ipn/ipnlocal: fix taildrop target list UI bug

The macOS and iOS apps that used the /localapi/v0/file-targets handler
were getting too many candidate targets. They wouldn't actually accept
the file. This is effectively just a UI glitch in the wrong hosts
being listed as valid targets from the source side.

Change-Id: I6907a5a1c3c66920e5ec71601c044e722e7cb888
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -3419,10 +3419,7 @@ func (b *LocalBackend) FileTargets() ([]*apitype.FileTarget, error) {
 		return nil, errors.New("file sharing not enabled by Tailscale admin")
 	}
 	for _, p := range nm.Peers {
-		if len(p.Addresses) == 0 {
-			continue
-		}
-		if p.User != nm.User && b.peerHasCapLocked(p.Addresses[0].Addr(), tailcfg.CapabilityFileSharing) {
+		if !b.peerIsTaildropTargetLocked(p) {
 			continue
 		}
 		peerAPI := peerAPIBase(b.netMap, p)
@@ -3438,6 +3435,26 @@ func (b *LocalBackend) FileTargets() ([]*apitype.FileTarget, error) {
 	return ret, nil
 }
 
+// peerIsTaildropTargetLocked reports whether p is a valid Taildrop file
+// recipient from this node according to its ownership and the capabilities in
+// the netmap.
+//
+// b.mu must be locked.
+func (b *LocalBackend) peerIsTaildropTargetLocked(p *tailcfg.Node) bool {
+	if b.netMap == nil || p == nil {
+		return false
+	}
+	if b.netMap.User == p.User {
+		return true
+	}
+	if len(p.Addresses) > 0 &&
+		b.peerHasCapLocked(p.Addresses[0].Addr(), tailcfg.CapabilityFileSharingTarget) {
+		// Explicitly noted in the netmap ACL caps as a target.
+		return true
+	}
+	return false
+}
+
 func (b *LocalBackend) peerHasCapLocked(addr netip.Addr, wantCap string) bool {
 	for _, hasCap := range b.peerCapsLocked(addr) {
 		if hasCap == wantCap {