prober: run all probes once on initial registration.

Turns out, it's annoying to have to wait the entire interval
before getting any monitorable data, especially for very long
interval probes like hourly/daily checks.

Signed-off-by: David Anderson <danderson@tailscale.com>

prober/prober.go

@@ -126,6 +126,8 @@ func (p *Prober) probeLoop(ctx context.Context, name string, interval time.Durat
 		tick.Stop()
 	}()
 
+	// Do a first probe right away, so that the prober immediately exports results for everything.
+	p.runProbe(ctx, name, interval, fun)
 	for {
 		select {
 		case <-tick.Chan():

prober/prober_test.go

@@ -19,7 +19,7 @@ import (
 )
 
 const (
-	probeInterval        = 10 * time.Second // So expvars that are integer numbers of seconds change
+	probeInterval        = 8 * time.Second // So expvars that are integer numbers of seconds change
 	halfProbeInterval    = probeInterval / 2
 	quarterProbeInterval = probeInterval / 4
 	convergenceTimeout   = time.Second
@@ -58,6 +58,7 @@ func TestProberTiming(t *testing.T) {
 
 	waitActiveProbes(t, p, 1)
 
+	called()
 	notCalled()
 	clk.Advance(probeInterval + halfProbeInterval)
 	called()
@@ -106,6 +107,7 @@ func TestProberRun(t *testing.T) {
 	}
 
 	waitActiveProbes(t, p, startingProbes)
+	checkCnt(startingProbes)
 	clk.Advance(probeInterval + halfProbeInterval)
 	checkCnt(startingProbes)
 
@@ -135,19 +137,18 @@ func TestExpvar(t *testing.T) {
 	})
 
 	waitActiveProbes(t, p, 1)
-	clk.Advance(probeInterval + halfProbeInterval)
 
-	waitExpInt(t, p, "start_secs/probe", int((probeInterval + halfProbeInterval).Seconds()))
-	waitExpInt(t, p, "end_secs/probe", int((probeInterval + halfProbeInterval).Seconds()))
+	waitExpInt(t, p, "start_secs/probe", 0)
+	waitExpInt(t, p, "end_secs/probe", 0)
 	waitExpInt(t, p, "interval_secs/probe", int(probeInterval.Seconds()))
 	waitExpInt(t, p, "latency_millis/probe", int(aFewMillis.Milliseconds()))
 	waitExpInt(t, p, "result/probe", 0)
 
 	succeed.Set(true)
-	clk.Advance(probeInterval)
+	clk.Advance(probeInterval + halfProbeInterval)
 
-	waitExpInt(t, p, "start_secs/probe", int((probeInterval + probeInterval + halfProbeInterval).Seconds()))
-	waitExpInt(t, p, "end_secs/probe", int((probeInterval + probeInterval + halfProbeInterval).Seconds()))
+	waitExpInt(t, p, "start_secs/probe", int((probeInterval + halfProbeInterval).Seconds()))
+	waitExpInt(t, p, "end_secs/probe", int((probeInterval + halfProbeInterval).Seconds()))
 	waitExpInt(t, p, "interval_secs/probe", int(probeInterval.Seconds()))
 	waitExpInt(t, p, "latency_millis/probe", int(aFewMillis.Milliseconds()))
 	waitExpInt(t, p, "result/probe", 1)