                                if let Err(e) = event_store.upsert_guild_roster(id, &json, t).await
+                                {
+                                    tracing::warn!(error = %e, "failed to upsert guild roster");
+                                }
+                            }
+                        }
+                    }
                     "guild.remove" => {
                         if let Some(id) = ev.value.get("id").and_then(|v| v.as_i64()) {
                             if let Err(e) = event_store.delete_guild(id).await {
@@ -279,6 +321,10 @@ where
             // with announce=false so a restart does not re-proclaim every article at once. news.add
             // is idempotent by id, so replaying to a still-populated shard is harmless.
             if ev.kind == "server.hello" {
+                // A (re)connected shard restarts every roster from `seq` 0, so any half-received
+                // one belongs to the previous connection and can never be completed.
+                roster_parts.clear();
+
                 match event_store.news_all().await {
                     Ok(items) => {
                         for mut item in items {
@@ -327,3 +373,199 @@ pub fn now_ms() -> i64 {
         .map(|d| d.as_millis() as i64)
         .unwrap_or(0)
 }
+
+/// A guild roster that has arrived in part: the `seq` expected next, and what has accumulated.
+struct RosterParts {
+    next_seq: i64,
+    members: Vec<Value>,
+}
+
+/// Refuses to accumulate a roster past this many members. The shard caps its own frames, so
+/// exceeding this means a shard that is buggy or not what it claims to be — and the one thing this
+/// buffer must not do is grow without bound on its say-so.
+const MAX_ROSTER_MEMBERS: usize = 50_000;
+
+/// Reassembles a `guild.roster` that the shard split across frames, returning the whole member list
+/// once the final frame arrives and `None` while one is still incomplete.
+///
+/// Reassembly happens **here, in memory, before the store** rather than by appending to the
+/// `members` column, for two reasons. Appending would make the write a read-modify-write — the exact
+/// thing the two-column board design exists to avoid — and it would publish a torn roster: a reader
+/// hitting `GET /guilds` between frames would see a partial member list as though it were the truth.
+/// Buffering keeps the store's write a single atomic upsert of a complete roster.
+///
+/// This is transport-level reassembly, not domain state: it is the same category of work as turning
+/// bytes into a line, and it holds nothing once a roster is complete. That is what keeps it
+/// compatible with the sidecar being a forwarder.
+///
+/// The ordinary case — a guild inside the shard's per-line cap, which is every realistic one —
+/// arrives as `seq` 0 with `more` false and is returned immediately without ever touching the map.
+fn accumulate_roster(
+    parts: &mut HashMap<i64, RosterParts>,
+    id: i64,
+    seq: i64,
+    more: bool,
+    members: Vec<Value>,
+) -> Option<Vec<Value>> {
+    if seq == 0 {
+        // A fresh roster supersedes any partial one: the shard restarts at 0 every time it emits,
+        // so a leftover buffer is from an emission that was interrupted and will never finish.
+        parts.remove(&id);
+
+        if !more {
+            return Some(members);
+        }
+
+        parts.insert(
+            id,
+            RosterParts {
+                next_seq: 1,
+                members,
+            },
+        );
+        return None;
+    }
+
+    let entry = match parts.get_mut(&id) {
+        Some(entry) => entry,
+        // A continuation with nothing to continue: the sidecar started, or the shard reconnected,
+        // midway through an emission. Dropping it is right — the next full roster is complete.
