feature/logtail: pull logtail + netlog out to modular features

Removes 434 KB from the minimal Linux binary, or ~3%.

Primarily this comes from not linking in the zstd encoding code.

Fixes #17323

Change-Id: I0a90de307dfa1ad7422db7aa8b1b46c782bfaaf7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -202,7 +202,7 @@ type LocalBackend struct {
 	store                    ipn.StateStore  // non-nil; TODO(bradfitz): remove; use sys
 	dialer                   *tsdial.Dialer  // non-nil; TODO(bradfitz): remove; use sys
 	pushDeviceToken          syncs.AtomicValue[string]
-	backendLogID             logid.PublicID
+	backendLogID             logid.PublicID // or zero value if logging not in use
 	unregisterSysPolicyWatch func()
 	varRoot                  string         // or empty if SetVarRoot never called
 	logFlushFunc             func()         // or nil if SetLogFlusher wasn't called
@@ -456,6 +456,8 @@ type clientGen func(controlclient.Options) (controlclient.Client, error)
 // but is not actually running.
 //
 // If dialer is nil, a new one is made.
+//
+// The logID may be the zero value if logging is not in use.
 func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, loginFlags controlclient.LoginFlags) (_ *LocalBackend, err error) {
 	e := sys.Engine.Get()
 	store := sys.StateStore.Get()

ipn/localapi/localapi.go

@@ -28,6 +28,7 @@ import (
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/clientupdate"
 	"tailscale.com/envknob"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/health/healthmsg"
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn"
@@ -575,6 +576,15 @@ func (h *Handler) serveGoroutines(w http.ResponseWriter, r *http.Request) {
 func (h *Handler) serveLogTap(w http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 
+	if !buildfeatures.HasLogTail {
+		// TODO(bradfitz): separate out logtail tap functionality from upload
+		// functionality to make this possible? But seems unlikely people would
+		// want just this. They could "tail -f" or "journalctl -f" their logs
+		// themselves.
+		http.Error(w, "logtap not supported in this build", http.StatusNotImplemented)
+		return
+	}
+
 	// Require write access (~root) as the logs could contain something
 	// sensitive.
 	if !h.PermitWrite {