perf: coalesce polling snapshots

backend/src/http.ts

@@ -1,3 +1,5 @@
+import { createHash } from "node:crypto";
+
 import type { Server } from "bun";
 
 import { type AuthSession, type AuthUser, getAuthSessionFromSessionId } from "./auth.ts";
@@ -56,6 +58,38 @@ export function json(data: unknown, init: BunResponseInit = {}): Response {
     return Response.json(data, { ...init, headers });
 }
 
+function hasMatchingEtag(request: Request, etag: string): boolean {
+    const candidates = request.headers.get("if-none-match");
+    if (!candidates) return false;
+    const normalizedEtag = etag.replace(/^W\//u, "");
+    return candidates.split(",").some((candidate) => {
+        const normalizedCandidate = candidate.trim().replace(/^W\//u, "");
+        return normalizedCandidate === "*" || normalizedCandidate === normalizedEtag;
+    });
+}
+
+/**
+ * Serves private JSON with a strong validator so repeated polling can reuse the
+ * browser's response body even when it still revalidates with the backend.
+ */
+export function jsonWithEtag(
+    request: Request,
+    data: unknown,
+    init: BunResponseInit = {}
+): Response {
+    const body = JSON.stringify(data);
+    const etag = `"${createHash("sha256").update(body).digest("base64url")}"`;
+    const headers = new Headers(init.headers);
+    headers.set("Cache-Control", "private, no-cache");
+    headers.set("Content-Type", "application/json");
+    headers.set("ETag", etag);
+    headers.set("Vary", "Cookie, Authorization");
+    if (hasMatchingEtag(request, etag)) {
+        return new Response(undefined, { ...init, headers, status: 304 });
+    }
+    return new Response(body, { ...init, headers });
+}
+
 export function text(
     body: string,
     {

backend/src/lib/coalescedSnapshot.ts

@@ -0,0 +1,238 @@
+export interface CoalescedSnapshotMetrics {
+    activeLoads: number;
+    averageLoadMs: number;
+    coalescedHits: number;
+    failures: number;
+    freshHits: number;
+    lastLoadMs: number;
+    loads: number;
+    name: string;
+    requests: number;
+    staleHits: number;
+}
+
+interface CoalescedSnapshotOptions<T> {
+    freshForMs: number;
+    load: () => Promise<T>;
+    name: string;
+    now?: () => number;
+    retryAfterMs?: number;
+    staleForMs: number;
+}
+
+interface SnapshotEntry<T> {
+    inFlight?: Promise<T>;
+    loadedAt?: number;
+    nextRetryAt?: number;
+    value?: T;
+}
+
+interface MutableSnapshotMetrics {
+    activeLoads: number;
+    coalescedHits: number;
+    failures: number;
+    freshHits: number;
+    lastLoadMs: number;
+    loads: number;
+    requests: number;
+    staleHits: number;
+    totalLoadMs: number;
+}
+
+const snapshotRegistry = new Map<
+    string,
+    {
+        metrics: MutableSnapshotMetrics;
+        name: string;
+        reset: () => void;
+    }
+>();
+
+function emptyMetrics(): MutableSnapshotMetrics {
+    return {
+        activeLoads: 0,
+        coalescedHits: 0,
+        failures: 0,
+        freshHits: 0,
+        lastLoadMs: 0,
+        loads: 0,
+        requests: 0,
+        staleHits: 0,
+        totalLoadMs: 0,
+    };
+}
+
+function positiveDuration(value: number, name: string): number {
+    if (!Number.isFinite(value) || value <= 0) {
+        throw new TypeError(`${name} must be a positive finite number`);
+    }
+    return value;
+}
+
+/**
+ * Shares one read-only producer across callers and keeps a bounded stale value
+ * available while the next snapshot is refreshed.
+ */
+export class CoalescedSnapshot<T> {
+    #entry: SnapshotEntry<T> = {};
+    #metricsGeneration = 0;
+    readonly #freshForMs: number;
+    readonly #load: () => Promise<T>;
+    readonly #metrics = emptyMetrics();
+    readonly #name: string;
+    readonly #now: () => number;
+    readonly #retryAfterMs: number;
+    readonly #staleForMs: number;
+
+    constructor(options: CoalescedSnapshotOptions<T>) {
+        const name = options.name.trim();
+        if (!name) throw new TypeError("Snapshot name is required");
+        this.#name = name;
+        this.#freshForMs = positiveDuration(options.freshForMs, "freshForMs");
+        this.#staleForMs = positiveDuration(options.staleForMs, "staleForMs");
+        if (this.#staleForMs < this.#freshForMs) {
+            throw new TypeError("staleForMs must be greater than or equal to freshForMs");
+        }
+        this.#retryAfterMs = positiveDuration(
+            options.retryAfterMs ?? options.freshForMs,
+            "retryAfterMs"
+        );
+        this.#load = options.load;
+        this.#now = options.now ?? Date.now;
+        snapshotRegistry.set(name, {
+            metrics: this.#metrics,
+            name,
+            reset: () => this.reset(),
+        });
+    }
+
+    #startLoad(entry: SnapshotEntry<T>): Promise<T> {
+        const startedAt = this.#now();
+        const metricsGeneration = this.#metricsGeneration;
+        this.#metrics.activeLoads += 1;
+        this.#metrics.loads += 1;
+
+        const load = async () => {
+            try {
+                const value = await this.#load();
+                if (this.#entry === entry) {
+                    entry.loadedAt = this.#now();
+                    entry.nextRetryAt = undefined;
+                    entry.value = value;
+                }
+                return value;
+            } catch (error) {
+                if (this.#metricsGeneration === metricsGeneration) {
+                    this.#metrics.failures += 1;
+                }
+                if (this.#entry === entry) {
+                    entry.nextRetryAt = this.#now() + this.#retryAfterMs;
+                }
+                throw error;
+            }
+        };
+        const inFlight = load();
+        entry.inFlight = inFlight;
+        const recordSettlement = async () => {
+            try {
+                await inFlight;
+            } catch {
+                // The reader or background refresh handler observes the load error.
+            } finally {
+                const elapsedMs = Math.max(0, this.#now() - startedAt);
+                if (this.#metricsGeneration === metricsGeneration) {
+                    this.#metrics.activeLoads = Math.max(
+                        0,
+                        this.#metrics.activeLoads - 1
+                    );
+                    this.#metrics.lastLoadMs = elapsedMs;
+                    this.#metrics.totalLoadMs += elapsedMs;
+                }
+                if (this.#entry === entry && entry.inFlight === inFlight) {
+                    entry.inFlight = undefined;
+                }
+            }
+        };
+        void recordSettlement();
+        return inFlight;
+    }
+
+    /** Returns the shared snapshot for this fixed read path. */
+    async read(): Promise<T> {
+        this.#metrics.requests += 1;
+        const entry = this.#entry;
+        const now = this.#now();
+        const age =
+            entry.value === undefined || entry.loadedAt === undefined
+                ? Infinity
+                : Math.max(0, now - entry.loadedAt);
+
+        if (age <= this.#freshForMs && entry.value !== undefined) {
+            this.#metrics.freshHits += 1;
+            return entry.value;
+        }
+
+        if (age <= this.#staleForMs && entry.value !== undefined) {
+            this.#metrics.staleHits += 1;
+            if (entry.inFlight) {
+                this.#metrics.coalescedHits += 1;
+            } else if ((entry.nextRetryAt ?? 0) <= now) {
+                const refresh = this.#startLoad(entry);
+                void refresh.catch((error: unknown) => {
+                    console.warn(
+                        `[PollingSnapshot:${this.#name}] Background refresh failed`,
+                        error
+                    );
+                });
+            }
+            return entry.value;
+        }
+
+        if (entry.inFlight) {
+            this.#metrics.coalescedHits += 1;
+            return await entry.inFlight;
+        }
+
+        return await this.#startLoad(entry);
+    }
+
+    /** Invalidates the visible value and detaches any older in-flight producer. */
+    invalidate(): void {
+        this.#entry = {};
+    }
+
+    /** Clears cached values and counters. Intended for deterministic tests. */
+    reset(): void {
+        this.invalidate();
+        this.#metricsGeneration += 1;
+        Object.assign(this.#metrics, emptyMetrics());
+    }
+}
+
+/** Returns process-local coalescing telemetry without cached payloads or keys. */
+export function getCoalescedSnapshotMetrics(): CoalescedSnapshotMetrics[] {
+    return snapshotRegistry
+        .values()
+        .map(({ metrics, name }) => ({
+            activeLoads: metrics.activeLoads,
+            averageLoadMs:
+                metrics.loads === 0
+                    ? 0
+                    : Math.round((metrics.totalLoadMs / metrics.loads) * 100) / 100,
+            coalescedHits: metrics.coalescedHits,
+            failures: metrics.failures,
+            freshHits: metrics.freshHits,
+            lastLoadMs: metrics.lastLoadMs,
+            loads: metrics.loads,
+            name,
+            requests: metrics.requests,
+            staleHits: metrics.staleHits,
+        }))
+        .toArray()
+        .toSorted((left, right) => left.name.localeCompare(right.name));
+}
+
+/** Resets every registered snapshot between tests. */
+export function resetCoalescedSnapshotsForTests(): void {
+    for (const snapshot of snapshotRegistry.values()) snapshot.reset();
+}

