build: add generating compat.json (#9700)

This is to add the generation of `compat.json` as a release artifact. It
describes the runtime requirements of the release in question. The next
step is to have the upgrade server use this information to filter
releases provided to clients. This is per the discussion in #9656

---------

Co-authored-by: Ross Smith II <ross@smithii.com>

build.go

@@ -4,8 +4,8 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at https://mozilla.org/MPL/2.0/.
 
-//go:build ignore
-// +build ignore
+//go:build tools
+// +build tools
 
 package main
 
@@ -34,6 +34,8 @@ import (
 	"time"
 
 	buildpkg "github.com/syncthing/syncthing/lib/build"
+	"github.com/syncthing/syncthing/lib/upgrade"
+	"sigs.k8s.io/yaml"
 )
 
 var (
@@ -342,9 +344,11 @@ func runCommand(cmd string, target target) {
 
 	case "tar":
 		buildTar(target, tags)
+		writeCompatJSON()
 
 	case "zip":
 		buildZip(target, tags)
+		writeCompatJSON()
 
 	case "deb":
 		buildDeb(target)
@@ -1557,3 +1561,29 @@ func nextPatchVersion(ver string) string {
 	digits[len(digits)-1] = strconv.Itoa(n + 1)
 	return strings.Join(digits, ".")
 }
+
+func writeCompatJSON() {
+	bs, err := os.ReadFile("compat.yaml")
+	if err != nil {
+		log.Fatal("Reading compat.yaml:", err)
+	}
+
+	var entries []upgrade.ReleaseCompatibility
+	if err := yaml.Unmarshal(bs, &entries); err != nil {
+		log.Fatal("Parsing compat.yaml:", err)
+	}
+
+	rt := runtime.Version()
+	for _, e := range entries {
+		if !strings.HasPrefix(rt, e.Runtime) {
+			continue
+		}
+		bs, _ := json.MarshalIndent(e, "", "  ")
+		if err := os.WriteFile("compat.json", bs, 0o644); err != nil {
+			log.Fatal("Writing compat.json:", err)
+		}
+		return
+	}
+
+	log.Fatalf("runtime %v not found in compat.yaml", rt)
+}