ipn,tailfs: tie TailFS share configuration to user profile

Previously, the configuration of which folders to share persisted across
profile changes. Now, it is tied to the user's profile.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

ipn/ipnlocal/local.go

@@ -309,9 +309,9 @@ type LocalBackend struct {
 	// Last ClientVersion received in MapResponse, guarded by mu.
 	lastClientVersion *tailcfg.ClientVersion
 
-	// notifyTailFSSharesOnce is used to only send one initial notification
-	// with the latest set of TailFS shares.
-	notifyTailFSSharesOnce sync.Once
+	// lastNotifiedTailFSShares keeps track of the last set of shares that we
+	// notified about.
+	lastNotifiedTailFSShares atomic.Pointer[views.SliceView[*tailfs.Share, tailfs.ShareView]]
 }
 
 type updateStatus struct {
@@ -435,8 +435,12 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 	// initialize TailFS shares from saved state
 	fs, ok := b.sys.TailFSForRemote.GetOK()
 	if ok {
-		shares, err := b.TailFSGetShares()
-		if err == nil && len(shares) > 0 {
+		currentShares := b.pm.prefs.TailFSShares()
+		if currentShares.Len() > 0 {
+			var shares []*tailfs.Share
+			for i := 0; i < currentShares.Len(); i++ {
+				shares = append(shares, currentShares.At(i).AsStruct())
+			}
 			fs.SetShares(shares)
 		}
 	}
@@ -2285,15 +2289,7 @@ func (b *LocalBackend) WatchNotifications(ctx context.Context, mask ipn.NotifyWa
 			ini.NetMap = b.netMap
 		}
 		if mask&ipn.NotifyInitialTailFSShares != 0 && b.tailFSSharingEnabledLocked() {
-			shares, err := b.TailFSGetShares()
-			if err != nil {
-				b.logf("unable to notify initial tailfs shares: %v", err)
-			} else {
-				ini.TailFSShares = make(map[string]*tailfs.Share, len(shares))
-				for _, share := range shares {
-					ini.TailFSShares[share.Name] = share
-				}
-			}
+			ini.TailFSShares = b.pm.prefs.TailFSShares()
 		}
 	}
 
@@ -4669,10 +4665,8 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 		}
 	}
 
-	if b.tailFSSharingEnabledLocked() {
-		b.updateTailFSPeersLocked(nm)
-		b.tailFSNotifyCurrentSharesOnce()
-	}
+	b.updateTailFSPeersLocked(nm)
+	b.tailFSNotifyCurrentSharesLocked()
 }
 
 func (b *LocalBackend) updatePeersFromNetmapLocked(nm *netmap.NetworkMap) {