build: add build tag noselfupdate

Allow downstream packaging to build rclone without selfupdate command:
$ go build -tags noselfupdate

Fixes #5187

cmd/cmd.go

@@ -553,7 +553,7 @@ func Main() {
 	setupRootCommand(Root)
 	AddBackendFlags()
 	if err := Root.Execute(); err != nil {
-		if strings.HasPrefix(err.Error(), "unknown command") {
+		if strings.HasPrefix(err.Error(), "unknown command") && selfupdateEnabled {
 			Root.PrintErrf("You could use '%s selfupdate' to get latest features.\n\n", Root.CommandPath())
 		}
 		log.Fatalf("Fatal error: %v", err)

cmd/selfupdate/help.go

@@ -1,3 +1,5 @@
+// +build !noselfupdate
+
 package selfupdate
 
 // Note: "|" will be replaced by backticks in the help string below

cmd/selfupdate/noselfupdate.go

@@ -0,0 +1,11 @@
+// +build noselfupdate
+
+package selfupdate
+
+import (
+	"github.com/rclone/rclone/lib/buildinfo"
+)
+
+func init() {
+	buildinfo.Tags = append(buildinfo.Tags, "noselfupdate")
+}

cmd/selfupdate/selfupdate.go

@@ -1,3 +1,5 @@
+// +build !noselfupdate
+
 package selfupdate
 
 import (

cmd/selfupdate/selfupdate_test.go

@@ -1,3 +1,5 @@
+// +build !noselfupdate
+
 package selfupdate
 
 import (

cmd/selfupdate/verify.go

@@ -1,3 +1,5 @@
+// +build !noselfupdate
+
 package selfupdate
 
 import (

cmd/selfupdate/writable_unix.go

@@ -1,4 +1,5 @@
 // +build !windows,!plan9,!js
+// +build !noselfupdate
 
 package selfupdate
 

cmd/selfupdate/writable_unsupported.go

@@ -1,4 +1,5 @@
 // +build plan9 js
+// +build !noselfupdate
 
 package selfupdate
 

cmd/selfupdate/writable_windows.go

@@ -1,4 +1,5 @@
 // +build windows
+// +build !noselfupdate
 
 package selfupdate
 

cmd/selfupdate_disabled.go

@@ -0,0 +1,5 @@
+// +build noselfupdate
+
+package cmd
+
+const selfupdateEnabled = false

cmd/selfupdate_enabled.go

@@ -0,0 +1,7 @@
+// +build !noselfupdate
+
+package cmd
+
+// This constant must be in the `cmd` package rather than `cmd/selfupdate`
+// to prevent build failure due to dependency loop.
+const selfupdateEnabled = true

lib/buildinfo/tags.go

@@ -5,8 +5,9 @@ import (
 	"strings"
 )
 
-// Tags contains slice of build tags
+// Tags contains slice of build tags.
 // The `cmount` tag is added by cmd/cmount/mount.go only if build is static.
+// The `noselfupdate` tag is added by cmd/selfupdate/noselfupdate.go
 // Other tags including `cgo` are detected in this package.
 var Tags []string