cmd/derper, types/logger: move log filter to shared package

So we can use it in trunkd to quiet down the logs there.

Updates #5563

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ie3177dc33f5ad103db832aab5a3e0e4f128f973f

types/logger/logger.go

@@ -20,6 +20,7 @@ import (
 
 	"context"
 
+	"go4.org/mem"
 	"tailscale.com/envknob"
 	"tailscale.com/util/ctxkey"
 )
@@ -393,3 +394,25 @@ func TestLogger(tb TBLogger) Logf {
 		tb.Logf("    ... "+format, args...)
 	}
 }
+
+// HTTPServerLogFilter is an io.Writer that can be used as the
+// net/http.Server.ErrorLog logger, and will filter out noisy, low-signal
+// messages that clutter up logs.
+type HTTPServerLogFilter struct {
+	Inner Logf
+}
+
+func (lf HTTPServerLogFilter) Write(p []byte) (int, error) {
+	b := mem.B(p)
+	if mem.HasSuffix(b, mem.S(": EOF\n")) ||
+		mem.HasSuffix(b, mem.S(": i/o timeout\n")) ||
+		mem.HasSuffix(b, mem.S(": read: connection reset by peer\n")) ||
+		mem.HasSuffix(b, mem.S(": remote error: tls: bad certificate\n")) ||
+		mem.HasSuffix(b, mem.S(": tls: first record does not look like a TLS handshake\n")) {
+		// Skip this log message, but say that we processed it
+		return len(p), nil
+	}
+
+	lf.Inner("%s", p)
+	return len(p), nil
+}

types/logger/logger_test.go

@@ -258,3 +258,23 @@ func TestAsJSON(t *testing.T) {
 		t.Errorf("allocs = %v; want max 2", n)
 	}
 }
+
+func TestHTTPServerLogFilter(t *testing.T) {
+	var buf bytes.Buffer
+	logf := func(format string, args ...any) {
+		t.Logf("[logf] "+format, args...)
+		fmt.Fprintf(&buf, format, args...)
+	}
+
+	lf := HTTPServerLogFilter{logf}
+	quietLogger := log.New(lf, "", 0)
+
+	quietLogger.Printf("foo bar")
+	quietLogger.Printf("http: TLS handshake error from %s:%d: EOF", "1.2.3.4", 9999)
+	quietLogger.Printf("baz")
+
+	const want = "foo bar\nbaz\n"
+	if s := buf.String(); s != want {
+		t.Errorf("got buf=%q, want %q", s, want)
+	}
+}