+        None => {
+            tracing::debug!(
+                guild = id,
+                seq,
+                "roster continuation with no start; ignoring"
+            );
+            return None;
+        }
+    };
+
+    if entry.next_seq != seq {
+        tracing::warn!(
+            guild = id,
+            expected = entry.next_seq,
+            got = seq,
+            "roster frames out of order; discarding the partial roster"
+        );
+        parts.remove(&id);
+        return None;
+    }
+
+    entry.members.extend(members);
+
+    if entry.members.len() > MAX_ROSTER_MEMBERS {
+        tracing::warn!(
+            guild = id,
+            len = entry.members.len(),
+            "roster exceeded the reassembly cap; discarding"
+        );
+        parts.remove(&id);
+        return None;
+    }
+
+    if more {
+        entry.next_seq = seq + 1;
+        return None;
+    }
+
+    parts.remove(&id).map(|done| done.members)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn members(names: &[&str]) -> Vec<Value> {
+        names
+            .iter()
+            .map(|n| serde_json::json!({"name": n}))
+            .collect()
+    }
+
+    fn names(vs: &[Value]) -> Vec<String> {
+        vs.iter()
+            .map(|v| v["name"].as_str().unwrap_or_default().to_string())
+            .collect()
+    }
+
+    #[test]
+    fn a_single_frame_roster_is_returned_immediately() {
+        // Every realistic guild takes this path, and it must not depend on the buffer at all.
+        let mut parts = HashMap::new();
+        let out = accumulate_roster(&mut parts, 1, 0, false, members(&["Ada", "Bo"]));
+
+        assert_eq!(names(&out.expect("complete")), ["Ada", "Bo"]);
+        assert!(parts.is_empty(), "nothing should be buffered");
+    }
+
+    #[test]
+    fn a_chunked_roster_reassembles_in_order() {
+        // The case the live rig caught: without this, only the final frame survived and a
+        // 155-member guild appeared on the board with 3 members.
+        let mut parts = HashMap::new();
+
+        assert!(accumulate_roster(&mut parts, 1, 0, true, members(&["Ada"])).is_none());
+        assert!(accumulate_roster(&mut parts, 1, 1, true, members(&["Bo"])).is_none());
+        let out = accumulate_roster(&mut parts, 1, 2, false, members(&["Cy"]));
+
+        assert_eq!(names(&out.expect("complete")), ["Ada", "Bo", "Cy"]);
+        assert!(parts.is_empty(), "buffer is released once complete");
+    }
+
+    #[test]
+    fn a_restarted_roster_supersedes_a_partial_one() {
+        // A shard that reconnects mid-emission starts again at seq 0. The abandoned frames must not
+        // end up spliced onto the front of the new roster.
+        let mut parts = HashMap::new();
+
+        assert!(accumulate_roster(&mut parts, 1, 0, true, members(&["Stale"])).is_none());
+        let out = accumulate_roster(&mut parts, 1, 0, false, members(&["Fresh"]));
+
+        assert_eq!(names(&out.expect("complete")), ["Fresh"]);
+    }
+
+    #[test]
+    fn an_out_of_order_frame_discards_the_partial_roster() {
+        // Better to publish nothing and wait for the next full emission than to store a roster with
+        // a hole in it that nothing downstream could detect.
+        let mut parts = HashMap::new();
+
+        assert!(accumulate_roster(&mut parts, 1, 0, true, members(&["Ada"])).is_none());
+        assert!(accumulate_roster(&mut parts, 1, 2, false, members(&["Skipped"])).is_none());
+        assert!(parts.is_empty());
+    }
+
+    #[test]
+    fn a_continuation_with_no_start_is_ignored() {
+        // The sidecar restarting midway through a shard's emission.
+        let mut parts = HashMap::new();
+
+        assert!(accumulate_roster(&mut parts, 1, 3, false, members(&["Orphan"])).is_none());
+        assert!(parts.is_empty());
+    }
+
+    #[test]
+    fn two_guilds_reassemble_independently() {
+        // Rosters for different guilds interleave freely — the sweep emits one guild after another
+        // and nothing serialises them on the wire.
