cmd/tailscale: don't depend on regexp in minbox builds

Updates #12614
Updates #18562

Change-Id: Ife4f10c55d1d68569938ffd68ffe72eef889e200
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -6,6 +6,7 @@
 package cli
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -14,7 +15,6 @@ import (
 	"io"
 	"log"
 	"os"
-	"regexp"
 	"runtime"
 	"strings"
 	"sync"
@@ -582,11 +582,32 @@ type sanitizeWriter struct {
 	w io.Writer
 }
 
-var rxTskey = regexp.MustCompile(`tskey-[\w-]+`)
-
+// Write logically replaces /tskey-[A-Za-z0-9-]+/ with /tskey-XXXX.../ in buf
+// before writing to the underlying writer.
+//
+// We avoid the "regexp" package to not bloat the minbox build, and without
+// making this a featuretag-omittable protection.
 func (w sanitizeWriter) Write(buf []byte) (int, error) {
-	sanitized := rxTskey.ReplaceAll(buf, []byte("tskey-REDACTED"))
-	diff := len(sanitized) - len(buf)
-	n, err := w.w.Write(sanitized)
-	return n - diff, err
+	const prefix = "tskey-"
+	scrub := buf
+	for {
+		i := bytes.Index(scrub, []byte(prefix))
+		if i == -1 {
+			break
+		}
+		scrub = scrub[i+len(prefix):]
+
+		for i, b := range scrub {
+			if (b >= 'a' && b <= 'z') ||
+				(b >= 'A' && b <= 'Z') ||
+				(b >= '0' && b <= '9') ||
+				b == '-' {
+				scrub[i] = 'X'
+			} else {
+				break
+			}
+		}
+	}
+
+	return w.w.Write(buf)
 }

cmd/tailscale/cli/cli_test.go

@@ -1804,8 +1804,8 @@ func TestSanitizeWriter(t *testing.T) {
 	buf := new(bytes.Buffer)
 	w := sanitizeOutput(buf)
 
-	in := []byte(`my auth key is tskey-auth-abc123-def456, what's yours?`)
-	want := []byte(`my auth key is tskey-REDACTED, what's yours?`)
+	in := []byte(`my auth key is tskey-auth-abc123-def456 and tskey-foo, what's yours?`)
+	want := []byte(`my auth key is tskey-XXXXXXXXXXXXXXXXXX and tskey-XXX, what's yours?`)
 	n, err := w.Write(in)
 	if err != nil {
 		t.Fatal(err)

cmd/tailscaled/depaware-minbox.txt

@@ -420,8 +420,6 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         path                                                         from io/fs+
         path/filepath                                                from crypto/x509+
         reflect                                                      from crypto/x509+
-        regexp                                                       from tailscale.com/cmd/tailscale/cli
-        regexp/syntax                                                from regexp
         runtime                                                      from crypto/internal/fips140+
         runtime/debug                                                from github.com/klauspost/compress/zstd+
         slices                                                       from crypto/tls+

cmd/tailscaled/deps_test.go

@@ -294,6 +294,7 @@ func TestMinTailscaledWithCLI(t *testing.T) {
 			"tailscale.com/clientupdate/distsign":    "unexpected distsign dep",
 			"archive/tar":                            "unexpected archive/tar dep",
 			"tailscale.com/feature/conn25":           "unexpected conn25 dep",
+			"regexp":                                 "unexpected regexp dep; bloats binary",
 		},
 	}.Check(t)
 }