logpolicy: export NewLogtailTransport for Android

Android doesn't use logpolicy and currently has enough
unique stuff about its logging that makes it difficult to
do so. For example, its logsDir comes from Gio.

Export NewLogtailTransport to let Android use it.

Updates https://github.com/tailscale/tailscale/issues/3046

Signed-off-by: Denton Gentry <[email protected]>

logpolicy/logpolicy.go

@@ -501,7 +501,7 @@ func New(collection string) *Policy {
 			}
 			return w
 		},
-		HTTPC: &http.Client{Transport: newLogtailTransport(logtail.DefaultHost)},
+		HTTPC: &http.Client{Transport: NewLogtailTransport(logtail.DefaultHost)},
 	}
 	if collection == logtail.CollectionNode {
 		c.MetricsDelta = clientmetric.EncodeLogTailMetricsDelta
@@ -511,7 +511,7 @@ func New(collection string) *Policy {
 		log.Println("You have enabled a non-default log target. Doing without being told to by Tailscale staff or your network administrator will make getting support difficult.")
 		c.BaseURL = val
 		u, _ := url.Parse(val)
-		c.HTTPC = &http.Client{Transport: newLogtailTransport(u.Host)}
+		c.HTTPC = &http.Client{Transport: NewLogtailTransport(u.Host)}
 	}
 
 	filchBuf, filchErr := filch.New(filepath.Join(dir, cmdName), filch.Options{
@@ -571,9 +571,12 @@ func (p *Policy) Shutdown(ctx context.Context) error {
 	return nil
 }
 
-// newLogtailTransport returns the HTTP Transport we use for uploading
-// logs to the given host name.
-func newLogtailTransport(host string) *http.Transport {
+// NewLogtailTransport returns an HTTP Transport particularly suited to uploading
+// logs to the given host name. This includes:
+//   - If DNS lookup fails, consult the bootstrap DNS list of Tailscale hostnames.
+//   - If TLS connection fails, try again using LetsEncrypt's built-in root certificate,
+//     for the benefit of older OS platforms which might not include it.
+func NewLogtailTransport(host string) *http.Transport {
 	// Start with a copy of http.DefaultTransport and tweak it a bit.
 	tr := http.DefaultTransport.(*http.Transport).Clone()