+        let mut parts = HashMap::new();
+
+        assert!(accumulate_roster(&mut parts, 1, 0, true, members(&["A1"])).is_none());
+        assert!(accumulate_roster(&mut parts, 2, 0, true, members(&["B1"])).is_none());
+        let g2 = accumulate_roster(&mut parts, 2, 1, false, members(&["B2"]));
+        let g1 = accumulate_roster(&mut parts, 1, 1, false, members(&["A2"]));
+
+        assert_eq!(names(&g2.expect("guild 2")), ["B1", "B2"]);
+        assert_eq!(names(&g1.expect("guild 1")), ["A1", "A2"]);
+    }
+
+    #[test]
+    fn an_empty_roster_is_a_complete_roster() {
+        // A guild whose last member left emits one frame with an empty array. Treating that as
+        // "nothing to store" would leave the board showing the roster it had before.
+        let mut parts = HashMap::new();
+        let out = accumulate_roster(&mut parts, 1, 0, false, vec![]);
+
+        assert_eq!(out.expect("complete").len(), 0);
+    }
+}

sidecar/src/main.rs

@@ -46,7 +46,13 @@ use tracing_subscriber::EnvFilter;
 /// `vendor.listing.remove`, with the `GET /ruleset`, `/points` and `/market` reads that serve them
 /// from the store. Same shape as the v2 bump — the kinds are additive, the endpoints are not — and
 /// there is deliberately no feature-negotiation array: v3 implies all three kinds.
-pub const PROTOCOL_VERSION: u32 = 3;
+///
+/// v4 (Protocol 4): adds `guild.roster` and `guild.leave`, giving the guild board a real member list
+/// instead of the member *count* that was all v2 could express. Additive in the same way again — the
+/// kinds are new, `GET /guilds` grows a `roster` key, and nothing existing changed shape. This is the
+/// first bump that also needed a **store migration** (`guilds.members`), because it is the first to
+/// add a column to a table that already exists rather than a whole new table; see `store::migrate`.
+pub const PROTOCOL_VERSION: u32 = 4;
 
 // Not `#[tokio::main]`: on Windows the SCM dispatcher takes over this thread and starts the runtime
 // itself, on its own thread, once the service actually begins. The runtime is built by whichever

sidecar/src/store.rs

@@ -45,6 +45,7 @@ impl Store {
             .await?;
 
         sqlx::query(SCHEMA).execute(&pool).await?;
+        migrate(&pool).await?;
         info!(%path, "store ready");
         Ok(Self { pool })
     }
@@ -236,12 +237,61 @@ impl Store {
         Ok(())
     }
 
+    /// Upserts one guild's member roster (Protocol 4), keyed by guild id, touching **only** the
+    /// `members` column.
+    ///
+    /// Deliberately not a write to `json`. That column holds the verbatim `guild.update` line, and a
+    /// roster arriving as its own event must not clobber the snapshot — name, abbreviation, leader,
+    /// online count — that `guild.update` owns. Splitting the two writers across two columns of one
+    /// row is what lets both be plain upserts: neither needs to read the other's value first, so
+    /// there is no read-modify-write and no ordering requirement between the two kinds.
+    ///
+    /// The `INSERT` half is not redundant: a roster can arrive before the first `guild.update` for a
+    /// guild, and the row it creates then carries `'{}'` until that update fills it in.
+    pub async fn upsert_guild_roster(
+        &self,
+        id: i64,
+        members_json: &str,
+        t: i64,
+    ) -> anyhow::Result<()> {
+        sqlx::query(
+            "INSERT INTO guilds (id, name, json, updated_t, members) VALUES (?, NULL, '{}', ?, ?)
+             ON CONFLICT(id) DO UPDATE SET members = excluded.members, updated_t = excluded.updated_t",
+        )
+        .bind(id)
+        .bind(t)
+        .bind(members_json)
+        .execute(&self.pool)
+        .await?;
+        Ok(())
+    }
+
     /// The full guild board: every guild's latest snapshot, ordered by name.
