fix: preserve bounded map updates

src/handlers/activity.ts

@@ -28,14 +28,7 @@ export function handleSessionDiff(e: EventSessionDiff, ctx: HandlerContext) {
   const deltaAdded = totalAdded - prev.additions
   const deltaRemoved = totalRemoved - prev.deletions
   const nextTotals = { additions: totalAdded, deletions: totalRemoved }
-  if (ctx.sessionDiffTotals.has(sessionID)) {
-    // Existing session: update in place. Calling setBoundedMap on a full map would
-    // evict an unrelated session here, and that session's next session.diff would
-    // be treated as first-seen — reintroducing the cumulative double-count bug.
-    ctx.sessionDiffTotals.set(sessionID, nextTotals)
-  } else {
-    setBoundedMap(ctx.sessionDiffTotals, sessionID, nextTotals)
-  }
+  setBoundedMap(ctx.sessionDiffTotals, sessionID, nextTotals)
 
   const baseAttrs = { ...ctx.commonAttrs, "session.id": sessionID }
 

src/util.ts

@@ -16,7 +16,7 @@ export function errorSummary(err: { name: string; data?: unknown } | undefined):
  * has reached `MAX_PENDING` capacity to prevent unbounded memory growth.
  */
 export function setBoundedMap<K, V>(map: Map<K, V>, key: K, value: V) {
-  if (map.size >= MAX_PENDING) {
+  if (!map.has(key) && map.size >= MAX_PENDING) {
     const [firstKey] = map.keys()
     if (firstKey !== undefined) map.delete(firstKey)
   }

tests/util.test.ts

@@ -62,6 +62,19 @@ describe("setBoundedMap", () => {
     expect(map.get("a")).toBe(2)
     expect(map.size).toBe(1)
   })
+
+  test("updates an existing key at capacity without evicting another entry", () => {
+    const map = new Map<string, number>()
+    for (let i = 0; i < MAX_PENDING; i++) {
+      setBoundedMap(map, `key-${i}`, i)
+    }
+
+    setBoundedMap(map, "key-10", 1000)
+
+    expect(map.size).toBe(MAX_PENDING)
+    expect(map.get("key-10")).toBe(1000)
+    expect(map.has("key-0")).toBe(true)
+  })
 })
 
 describe("isMetricEnabled", () => {