logtail, net/portmapper, wgengine/magicsock: use fmt.Appendf

Fixes #5206

Change-Id: I490bb92e774ce7c044040537e2cd864fcf1dbe5a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/buffer.go

@@ -5,7 +5,6 @@
 package logtail
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"sync"
@@ -51,9 +50,7 @@ func (m *memBuffer) TryReadLine() ([]byte, error) {
 	case ent := <-m.pending:
 		if ent.dropCount > 0 {
 			m.next = ent.msg
-			buf := new(bytes.Buffer)
-			fmt.Fprintf(buf, "----------- %d logs dropped ----------", ent.dropCount)
-			return buf.Bytes(), nil
+			return fmt.Appendf(nil, "----------- %d logs dropped ----------", ent.dropCount), nil
 		}
 		return ent.msg, nil
 	default:

logtail/logtail.go

@@ -270,7 +270,7 @@ func (l *Logger) drainPending(scratch []byte) (res []byte) {
 		if err == io.EOF {
 			break
 		} else if err != nil {
-			b = []byte(fmt.Sprintf("reading ringbuffer: %v", err))
+			b = fmt.Appendf(nil, "reading ringbuffer: %v", err)
 			batchDone = true
 		} else if b == nil {
 			if entries > 0 {

net/portmapper/igd_test.go

@@ -143,7 +143,7 @@ func (d *TestIGD) serveUPnPDiscovery() {
 		pkt := buf[:n]
 		if bytes.Equal(pkt, uPnPPacket) { // a super lazy "parse"
 			d.inc(&d.counters.numUPnPDiscoRecv)
-			resPkt := []byte(fmt.Sprintf("HTTP/1.1 200 OK\r\nCACHE-CONTROL: max-age=120\r\nST: urn:schemas-upnp-org:device:InternetGatewayDevice:1\r\nUSN: uuid:bee7052b-49e8-3597-b545-55a1e38ac11::urn:schemas-upnp-org:device:InternetGatewayDevice:1\r\nEXT:\r\nSERVER: Tailscale-Test/1.0 UPnP/1.1 MiniUPnPd/2.2.1\r\nLOCATION: %s\r\nOPT: \"http://schemas.upnp.org/upnp/1/0/\"; ns=01\r\n01-NLS: 1627958564\r\nBOOTID.UPNP.ORG: 1627958564\r\nCONFIGID.UPNP.ORG: 1337\r\n\r\n", d.ts.URL+"/rootDesc.xml"))
+			resPkt := fmt.Appendf(nil, "HTTP/1.1 200 OK\r\nCACHE-CONTROL: max-age=120\r\nST: urn:schemas-upnp-org:device:InternetGatewayDevice:1\r\nUSN: uuid:bee7052b-49e8-3597-b545-55a1e38ac11::urn:schemas-upnp-org:device:InternetGatewayDevice:1\r\nEXT:\r\nSERVER: Tailscale-Test/1.0 UPnP/1.1 MiniUPnPd/2.2.1\r\nLOCATION: %s\r\nOPT: \"http://schemas.upnp.org/upnp/1/0/\"; ns=01\r\n01-NLS: 1627958564\r\nBOOTID.UPNP.ORG: 1627958564\r\nCONFIGID.UPNP.ORG: 1337\r\n\r\n", d.ts.URL+"/rootDesc.xml")
 			if d.doUPnP {
 				_, err = d.upnpConn.WriteTo(resPkt, src)
 				if err != nil {

wgengine/magicsock/magicsock.go

@@ -811,7 +811,7 @@ func (c *Conn) pickDERPFallback() int {
 	}
 
 	h := fnv.New64()
-	h.Write([]byte(fmt.Sprintf("%p/%d", c, processStartUnixNano))) // arbitrary
+	fmt.Fprintf(h, "%p/%d", c, processStartUnixNano) // arbitrary
 	return ids[rand.New(rand.NewSource(int64(h.Sum64()))).Intn(len(ids))]
 }