+    ///
+    /// The roster is stored in its own column (see [`Self::upsert_guild_roster`]) and folded into
+    /// the projected object as `roster` here, at read time. A guild that has had a `guild.update`
+    /// but no `guild.roster` yet simply has no `roster` key, which is the honest representation of
+    /// "not known" and distinct from a guild whose roster is genuinely empty.
     pub async fn guilds_all(&self) -> anyhow::Result<Vec<Value>> {
-        let rows = sqlx::query("SELECT json FROM guilds ORDER BY name, id")
+        let rows = sqlx::query("SELECT json, members FROM guilds ORDER BY name, id")
             .fetch_all(&self.pool)
             .await?;
-        Ok(parse_json_column(rows))
+
+        Ok(rows
+            .into_iter()
+            .filter_map(|r| {
+                let mut v: Value = serde_json::from_str(&r.get::<String, _>("json")).ok()?;
+                let members: Option<String> = r.get("members");
+
+                if let (Some(obj), Some(raw)) = (v.as_object_mut(), members) {
+                    if let Ok(list) = serde_json::from_str::<Value>(&raw) {
+                        obj.insert("roster".into(), list);
+                    }
+                }
+
+                Some(v)
+            })
+            .collect())
     }
 
     // ---- governor board (Protocol 2.0) ----
@@ -513,6 +563,75 @@ impl Store {
     }
 }
 
+/// The schema version this build expects. Bump it, and add the matching arm to [`migrate`], for
+/// every change that `SCHEMA` alone cannot make to a database that already exists.
+const SCHEMA_VERSION: i64 = 1;
+
+/// Brings an existing database forward to [`SCHEMA_VERSION`].
+///
+/// `SCHEMA` is `CREATE TABLE IF NOT EXISTS` only, which is enough to *add a table* but cannot add a
+/// column to a table that is already there. Every schema change up to and including Protocol 3.0
+/// happened to add whole tables, so this never mattered and `ALTER TABLE` appears nowhere in this
+/// repo's history. `guilds.members` (Protocol 4) is the first column added to an existing table, so
+/// the mechanism has to exist now.
+///
+/// The version counter is SQLite's own `PRAGMA user_version`: an integer in the database header, so
+/// it needs no table of its own and cannot be separated from the file it describes. Each step runs
+/// in a transaction **together with** the bump that records it, so a step either lands completely or
+/// not at all, and an interrupted run resumes at the right place rather than re-applying half of one.
+///
+/// A failure here propagates and aborts startup, deliberately. A half-migrated store answers the
+/// website with confusing partial data, which is worse than being plainly absent — and the shard
+/// dials *out* to the sidecar, so a sidecar that refuses to start never stalls the game.
+async fn migrate(pool: &SqlitePool) -> anyhow::Result<()> {
+    let mut version: i64 = sqlx::query_scalar("PRAGMA user_version")
+        .fetch_one(pool)
+        .await?;
+
+    // A database written by a *newer* sidecar than this binary. This is not an error: every step
+    // here is additive, so a newer schema has only columns and tables an older reader ignores, and
+    // refusing to start would turn "roll the binary back" — a recovery path — into a dead end.
+    if version > SCHEMA_VERSION {
+        tracing::warn!(
+            found = version,
+            expected = SCHEMA_VERSION,
+            "store was written by a newer sidecar; continuing, as migrations are additive"
+        );
+        return Ok(());
+    }
+
+    while version < SCHEMA_VERSION {
+        let next = version + 1;
+        let mut tx = pool.begin().await?;
+
+        match next {
+            // Protocol 4: the guild board carries a member roster. Its own column rather than a
+            // field folded into `json`, because `json` holds the verbatim `guild.update` line and
+            // the two writers must not overwrite each other — see `upsert_guild_roster`.
+            1 => {
+                sqlx::query("ALTER TABLE guilds ADD COLUMN members TEXT")
+                    .execute(&mut *tx)
+                    .await?;
+            }
+            // Unreachable while SCHEMA_VERSION and this match are edited together, which is the
+            // point of failing loudly rather than silently leaving the counter short.
