dkim: Implement internal dkim signing and verification

This patch implements internal DKIM signing and verification.

internal/dkim/context_test.go

@@ -0,0 +1,67 @@
+package dkim
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"testing"
+)
+
+func TestTraceNoCtx(t *testing.T) {
+	// Call trace() on a context without a trace function, to check it doesn't
+	// panic.
+	ctx := context.Background()
+	trace(ctx, "test")
+}
+
+func TestTrace(t *testing.T) {
+	s := ""
+	traceF := func(f string, a ...interface{}) {
+		s = fmt.Sprintf(f, a...)
+	}
+	ctx := WithTraceFunc(context.Background(), traceF)
+	trace(ctx, "test %d", 1)
+	if s != "test 1" {
+		t.Errorf("trace function not called")
+	}
+}
+
+func TestLookupTXTNoCtx(t *testing.T) {
+	// Call lookupTXT() on a context without an override, to check it calls
+	// the real function.
+	// We just check there is a reasonable error.
+	// We don't specifically check that it's NXDOMAIN because if we don't have
+	// internet access, the error may be different.
+	ctx := context.Background()
+	_, err := lookupTXT(ctx, "does.not.exist.example.com")
+	if _, ok := err.(*net.DNSError); !ok {
+		t.Fatalf("expected *net.DNSError, got %T", err)
+	}
+}
+
+func TestLookupTXT(t *testing.T) {
+	called := false
+	lookupTXTF := func(ctx context.Context, name string) ([]string, error) {
+		called = true
+		return nil, nil
+	}
+	ctx := WithLookupTXTFunc(context.Background(), lookupTXTF)
+	lookupTXT(ctx, "example.com")
+	if !called {
+		t.Errorf("lookupTXT function not called")
+	}
+}
+
+func TestMaxHeaders(t *testing.T) {
+	// First without an override, check we return the default.
+	ctx := context.Background()
+	if m := maxHeaders(ctx); m != 5 {
+		t.Errorf("expected 5, got %d", m)
+	}
+
+	// Now with an override.
+	ctx = WithMaxHeaders(ctx, 10)
+	if m := maxHeaders(ctx); m != 10 {
+		t.Errorf("expected 10, got %d", m)
+	}
+}