native: count frames on the pprof server

docs/native-window.md

@@ -56,8 +56,9 @@ from `main` either drowns in merges or freezes the web app for months.
   `task gallery` builds it into `bin/` and opens it.
 - **`LICH_NATIVE_PPROF=127.0.0.1:6060`**, set where `lich native` starts, has `lich-native` serve Go's
   runtime profiles (`net/http/pprof`) on that address, for a heap or CPU profile of a real window:
-  `go tool pprof http://127.0.0.1:6060/debug/pprof/heap`. Only a loopback address is accepted, and any
-  process on the machine can read what the profiles carry.
+  `go tool pprof http://127.0.0.1:6060/debug/pprof/heap`. `/debug/vars` on the same address carries `frames`,
+  the frames drawn since the window opened: two reads a few seconds apart give the window's frame rate. Only a
+  loopback address is accepted, and any process on the machine can read what the profiles carry.
 - **`GOGC`** left unset, `lich-native` collects at 50 instead of Go's 100: a window with many sessions keeps a
   live heap of hundreds of MB, and at 100 half the resident size is slack the system swaps out. Set `GOGC` to
   choose another value.

native/cmd/lich-native/main.go

@@ -376,6 +376,7 @@ func run(w *app.Window, rt backend.Runtime) error {
 				}
 			}
 			e.Frame(gtx.Ops)
+			frames.Add(1)
 		}
 	}
 }

native/cmd/lich-native/pprof.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"expvar"
 	"fmt"
 	"log"
 	"net"
@@ -29,9 +30,16 @@ func pprofListener(addr string) (net.Listener, error) {
 	return ln, nil
 }
 
-// servePprof answers net/http/pprof's endpoints on ln until it closes.
+// frames counts the frames the window drew, served at /debug/vars beside
+// the profiles: two reads a few seconds apart give the window's frame rate,
+// which a profile alone does not show.
+var frames = expvar.NewInt("frames")
+
+// servePprof answers net/http/pprof's endpoints and /debug/vars on ln until
+// it closes.
 func servePprof(ln net.Listener) {
 	mux := http.NewServeMux()
+	mux.Handle("/debug/vars", expvar.Handler())
 	mux.HandleFunc("/debug/pprof/", pprof.Index)
 	mux.HandleFunc("/debug/pprof/cmdline", pprof.Cmdline)
 	mux.HandleFunc("/debug/pprof/profile", pprof.Profile)

native/cmd/lich-native/pprof_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"io"
 	"net/http"
 	"strings"
@@ -36,3 +37,25 @@ func TestPprofServesTheHeapOnLoopback(t *testing.T) {
 		t.Fatalf("status %d, body starts %.80q", resp.StatusCode, body)
 	}
 }
+
+func TestPprofServesTheFrameCount(t *testing.T) {
+	ln, err := pprofListener("127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+	go servePprof(ln)
+	frames.Add(3)
+	resp, err := http.Get("http://" + ln.Addr().String() + "/debug/vars")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+	var vars struct{ Frames int64 }
+	if err := json.NewDecoder(resp.Body).Decode(&vars); err != nil {
+		t.Fatal(err)
+	}
+	if vars.Frames < 3 {
+		t.Fatalf("frames = %d, want the 3 drawn", vars.Frames)
+	}
+}