Use header-only usage server timing

frontend/src/features/usage/composables/__tests__/useServerClock.spec.ts

@@ -1,15 +1,20 @@
 import { describe, expect, it } from 'vitest'
-import { calculateServerClockOffsetMs, useServerClock } from '../useServerClock'
+import {
+  calculateServerClockOffsetMs,
+  shouldUseServerClockSample,
+  useServerClock
+} from '../useServerClock'
 
 describe('useServerClock', () => {
-  it('calculates offset from the request midpoint', () => {
+  it('calculates offset from the response receive time', () => {
     const offset = calculateServerClockOffsetMs({
       server_now_unix_ms: 10_500,
       client_send_unix_ms: 20_000,
       client_receive_unix_ms: 20_200,
+      round_trip_ms: 200,
     })
 
-    expect(offset).toBe(-9_600)
+    expect(offset).toBe(-9_700)
   })
 
   it('ignores missing or invalid timing samples', () => {
@@ -18,11 +23,19 @@ describe('useServerClock', () => {
       server_now_unix_ms: Number.NaN,
       client_send_unix_ms: 20_000,
       client_receive_unix_ms: 20_200,
+      round_trip_ms: 200,
     })).toBeNull()
     expect(calculateServerClockOffsetMs({
       server_now_unix_ms: 10_500,
       client_send_unix_ms: 20_200,
       client_receive_unix_ms: 20_000,
+      round_trip_ms: 200,
+    })).toBeNull()
+    expect(calculateServerClockOffsetMs({
+      server_now_unix_ms: 10_500,
+      client_send_unix_ms: 20_000,
+      client_receive_unix_ms: 20_200,
+      round_trip_ms: Number.NaN,
     })).toBeNull()
   })
 
@@ -33,10 +46,57 @@ describe('useServerClock', () => {
       server_now_unix_ms: 10_500,
       client_send_unix_ms: 20_000,
       client_receive_unix_ms: 20_200,
+      round_trip_ms: 200,
     })
     clock.updateServerClockOffset(undefined)
 
     expect(clock.hasServerClockOffset.value).toBe(true)
-    expect(clock.serverClockOffsetMs.value).toBe(-9_600)
+    expect(clock.serverClockOffsetMs.value).toBe(-9_700)
+    expect(clock.serverClockSampleRoundTripMs.value).toBe(200)
+  })
+
+  it('does not let a much slower sample overwrite a better clock offset', () => {
+    const clock = useServerClock()
+
+    clock.updateServerClockOffset({
+      server_now_unix_ms: 10_500,
+      client_send_unix_ms: 20_000,
+      client_receive_unix_ms: 20_050,
+      round_trip_ms: 50,
+    })
+    clock.updateServerClockOffset({
+      server_now_unix_ms: 20_500,
+      client_send_unix_ms: 30_000,
+      client_receive_unix_ms: 30_500,
+      round_trip_ms: 500,
+    })
+
+    expect(clock.serverClockOffsetMs.value).toBe(-9_550)
+    expect(clock.serverClockSampleRoundTripMs.value).toBe(50)
+  })
+
+  it('accepts a faster sample after an initial slow sample', () => {
+    const clock = useServerClock()
+
+    clock.updateServerClockOffset({
+      server_now_unix_ms: 10_500,
+      client_send_unix_ms: 20_000,
+      client_receive_unix_ms: 20_500,
+      round_trip_ms: 500,
+    })
+    clock.updateServerClockOffset({
+      server_now_unix_ms: 20_500,
+      client_send_unix_ms: 30_000,
+      client_receive_unix_ms: 30_050,
+      round_trip_ms: 50,
+    })
+
+    expect(clock.serverClockOffsetMs.value).toBe(-9_550)
+    expect(clock.serverClockSampleRoundTripMs.value).toBe(50)
+  })
+
+  it('allows small RTT regressions so the offset can stay fresh', () => {
+    expect(shouldUseServerClockSample(140, 50)).toBe(true)
+    expect(shouldUseServerClockSample(151, 50)).toBe(false)
   })
 })