fix(agent): skip first disk I/O sample of an interval when its window is too short

Right after agent start the hub requests stats immediately, and the first
sample of the interval was measured from the baseline taken at startup, only
a second or so earlier. A burst of startup I/O was then stored as the rate
for the whole minute, causing large spikes in disk I/O charts.

The first sample of an interval now must span at least half the interval.
Otherwise it only stores the snapshot, and the next sample is measured from it.

agent/disk_io_linux_test.go

@@ -75,17 +75,30 @@ func TestUpdateDiskIoTimeCounterWrap(t *testing.T) {
 	}
 }
 
-// The first sample of a cache interval has no snapshot of its own. It must
-// measure the time counters from the same baseline as the byte counters.
-func TestUpdateDiskIoFirstSampleOfInterval(t *testing.T) {
+// backdateDiskBaseline moves the baseline of a device into the past so the
+// next seeded sample of an interval spans d.
+func backdateDiskBaseline(a *Agent, name string, d time.Duration) {
+	b := a.diskBaseline[name]
+	b.at = time.Now().Add(-d)
+	a.diskBaseline[name] = b
+}
+
+// setupDiskstats points gopsutil at a temp dir and returns a writer for its diskstats file.
+func setupDiskstats(t *testing.T) func(line string) {
 	dir := t.TempDir()
 	t.Setenv("HOST_PROC", dir)
 	t.Setenv("HOST_SYS", dir)
 	t.Setenv("HOST_DEV", dir)
 	t.Setenv("HOST_RUN", dir)
-	writeDiskstats := func(line string) {
+	return func(line string) {
 		require.NoError(t, os.WriteFile(filepath.Join(dir, "diskstats"), []byte(line), 0o644))
 	}
+}
+
+// The first sample of a cache interval has no snapshot of its own. It must
+// measure the time counters from the same baseline as the byte counters.
+func TestUpdateDiskIoFirstSampleOfInterval(t *testing.T) {
+	writeDiskstats := setupDiskstats(t)
 
 	writeDiskstats("   8       0 sda 1000 0 20000 900 500 0 10000 700 0 400 0\n")
 	counters, err := disk.IOCounters("sda")
@@ -97,9 +110,7 @@ func TestUpdateDiskIoFirstSampleOfInterval(t *testing.T) {
 		diskPrev: map[uint16]map[string]prevDisk{},
 	}
 	a.initializeDiskIoStats(counters)
-
-	// updateDiskIo skips samples less than 100ms apart.
-	time.Sleep(150 * time.Millisecond)
+	backdateDiskBaseline(a, "sda", 60*time.Second)
 
 	// Deltas: read 300ms / 10 ops, write 400ms / 20 ops, io time 1200ms, weighted io 3000ms.
 	writeDiskstats("   8       0 sda 1010 0 21200 1200 520 0 10400 1100 0 1600 3000\n")
@@ -115,7 +126,7 @@ func TestUpdateDiskIoFirstSampleOfInterval(t *testing.T) {
 	assert.NotZero(t, fs.DiskIoStats[5], "weighted io")
 
 	// A second interval starts from the latest counters, not from the ones at start.
-	time.Sleep(150 * time.Millisecond)
+	backdateDiskBaseline(a, "sda", time.Second)
 	// Deltas: read 100ms / 10 ops, write 100ms / 20 ops.
 	writeDiskstats("   8       0 sda 1020 0 22400 1300 540 0 10800 1200 0 1800 3500\n")
 	a.updateDiskIo(1000, &stats)
@@ -123,3 +134,41 @@ func TestUpdateDiskIoFirstSampleOfInterval(t *testing.T) {
 	assert.InDelta(t, 10, fs.DiskIoStats[3], 0.01, "r_await")
 	assert.InDelta(t, 5, fs.DiskIoStats[4], 0.01, "w_await")
 }
+
+// Right after agent start the baseline is too recent to stand for a whole
+// interval. The first sample only stores a snapshot, and the next one is
+// measured from it.
+func TestUpdateDiskIoSkipsShortSeededSample(t *testing.T) {
+	writeDiskstats := setupDiskstats(t)
+
+	writeDiskstats("   8       0 sda 1000 0 20000 900 500 0 10000 700 0 400 0\n")
+	counters, err := disk.IOCounters("sda")
+	require.NoError(t, err)
+
+	fs := &system.FsStats{Root: true}
+	a := &Agent{
+		fsStats:  map[string]*system.FsStats{"sda": fs},
+		diskPrev: map[uint16]map[string]prevDisk{},
+	}
+	a.initializeDiskIoStats(counters)
+	backdateDiskBaseline(a, "sda", 2*time.Second)
+
+	// 1000 MB read in the 2s after start.
+	writeDiskstats("   8       0 sda 2000 0 2068000 900 500 0 10000 700 0 400 0\n")
+	var stats system.Stats
+	a.updateDiskIo(60000, &stats)
+
+	assert.Zero(t, fs.DiskReadBytes)
+	assert.Zero(t, stats.DiskIO[0])
+	require.Contains(t, a.diskPrev[60000], "sda", "snapshot is stored")
+
+	// Next sample is measured from the stored snapshot: 60 MB over 60s.
+	snap := a.diskPrev[60000]["sda"]
+	snap.at = time.Now().Add(-60 * time.Second)
+	a.diskPrev[60000]["sda"] = snap
+	writeDiskstats("   8       0 sda 3000 0 2190880 900 500 0 10000 700 0 400 0\n")
+	stats = system.Stats{}
+	a.updateDiskIo(60000, &stats)
+
+	assert.InDelta(t, 1_048_576, float64(stats.DiskIO[0]), 20_000)
+}