types/logger: add AsJSON

Printing out JSON representation things in log output is pretty common.

Updates #cleanup

Change-Id: Ife2d2e321a18e6e1185efa8b699a23061ac5e5a4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -21,6 +21,7 @@
 	"tailscale.com/tailcfg"
 	"tailscale.com/tka"
 	"tailscale.com/tstest"
+	"tailscale.com/types/logger"
 	"tailscale.com/types/persist"
 	"tailscale.com/types/preftype"
 	"tailscale.com/util/cmpx"
@@ -1151,18 +1152,13 @@ func TestUpdatePrefs(t *testing.T) {
 				justEditMP.Prefs = ipn.Prefs{} // uninteresting
 			}
 			if !reflect.DeepEqual(justEditMP, tt.wantJustEditMP) {
-				t.Logf("justEditMP != wantJustEditMP; following diff omits the Prefs field, which was \n%v", asJSON(oldEditPrefs))
+				t.Logf("justEditMP != wantJustEditMP; following diff omits the Prefs field, which was \n%v", logger.AsJSON(oldEditPrefs))
 				t.Fatalf("justEditMP: %v\n\n: ", cmp.Diff(justEditMP, tt.wantJustEditMP, cmpIP))
 			}
 		})
 	}
 }
 
-func asJSON(v any) string {
-	b, _ := json.MarshalIndent(v, "", "\t")
-	return string(b)
-}
-
 var cmpIP = cmp.Comparer(func(a, b netip.Addr) bool {
 	return a == b
 })

cmd/tailscale/cli/serve_test.go

@@ -22,6 +22,7 @@
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/logger"
 )
 
 func TestCleanMountPoint(t *testing.T) {
@@ -737,8 +738,8 @@ type step struct {
 			got = lc.config
 		}
 		if !reflect.DeepEqual(got, st.want) {
-			t.Fatalf("[%d] %v: bad state. got:\n%s\n\nwant:\n%s\n",
-				i, st.command, asJSON(got), asJSON(st.want))
+			t.Fatalf("[%d] %v: bad state. got:\n%v\n\nwant:\n%v\n",
+				i, st.command, logger.AsJSON(got), logger.AsJSON(st.want))
 			// NOTE: asJSON will omit empty fields, which might make
 			// result in bad state got/want diffs being the same, even
 			// though the actual state is different. Use below to debug:

control/controlclient/map_test.go

@@ -4,7 +4,6 @@
 package controlclient
 
 import (
-	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -21,6 +20,7 @@
 	"tailscale.com/tstest"
 	"tailscale.com/tstime"
 	"tailscale.com/types/key"
+	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
 	"tailscale.com/types/ptr"
 	"tailscale.com/util/mak"
@@ -637,7 +637,7 @@ type step struct {
 			for stepi, s := range tt.steps {
 				nm := ms.netmapForResponse(&tailcfg.MapResponse{DERPMap: s.got})
 				if !reflect.DeepEqual(nm.DERPMap, s.want) {
-					t.Errorf("unexpected result at step index %v; got: %s", stepi, must.Get(json.Marshal(nm.DERPMap)))
+					t.Errorf("unexpected result at step index %v; got: %s", stepi, logger.AsJSON(nm.DERPMap))
 				}
 			}
 		})
@@ -740,17 +740,15 @@ func TestPeerChangeDiff(t *testing.T) {
 			pc, ok := peerChangeDiff(tt.a.View(), tt.b)
 			if tt.wantEqual {
 				if !ok || pc != nil {
-					t.Errorf("got (%p, %v); want (nil, true); pc=%v", pc, ok, must.Get(json.Marshal(pc)))
+					t.Errorf("got (%p, %v); want (nil, true); pc=%v", pc, ok, logger.AsJSON(pc))
 				}
 				return
 			}
 			if (pc != nil) != ok {
 				t.Fatalf("inconsistent ok=%v, pc=%p", ok, pc)
 			}
-			gotj := must.Get(json.Marshal(pc))
-			wantj := must.Get(json.Marshal(tt.want))
-			if !bytes.Equal(gotj, wantj) {
-				t.Errorf("mismatch\n got: %s\nwant: %s\n", gotj, wantj)
+			if !reflect.DeepEqual(pc, tt.want) {
+				t.Errorf("mismatch\n got: %v\nwant: %v\n", logger.AsJSON(pc), logger.AsJSON(tt.want))
 			}
 		})
 	}
@@ -762,7 +760,7 @@ func TestPeerChangeDiffAllocs(t *testing.T) {
 	n := testing.AllocsPerRun(10000, func() {
 		diff, ok := peerChangeDiff(a.View(), b)
 		if !ok || diff != nil {
-			t.Fatalf("unexpected result: (%s, %v)", must.Get(json.Marshal(diff)), ok)
+			t.Fatalf("unexpected result: (%s, %v)", logger.AsJSON(diff), ok)
 		}
 	})
 	if n != 0 {

ipn/ipnlocal/profiles_test.go

@@ -4,7 +4,6 @@
 package ipnlocal
 
 import (
-	"encoding/json"
 	"fmt"
 	"os/user"
 	"strconv"
@@ -312,10 +311,6 @@ type step struct {
 	}
 }
 
-func asJSON(v any) []byte {
-	return must.Get(json.MarshalIndent(v, "", "  "))
-}
-
 // TestProfileManagement tests creating, loading, and switching profiles.
 func TestProfileManagement(t *testing.T) {
 	store := new(mem.Store)

types/logger/logger.go

@@ -353,3 +353,23 @@ func LogfCloser(logf Logf) (newLogf Logf, close func()) {
 	}
 	return newLogf, close
 }
+
+// AsJSON returns a formatter that formats v as JSON. The value is suitable to
+// passing to a regular %v printf argument. (%s is not required)
+//
+// If json.Marshal returns an error, the output is "%%!JSON-ERROR:" followed by
+// the error string.
+func AsJSON(v any) fmt.Formatter {
+	return asJSONResult{v}
+}
+
+type asJSONResult struct{ v any }
+
+func (a asJSONResult) Format(s fmt.State, verb rune) {
+	v, err := json.Marshal(a.v)
+	if err != nil {
+		fmt.Fprintf(s, "%%!JSON-ERROR:%v", err)
+		return
+	}
+	s.Write(v)
+}

types/logger/logger_test.go

@@ -221,3 +221,33 @@ func TestJSON(t *testing.T) {
 		t.Errorf("mismatch\n got: %q\nwant: %q\n", got, want)
 	}
 }
+
+func TestAsJSON(t *testing.T) {
+	got := fmt.Sprintf("got %v", AsJSON(struct {
+		Foo string
+		Bar int
+	}{"hi", 123}))
+	const want = `got {"Foo":"hi","Bar":123}`
+	if got != want {
+		t.Errorf("got %#q; want %#q", got, want)
+	}
+
+	got = fmt.Sprintf("got %v", AsJSON(func() {}))
+	const wantErr = `got %!JSON-ERROR:json: unsupported type: func()`
+	if got != wantErr {
+		t.Errorf("for marshal error, got %#q; want %#q", got, wantErr)
+	}
+
+	var buf bytes.Buffer
+	n := int(testing.AllocsPerRun(1000, func() {
+		buf.Reset()
+		fmt.Fprintf(&buf, "got %v", AsJSON("hi"))
+	}))
+	if n > 2 {
+		// the JSON AsMarshal itself + boxing
+		// the asJSONResult into an interface (which needs
+		// to happen at some point to get to fmt, so might
+		// as well return an interface from AsJSON))
+		t.Errorf("allocs = %v; want max 2", n)
+	}
+}