Unique subnets for exit nodes

server/lib/exitNodes/subnet.ts

@@ -1,30 +1,55 @@
-import { db, exitNodes } from "@server/db";
+import { db, exitNodes, Transaction } from "@server/db";
 import config from "@server/lib/config";
 import { findNextAvailableCidr } from "@server/lib/ip";
+import { lockManager } from "#dynamic/lib/lock";
 
-export async function getNextAvailableSubnet(): Promise<string> {
-    // Get all existing subnets from routes table
-    const existingAddresses = await db
-        .select({
-            address: exitNodes.address
-        })
-        .from(exitNodes);
-
-    const addresses = existingAddresses.map((a) => a.address);
-    let subnet = findNextAvailableCidr(
-        addresses,
-        config.getRawConfig().gerbil.block_size,
-        config.getRawConfig().gerbil.subnet_group
-    );
-    if (!subnet) {
-        throw new Error("No available subnets remaining in space");
+/**
+ * Reserves the next available exit node subnet.
+ *
+ * Exit node subnets must never overlap with one another - regardless of
+ * which org(s) they belong to - since HA exit nodes can end up routing for
+ * the same org. This acquires a lock that the caller MUST release (via the
+ * returned `release`) only after the chosen address has been durably
+ * persisted (e.g. after the enclosing transaction commits), otherwise
+ * concurrent callers can race and pick the same subnet.
+ */
+export async function getNextAvailableSubnet(
+    trx: Transaction | typeof db = db
+): Promise<{ value: string; release: () => Promise<void> }> {
+    const lockKey = "exit-node-subnet-allocation";
+    const acquired = await lockManager.acquireLockWithRetry(lockKey, 6000);
+    if (!acquired) {
+        throw new Error(`Failed to acquire lock: ${lockKey}`);
     }
+    const release = () => lockManager.releaseLock(lockKey, acquired);
 
-    // replace the last octet with 1
-    subnet =
-        subnet.split(".").slice(0, 3).join(".") +
-        ".1" +
-        "/" +
-        subnet.split("/")[1];
-    return subnet;
+    try {
+        // Get all existing subnets from routes table
+        const existingAddresses = await trx
+            .select({
+                address: exitNodes.address
+            })
+            .from(exitNodes);
+
+        const addresses = existingAddresses.map((a) => a.address);
+        let subnet = findNextAvailableCidr(
+            addresses,
+            config.getRawConfig().gerbil.block_size,
+            config.getRawConfig().gerbil.subnet_group
+        );
+        if (!subnet) {
+            throw new Error("No available subnets remaining in space");
+        }
+
+        // replace the last octet with 1
+        subnet =
+            subnet.split(".").slice(0, 3).join(".") +
+            ".1" +
+            "/" +
+            subnet.split("/")[1];
+        return { value: subnet, release };
+    } catch (e) {
+        await release();
+        throw e;
+    }
 }