ipn/localapi: refactor some cert code in prep for a move

I want to move the guts (after the HTTP layer) of the certificate
fetching into the ipnlocal package, out of localapi.

As prep, refactor a bit:

* add a method to do the fetch-from-cert-or-as-needed-with-refresh,
  rather than doing it in the HTTP hander

* convert two methods to funcs, taking the one extra field (LocalBackend)
  then needed from their method receiver. One of the methods needed
  nothing from its receiver.

This will make a future change easier to reason about.

Change-Id: I2a7811e5d7246139927bb86e7db8009bf09b3be3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/localapi/cert.go

@@ -34,6 +34,7 @@
 
 	"golang.org/x/crypto/acme"
 	"tailscale.com/envknob"
+	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/strs"
@@ -79,13 +80,6 @@ func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "cert access denied", http.StatusForbidden)
 		return
 	}
-	dir, err := h.certDir()
-	if err != nil {
-		h.logf("certDir: %v", err)
-		http.Error(w, "failed to get cert dir", 500)
-		return
-	}
-
 	domain, ok := strs.CutPrefix(r.URL.Path, "/localapi/v0/cert/")
 	if !ok {
 		http.Error(w, "internal handler config wired wrong", 500)
@@ -95,8 +89,24 @@ func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "invalid domain", 400)
 		return
 	}
-	now := time.Now()
+	pair, err := h.getCertPEM(r.Context(), domain)
+	if err != nil {
+		http.Error(w, fmt.Sprint(err), 500)
+		return
+	}
+	serveKeyPair(w, r, pair)
+}
+
+// getCertPEM gets the KeyPair for domain, either from cache, via the ACME
+// process, or from cache and kicking off an async ACME renewal.
+func (h *Handler) getCertPEM(ctx context.Context, domain string) (*keyPair, error) {
 	logf := logger.WithPrefix(h.logf, fmt.Sprintf("cert(%q): ", domain))
+	dir, err := h.certDir()
+	if err != nil {
+		logf("failed to get certDir: %v", err)
+		return nil, err
+	}
+	now := time.Now()
 	traceACME := func(v any) {
 		if !acmeDebug() {
 			return
@@ -105,24 +115,22 @@ func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
 		log.Printf("acme %T: %s", v, j)
 	}
 
-	if pair, ok := h.getCertPEMCached(dir, domain, now); ok {
+	if pair, ok := getCertPEMCached(dir, domain, now); ok {
 		future := now.AddDate(0, 0, 14)
 		if h.shouldStartDomainRenewal(dir, domain, future) {
 			logf("starting async renewal")
 			// Start renewal in the background.
-			go h.getCertPEM(context.Background(), logf, traceACME, dir, domain, future)
+			go getCertPEM(context.Background(), h.b, logf, traceACME, dir, domain, future)
 		}
-		serveKeyPair(w, r, pair)
-		return
+		return pair, nil
 	}
 
-	pair, err := h.getCertPEM(r.Context(), logf, traceACME, dir, domain, now)
+	pair, err := getCertPEM(ctx, h.b, logf, traceACME, dir, domain, now)
 	if err != nil {
 		logf("getCertPEM: %v", err)
-		http.Error(w, fmt.Sprint(err), 500)
-		return
+		return nil, err
 	}
-	serveKeyPair(w, r, pair)
+	return pair, nil
 }
 
 func (h *Handler) shouldStartDomainRenewal(dir, domain string, future time.Time) bool {
@@ -135,7 +143,7 @@ func (h *Handler) shouldStartDomainRenewal(dir, domain string, future time.Time)
 		return false
 	}
 	lastRenewCheck[domain] = now
-	_, ok := h.getCertPEMCached(dir, domain, future)
+	_, ok := getCertPEMCached(dir, domain, future)
 	return !ok
 }
 
@@ -154,10 +162,12 @@ func serveKeyPair(w http.ResponseWriter, r *http.Request, p *keyPair) {
 	}
 }
 
+// keyPair is a TLS public and private key, and whether they were obtained
+// from cache or freshly obtained.
 type keyPair struct {
-	certPEM []byte
-	keyPEM  []byte
-	cached  bool
+	certPEM []byte // public key, in PEM form
+	keyPEM  []byte // private key, in PEM form
+	cached  bool   // whether result came from cache
 }
 
 func keyFile(dir, domain string) string  { return filepath.Join(dir, domain+".key") }
@@ -166,7 +176,7 @@ func certFile(dir, domain string) string { return filepath.Join(dir, domain+".cr
 // getCertPEMCached returns a non-nil keyPair and true if a cached
 // keypair for domain exists on disk in dir that is valid at the
 // provided now time.
-func (h *Handler) getCertPEMCached(dir, domain string, now time.Time) (p *keyPair, ok bool) {
+func getCertPEMCached(dir, domain string, now time.Time) (p *keyPair, ok bool) {
 	if !validLookingCertDomain(domain) {
 		// Before we read files from disk using it, validate it's halfway
 		// reasonable looking.
@@ -181,11 +191,11 @@ func (h *Handler) getCertPEMCached(dir, domain string, now time.Time) (p *keyPai
 	return nil, false
 }
 
-func (h *Handler) getCertPEM(ctx context.Context, logf logger.Logf, traceACME func(any), dir, domain string, now time.Time) (*keyPair, error) {
+func getCertPEM(ctx context.Context, lb *ipnlocal.LocalBackend, logf logger.Logf, traceACME func(any), dir, domain string, now time.Time) (*keyPair, error) {
 	acmeMu.Lock()
 	defer acmeMu.Unlock()
 
-	if p, ok := h.getCertPEMCached(dir, domain, now); ok {
+	if p, ok := getCertPEMCached(dir, domain, now); ok {
 		return p, nil
 	}
 
@@ -223,7 +233,7 @@ func (h *Handler) getCertPEM(ctx context.Context, logf logger.Logf, traceACME fu
 	}
 
 	// Before hitting LetsEncrypt, see if this is a domain that Tailscale will do DNS challenges for.
-	st := h.b.StatusWithoutPeers()
+	st := lb.StatusWithoutPeers()
 	if err := checkCertDomain(st, domain); err != nil {
 		return nil, err
 	}
@@ -260,7 +270,7 @@ func (h *Handler) getCertPEM(ctx context.Context, logf logger.Logf, traceACME fu
 				}
 				if !ok {
 					logf("starting SetDNS call...")
-					err = h.b.SetDNS(ctx, key, rec)
+					err = lb.SetDNS(ctx, key, rec)
 					if err != nil {
 						return nil, fmt.Errorf("SetDNS %q => %q: %w", key, rec, err)
 					}