+            n => anyhow::bail!("no migration step defined for schema version {n}"),
+        }
+
+        // `PRAGMA` takes no bind parameters, so this is formatted — safe because `next` is an i64
+        // this loop produced, never anything from outside the process.
+        sqlx::query(&format!("PRAGMA user_version = {next}"))
+            .execute(&mut *tx)
+            .await?;
+
+        tx.commit().await?;
+        info!(version = next, "schema migration applied");
+        version = next;
+    }
+
+    Ok(())
+}
+
 fn parse_json_column(rows: Vec<sqlx::sqlite::SqliteRow>) -> Vec<Value> {
     rows.into_iter()
         .filter_map(|r| serde_json::from_str(&r.get::<String, _>("json")).ok())
@@ -611,3 +730,195 @@ CREATE TABLE IF NOT EXISTS ruleset (
     updated_t INTEGER NOT NULL
 );
 "#;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// A unique scratch database path. Matches `config`'s idiom — `std::env::temp_dir()` plus the
+    /// test name — so the cases stay independent under the parallel test runner.
+    fn scratch(name: &str) -> String {
+        let dir = std::env::temp_dir().join(format!("uo-link-store-test-{name}"));
+        let _ = std::fs::remove_dir_all(&dir);
+        std::fs::create_dir_all(&dir).expect("create scratch dir");
+        dir.join("uo-link.db").to_string_lossy().into_owned()
+    }
+
+    /// The `guilds` table exactly as a pre-Protocol-4 sidecar left it: no `members` column, and
+    /// `user_version` still 0. This is the shape a real operator's database is in before an update,
+    /// and the only starting point where the migration does anything.
+    async fn legacy_db(path: &str) -> SqlitePool {
+        let pool = SqlitePoolOptions::new()
+            .max_connections(1)
+            .connect_with(
+                SqliteConnectOptions::new()
+                    .filename(path)
+                    .create_if_missing(true),
+            )
+            .await
+            .expect("open legacy db");
+
+        sqlx::query(
+            "CREATE TABLE guilds (
+                 id        INTEGER PRIMARY KEY,
+                 name      TEXT,
+                 json      TEXT NOT NULL,
+                 updated_t INTEGER NOT NULL
+             )",
+        )
+        .execute(&pool)
+        .await
+        .expect("create legacy guilds table");
+
+        pool.close().await;
+        pool
+    }
+
+    async fn user_version(store: &Store) -> i64 {
+        sqlx::query_scalar("PRAGMA user_version")
+            .fetch_one(&store.pool)
+            .await
+            .expect("read user_version")
+    }
+
+    async fn guild_columns(store: &Store) -> Vec<String> {
+        sqlx::query("PRAGMA table_info(guilds)")
+            .fetch_all(&store.pool)
+            .await
+            .expect("table_info")
+            .into_iter()
+            .map(|r| r.get::<String, _>("name"))
+            .collect()
+    }
+
+    #[tokio::test]
+    async fn an_existing_pre_protocol_4_database_gains_the_members_column() {
+        // The case that matters: `SCHEMA`'s CREATE TABLE IF NOT EXISTS is a no-op against a table
+        // that is already there, so without `migrate` this database would never get the column and
+        // every roster write would fail against a live install.
+        let path = scratch("legacy-upgrade");
+        legacy_db(&path).await;
+
+        let store = Store::open(&path).await.expect("open migrates");
+
+        assert!(
+            guild_columns(&store).await.contains(&"members".to_string()),
+            "the migration must add guilds.members to a database that already had the table"
+        );
+        assert_eq!(user_version(&store).await, SCHEMA_VERSION);
+    }
+
+    #[tokio::test]
+    async fn a_fresh_database_lands_at_the_current_version() {
+        let path = scratch("fresh");
+        let store = Store::open(&path).await.expect("open");
+
+        assert!(guild_columns(&store).await.contains(&"members".to_string()));
+        assert_eq!(user_version(&store).await, SCHEMA_VERSION);
+    }
+
+    #[tokio::test]
+    async fn reopening_an_already_migrated_database_is_a_no_op() {
+        // Every sidecar restart re-runs this path, so a second run must not attempt the ALTER again
+        // — which would fail with "duplicate column name" and, since a migration failure aborts
+        // startup, would leave the sidecar unable to start at all after its first upgrade.
