caddy: Add InstanceID() method

Caddy can now generate and persist its own instance ID, a UUID that is stored in
the data directory.

This makes it possible to differentiate it from other instances in a cluster.

caddy.go

@@ -33,6 +33,7 @@ import (
 	"time"
 
 	"github.com/caddyserver/certmagic"
+	"github.com/google/uuid"
 	"go.uber.org/zap"
 )
 
@@ -662,6 +663,26 @@ func ParseDuration(s string) (time.Duration, error) {
 	return time.ParseDuration(s)
 }
 
+// InstanceID returns the UUID for this instance, and generates one if it
+// does not already exist. The UUID is stored in the local data directory,
+// regardless of storage configuration, since each instance is intended to
+// have its own unique ID.
+func InstanceID() (uuid.UUID, error) {
+	uuidFilePath := filepath.Join(AppDataDir(), "instance.uuid")
+	uuidFileBytes, err := os.ReadFile(uuidFilePath)
+	if os.IsNotExist(err) {
+		uuid, err := uuid.NewRandom()
+		if err != nil {
+			return uuid, err
+		}
+		err = ioutil.WriteFile(uuidFilePath, []byte(uuid.String()), 0644)
+		return uuid, err
+	} else if err != nil {
+		return [16]byte{}, err
+	}
+	return uuid.ParseBytes(uuidFileBytes)
+}
+
 // GoModule returns the build info of this Caddy
 // build from debug.BuildInfo (requires Go modules).
 // If no version information is available, a non-nil

go.mod

@@ -10,6 +10,7 @@ require (
 	github.com/dustin/go-humanize v1.0.1-0.20200219035652-afde56e7acac
 	github.com/go-chi/chi v4.1.2+incompatible
 	github.com/google/cel-go v0.6.0
+	github.com/google/uuid v1.2.0
 	github.com/klauspost/compress v1.11.3
 	github.com/klauspost/cpuid/v2 v2.0.1
 	github.com/lucas-clemente/quic-go v0.19.3

go.sum

@@ -304,8 +304,9 @@ github.com/google/renameio v0.1.0/go.mod h1:KWCgfxg9yswjAJkECMjeO8J8rahYeXnNhOm4
 github.com/google/trillian v1.2.2-0.20190612132142-05461f4df60a/go.mod h1:YPmUVn5NGwgnDUgqlVyFGMTgaWlnSvH7W5p+NdOG8UA=
 github.com/google/trillian-examples v0.0.0-20190603134952-4e75ba15216c/go.mod h1:WgL3XZ3pA8/9cm7yxqWrZE6iZkESB2ItGxy5Fo6k2lk=
 github.com/google/uuid v1.0.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
-github.com/google/uuid v1.1.1 h1:Gkbcsh/GbpXz7lPftLA3P6TYMwjCLYm83jiFQZF/3gY=
 github.com/google/uuid v1.1.1/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/google/uuid v1.2.0 h1:qJYtXnJRWmpe7m/3XlyhrsLrEURqHRM2kxzoxXqyUDs=
+github.com/google/uuid v1.2.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/googleapis/gax-go v2.0.0+incompatible h1:j0GKcs05QVmm7yesiZq2+9cxHkNK9YM6zKx4D2qucQU=
 github.com/googleapis/gax-go v2.0.0+incompatible/go.mod h1:SFVmujtThgffbyetf+mdk2eWhX2bMyUtNHzFKcPA9HY=
 github.com/googleapis/gax-go/v2 v2.0.3/go.mod h1:LLvjysVCY1JZeum8Z6l8qUty8fiNwE08qbEPm1M08qg=