util/httpm, all: add a test to make sure httpm is used consistently

Updates #cleanup

Change-Id: I7dbf8a02de22fc6b317ab5e29cc97792dd75352c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

clientupdate/distsign/distsign.go

@@ -57,6 +57,7 @@ import (
 	"golang.org/x/crypto/blake2s"
 	"tailscale.com/net/tshttpproxy"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/httpm"
 	"tailscale.com/util/must"
 )
 
@@ -335,7 +336,7 @@ func (c *Client) download(ctx context.Context, url, dst string, limit int64) ([]
 
 	quickCtx, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
-	headReq := must.Get(http.NewRequestWithContext(quickCtx, http.MethodHead, url, nil))
+	headReq := must.Get(http.NewRequestWithContext(quickCtx, httpm.HEAD, url, nil))
 
 	res, err := hc.Do(headReq)
 	if err != nil {
@@ -349,7 +350,7 @@ func (c *Client) download(ctx context.Context, url, dst string, limit int64) ([]
 	}
 	c.logf("Download size: %v", res.ContentLength)
 
-	dlReq := must.Get(http.NewRequestWithContext(ctx, http.MethodGet, url, nil))
+	dlReq := must.Get(http.NewRequestWithContext(ctx, httpm.GET, url, nil))
 	dlRes, err := hc.Do(dlReq)
 	if err != nil {
 		return nil, 0, err

ipn/ipnlocal/c2n.go

@@ -24,6 +24,7 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/goroutines"
+	"tailscale.com/util/httpm"
 	"tailscale.com/version"
 )
 
@@ -41,9 +42,9 @@ func (b *LocalBackend) handleC2N(w http.ResponseWriter, r *http.Request) {
 		w.Write(body)
 	case "/update":
 		switch r.Method {
-		case http.MethodGet:
+		case httpm.GET:
 			b.handleC2NUpdateGet(w, r)
-		case http.MethodPost:
+		case httpm.POST:
 			b.handleC2NUpdatePost(w, r)
 		default:
 			http.Error(w, "bad method", http.StatusMethodNotAllowed)

ssh/tailssh/tailssh.go

@@ -43,6 +43,7 @@ import (
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
 	"tailscale.com/util/clientmetric"
+	"tailscale.com/util/httpm"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/multierr"
 )
@@ -1752,7 +1753,7 @@ func (ss *sshSession) notifyControl(ctx context.Context, nodeKey key.NodePublic,
 		return
 	}
 
-	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(body))
+	req, err := http.NewRequestWithContext(ctx, httpm.POST, url, bytes.NewReader(body))
 	if err != nil {
 		ss.logf("notifyControl: unable to create request:", err)
 		return

util/httpm/httpm_test.go

@@ -0,0 +1,29 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package httpm
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestUsedConsistently(t *testing.T) {
+	cmd := exec.Command("git", "grep", "-l", "-F", "http.Method")
+	dir, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmd.Dir = filepath.Join(dir, "../..")
+	matches, _ := cmd.Output()
+	for _, fn := range strings.Split(strings.TrimSpace(string(matches)), "\n") {
+		switch fn {
+		case "util/httpm/httpm.go", "util/httpm/httpm_test.go":
+			continue
+		}
+		t.Errorf("http.MethodFoo constant used in %s; use httpm.FOO instead", fn)
+	}
+}