+        let path = scratch("idempotent");
+        legacy_db(&path).await;
+
+        Store::open(&path).await.expect("first open");
+        let store = Store::open(&path).await.expect("second open must succeed");
+
+        assert_eq!(user_version(&store).await, SCHEMA_VERSION);
+    }
+
+    #[tokio::test]
+    async fn a_roster_does_not_clobber_the_guild_update_snapshot() {
+        // The invariant the two-column split exists to give. `json` holds the verbatim guild.update
+        // line; if a roster write touched it, name/abbr/online would vanish from the board.
+        let path = scratch("no-clobber");
+        let store = Store::open(&path).await.expect("open");
+
+        store
+            .upsert_guild(
+                7,
+                Some("The Cartographers"),
+                r#"{"kind":"guild.update","id":7,"name":"The Cartographers","abbr":"MAP","members":2,"online":1}"#,
+                100,
+            )
+            .await
+            .expect("upsert guild");
+
+        store
+            .upsert_guild_roster(
+                7,
+                r#"[{"serial":"0x1","name":"Ada"},{"serial":"0x2","name":"Bo"}]"#,
+                200,
+            )
+            .await
+            .expect("upsert roster");
+
+        let guilds = store.guilds_all().await.expect("read board");
+        assert_eq!(guilds.len(), 1);
+        let g = &guilds[0];
+
+        assert_eq!(
+            g["name"], "The Cartographers",
+            "guild.update's name survived"
+        );
+        assert_eq!(g["abbr"], "MAP", "guild.update's abbr survived");
+        assert_eq!(g["online"], 1, "guild.update's online count survived");
+        assert_eq!(g["roster"].as_array().expect("roster is an array").len(), 2);
+        assert_eq!(g["roster"][0]["name"], "Ada");
+    }
+
+    #[tokio::test]
+    async fn the_two_writers_are_order_independent() {
+        // A roster can arrive before the first guild.update for a guild — on a reconnect the shard
+        // re-emits both and nothing orders them. Neither write may depend on the other's row.
+        let path = scratch("either-order");
+        let store = Store::open(&path).await.expect("open");
+
+        store
+            .upsert_guild_roster(9, r#"[{"serial":"0x3","name":"Cy"}]"#, 100)
+            .await
+            .expect("roster first");
+        store
+            .upsert_guild(
+                9,
+                Some("Late Arrivals"),
+                r#"{"kind":"guild.update","id":9,"name":"Late Arrivals","abbr":"LTE"}"#,
+                200,
+            )
+            .await
+            .expect("update second");
+
+        let guilds = store.guilds_all().await.expect("read board");
+        assert_eq!(guilds.len(), 1, "one row, not two");
+        assert_eq!(guilds[0]["name"], "Late Arrivals");
+        assert_eq!(guilds[0]["roster"].as_array().expect("roster").len(), 1);
+    }
+
+    #[tokio::test]
+    async fn a_guild_with_no_roster_yet_has_no_roster_key() {
+        // "Not known" and "known to be empty" are different, and the board must not conflate them:
+        // a website reading `roster: []` would render an empty roster as fact.
+        let path = scratch("absent-roster");
+        let store = Store::open(&path).await.expect("open");
+
+        store
+            .upsert_guild(
+                11,
+                Some("Unswept"),
+                r#"{"kind":"guild.update","id":11,"name":"Unswept"}"#,
+                100,
+            )
+            .await
+            .expect("upsert guild");
+
+        let guilds = store.guilds_all().await.expect("read board");
+        assert!(
+            guilds[0].get("roster").is_none(),
+            "a guild with no roster event must not grow a roster key"
+        );
+    }
+}