util/rands: add package with HexString func

We use it a number of places in different repos. Might as well make
one. Another use is coming.

Updates #cleanup

Change-Id: Ib7ce38de0db35af998171edee81ca875102349a4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -343,6 +343,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/util/osshare                                   from tailscale.com/ipn/ipnlocal+
    W    tailscale.com/util/pidowner                                  from tailscale.com/ipn/ipnauth
         tailscale.com/util/racebuild                                 from tailscale.com/logpolicy
+        tailscale.com/util/rands                                     from tailscale.com/ipn/localapi
         tailscale.com/util/ringbuffer                                from tailscale.com/wgengine/magicsock
         tailscale.com/util/set                                       from tailscale.com/health+
         tailscale.com/util/singleflight                              from tailscale.com/control/controlclient+

ipn/localapi/localapi.go

@@ -7,8 +7,6 @@
 import (
 	"bytes"
 	"context"
-	"crypto/rand"
-	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -49,6 +47,7 @@
 	"tailscale.com/util/httpm"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/osdiag"
+	"tailscale.com/util/rands"
 	"tailscale.com/version"
 )
 
@@ -118,12 +117,6 @@
 	"query-feature":               (*Handler).serveQueryFeature,
 }
 
-func randHex(n int) string {
-	b := make([]byte, n)
-	rand.Read(b)
-	return hex.EncodeToString(b)
-}
-
 var (
 	// The clientmetrics package is stateful, but we want to expose a simple
 	// imperative API to local clients, so we need to keep track of
@@ -318,7 +311,7 @@ func (h *Handler) serveBugReport(w http.ResponseWriter, r *http.Request) {
 	defer h.b.TryFlushLogs() // kick off upload after bugreport's done logging
 
 	logMarker := func() string {
-		return fmt.Sprintf("BUG-%v-%v-%v", h.backendLogID, h.clock.Now().UTC().Format("20060102150405Z"), randHex(8))
+		return fmt.Sprintf("BUG-%v-%v-%v", h.backendLogID, h.clock.Now().UTC().Format("20060102150405Z"), rands.HexString(16))
 	}
 	if envknob.NoLogsNoSupport() {
 		logMarker = func() string { return "BUG-NO-LOGS-NO-SUPPORT-this-node-has-had-its-logging-disabled" }

tstest/integration/testcontrol/testcontrol.go

@@ -7,7 +7,6 @@
 import (
 	"bytes"
 	"context"
-	crand "crypto/rand"
 	"encoding/binary"
 	"encoding/json"
 	"errors"
@@ -34,6 +33,7 @@
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/ptr"
+	"tailscale.com/util/rands"
 )
 
 const msgLimit = 1 << 20 // encrypted message length limit
@@ -569,9 +569,7 @@ func (s *Server) serveRegister(w http.ResponseWriter, r *http.Request, mkey key.
 
 	authURL := ""
 	if requireAuth {
-		randHex := make([]byte, 10)
-		crand.Read(randHex)
-		authPath := fmt.Sprintf("/auth/%x", randHex)
+		authPath := fmt.Sprintf("/auth/%s", rands.HexString(20))
 		s.addAuthPath(authPath, nk)
 		authURL = s.BaseURL() + authPath
 	}

util/rands/rands.go

@@ -0,0 +1,25 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package rands contains utility functions for randomness.
+package rands
+
+import (
+	crand "crypto/rand"
+	"encoding/hex"
+)
+
+// HexString returns a string of n cryptographically random lowercase
+// hex characters.
+//
+// That is, HexString(3) returns something like "0fc", containing 12
+// bits of randomness.
+func HexString(n int) string {
+	nb := n / 2
+	if n%2 == 1 {
+		nb++
+	}
+	b := make([]byte, nb)
+	crand.Read(b)
+	return hex.EncodeToString(b)[:n]
+}

util/rands/rands_test.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package rands
+
+import "testing"
+
+func TestHexString(t *testing.T) {
+	for i := 0; i <= 8; i++ {
+		s := HexString(i)
+		if len(s) != i {
+			t.Errorf("HexString(%v) = %q; want len %v, not %v", i, s, i, len(s))
+		}
+	}
+}