backend/src/lib/processes.ts

@@ -16,10 +16,23 @@ export interface RunProcessResult {
     stdout: string;
 }
 
+export interface ChildProcessMetrics {
+    active: number;
+    failed: number;
+    started: number;
+    succeeded: number;
+}
+
 export type BunProcess = ReturnType<typeof Bun.spawn>;
 
 const DEFAULT_MAX_BUFFER = 10 * 1024 * 1024;
 const DEFAULT_FORCE_KILL_GRACE_MS = 3000;
+const childProcessMetrics: ChildProcessMetrics = {
+    active: 0,
+    failed: 0,
+    started: 0,
+    succeeded: 0,
+};
 
 /** Returns the absolute Bun executable already running the Dashboard process. */
 export function resolveBunExecutable(): string {
@@ -64,7 +77,7 @@ export function spawnProcess(
         throw new DOMException("Process aborted before start", "AbortError");
     }
     const command = scopedJobProcessCommand(executable, arguments_);
-    return Bun.spawn({
+    const process = Bun.spawn({
         cmd: [command.executable, ...command.arguments],
         cwd: options.cwd,
         detached: options.detached ?? true,
@@ -73,6 +86,28 @@ export function spawnProcess(
         stdin: "ignore",
         stdout: "pipe",
     });
+    childProcessMetrics.active += 1;
+    childProcessMetrics.started += 1;
+    void process.exited
+        .then((code) => {
+            if (code === 0) {
+                childProcessMetrics.succeeded += 1;
+            } else {
+                childProcessMetrics.failed += 1;
+            }
+        })
+        .catch(() => {
+            childProcessMetrics.failed += 1;
+        })
+        .finally(() => {
+            childProcessMetrics.active = Math.max(0, childProcessMetrics.active - 1);
+        });
+    return process;
+}
+
+/** Returns aggregate process telemetry without command arguments or environment data. */
+export function getChildProcessMetrics(): ChildProcessMetrics {
+    return { ...childProcessMetrics };
 }
 
 export function killProcessGroup(process_: BunProcess, signal: NodeJS.Signals): void {