From 755396d6fe77cbee6a32937fea9e58d5c175d047 Mon Sep 17 00:00:00 2001
From: Denton Gentry <dgentry@tailscale.com>
Date: Mon, 11 Jul 2022 20:36:52 -0700
Subject: [PATCH] tsweb: add Float expvar support in varz

We make assertions about stringification of 0.5. IEEE floating point and
all reasonable proprietary floating point can exactly represent 0.5.
We don't make assertions about other floating point values, too brittle
in tests.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>
---
 tsweb/tsweb.go      |  6 ++++++
 tsweb/tsweb_test.go | 28 ++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/tsweb/tsweb.go b/tsweb/tsweb.go
index 278b9d111..c834f280b 100644
--- a/tsweb/tsweb.go
+++ b/tsweb/tsweb.go
@@ -466,6 +466,12 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 		}
 		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, typ, name, v.Value())
 		return
+	case *expvar.Float:
+		if typ == "" {
+			typ = "gauge"
+		}
+		fmt.Fprintf(w, "# TYPE %s %s\n%s %v\n", name, typ, name, v.Value())
+		return
 	case *metrics.Set:
 		v.Do(func(kv expvar.KeyValue) {
 			writePromExpVar(w, name+"_", kv)
diff --git a/tsweb/tsweb_test.go b/tsweb/tsweb_test.go
index 8aef7073c..2de762a96 100644
--- a/tsweb/tsweb_test.go
+++ b/tsweb/tsweb_test.go
@@ -334,6 +334,9 @@ func TestVarzHandler(t *testing.T) {
 		t.Logf("Got: %s", rec.Body.Bytes())
 	})
 
+	half := new(expvar.Float)
+	half.Set(0.5)
+
 	tests := []struct {
 		name string
 		k    string // key name
@@ -358,6 +361,31 @@ func TestVarzHandler(t *testing.T) {
 			new(expvar.Int),
 			"# TYPE foo gauge\nfoo 0\n",
 		},
+		{
+			// For a float = 0.0, Prometheus client_golang outputs "0"
+			"float_zero",
+			"foo",
+			new(expvar.Float),
+			"# TYPE foo gauge\nfoo 0\n",
+		},
+		{
+			"float_point_5",
+			"foo",
+			half,
+			"# TYPE foo gauge\nfoo 0.5\n",
+		},
+		{
+			"float_with_type_counter",
+			"counter_foo",
+			half,
+			"# TYPE foo counter\nfoo 0.5\n",
+		},
+		{
+			"float_with_type_gauge",
+			"gauge_foo",
+			half,
+			"# TYPE foo gauge\nfoo 0.5\n",
+		},
 		{
 			"metrics_set",
 			"s",