cmd/tailscale/cli/jsonoutput: extract routecheck’s output format

In PR #19641, we added the `tailscale routecheck` command that
supports both `--format=json` and `--format=json-line`. To allow other
packages to import and unmarshal that JSON structure, this patch
exports that format in a new tsroutecheckjsonv0 package.

Updates #17366
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

cmd/tailscale/cli/routecheck.go

@@ -13,12 +13,13 @@
 	"slices"
 	"strings"
 	"text/tabwriter"
-	"time"
 
 	jsonv2 "github.com/go-json-experiment/json"
 	"github.com/go-json-experiment/json/jsontext"
 	"github.com/peterbourgon/ff/v3/ffcli"
 
+	"tailscale.com/cmd/tailscale/cli/jsonoutput"
+	"tailscale.com/cmd/tailscale/tsroutecheckjsonv0"
 	"tailscale.com/net/routecheck"
 	"tailscale.com/tstime"
 )
@@ -41,13 +42,14 @@ func init() {
 var routecheckFlagSet = func() *flag.FlagSet {
 	fs := newFlagSet("routecheck")
 	fs.BoolVar(&routecheckArgs.probe, "probe", false, "probe now to generate a new reachability report")
-	fs.StringVar(&routecheckArgs.format, "format", "", `output format: empty (for human-readable), "json" or "json-line"`)
+	fs.Var(&routecheckArgs.format, "format", `output format: empty (for human-readable), "json" or "json-line"`)
+	fs.Var(routecheckArgs.format.JSONBool(), "json", "output in JSON format")
 	return fs
 }()
 
 var routecheckArgs struct {
 	probe  bool
-	format string
+	format jsonoutput.Format
 }
 
 func runRoutecheck(ctx context.Context, args []string) error {
@@ -67,7 +69,7 @@ func runRoutecheck(ctx context.Context, args []string) error {
 
 func printRouteCheckReport(rp *routecheck.Report) error {
 	var enc *jsontext.Encoder
-	switch routecheckArgs.format {
+	switch routecheckArgs.format.String() {
 	case "":
 	case "json":
 		enc = jsontext.NewEncoder(Stdout, jsontext.WithIndent("\t"))
@@ -90,10 +92,7 @@ func printRouteCheckReport(rp *routecheck.Report) error {
 	}
 
 	if enc != nil {
-		out := struct {
-			Done   time.Time                   `json:"done"`
-			Routes routecheck.RoutablePrefixes `json:"routes"`
-		}{
+		out := tsroutecheckjsonv0.ReportResponse{
 			Done:   rp.Done,
 			Routes: routes,
 		}

cmd/tailscale/depaware.txt

@@ -184,6 +184,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/cmd/tailscale/cli/ffcomplete                   from tailscale.com/cmd/tailscale/cli
         tailscale.com/cmd/tailscale/cli/ffcomplete/internal          from tailscale.com/cmd/tailscale/cli/ffcomplete
         tailscale.com/cmd/tailscale/cli/jsonoutput                   from tailscale.com/cmd/tailscale/cli
+        tailscale.com/cmd/tailscale/tsroutecheckjsonv0               from tailscale.com/cmd/tailscale/cli
         tailscale.com/control/controlbase                            from tailscale.com/control/controlhttp+
         tailscale.com/control/controlhttp                            from tailscale.com/control/ts2021
         tailscale.com/control/controlhttp/controlhttpcommon          from tailscale.com/control/controlhttp

cmd/tailscale/tsroutecheckjsonv0/tsroutecheck.go

@@ -0,0 +1,28 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_routecheck
+
+// Package tsroutecheckjsonv0 provides types for unmarshalling the JSON output of the
+// "tailscale routecheck --json" command:
+//
+//   - [ReportResponse] will unmarshal the output of "tailscale routecheck --json"
+//
+// # WARNING: unstable
+//
+// Format is "v0" and is subject to change.
+// There is no guarantee of backwards or forwards compatibility.
+package tsroutecheckjsonv0
+
+import (
+	"time"
+
+	"tailscale.com/net/routecheck"
+)
+
+// ReportResponse is the JSON form of [routecheck.Report].
+// Experimental: This output is not yet stable: tailscale/tailscale#17366.
+type ReportResponse struct {
+	Done   time.Time                   `json:"done"`
+	Routes routecheck.RoutablePrefixes `json:"routes"`
+}