NewClient panics if http.client is nil and LINODE_CA is set (#635)

I tried a new linodego with a custom CA and a nil HTTP client and encountered a panic.

Co-authored-by: Zhiwei Liang <121905282+zliang-akamai@users.noreply.github.com>

Author: kokes

client.go

@@ -738,7 +738,7 @@ func NewClient(hc *http.Client) (client Client) {
 
 	certPath, certPathExists := os.LookupEnv(APIHostCert)
 
-	if certPathExists && !isCustomTransport(hc.Transport) {
+	if certPathExists && !hasCustomTransport(hc) {
 		cert, err := os.ReadFile(filepath.Clean(certPath))
 		if err != nil {
 			log.Fatalf("[ERROR] Error when reading cert at %s: %s\n", certPath, err.Error())
@@ -881,8 +881,11 @@ func generateListCacheURL(endpoint string, opts *ListOptions) (string, error) {
 	return fmt.Sprintf("%s:%s", endpoint, hashedOpts), nil
 }
 
-func isCustomTransport(transport http.RoundTripper) bool {
-	if transport != http.DefaultTransport.(*http.Transport) {
+func hasCustomTransport(hc *http.Client) bool {
+	if hc == nil {
+		return false
+	}
+	if hc.Transport != http.DefaultTransport.(*http.Transport) {
 		log.Println("[WARN] Custom transport is not allowed with a custom root CA.")
 		return true
 	}

client_test.go

@@ -576,3 +576,30 @@ func TestClient_CustomRootCAWithCustomRoundTripper(t *testing.T) {
 
 	log.SetOutput(os.Stderr)
 }
+
+func TestClient_CustomRootCAWithoutCustomRoundTripper(t *testing.T) {
+	caFile, err := os.CreateTemp(t.TempDir(), "linodego_test_ca_*")
+	if err != nil {
+		t.Fatalf("Failed to create temp ca file: %s", err)
+	}
+	defer os.Remove(caFile.Name())
+
+	for _, setCA := range []bool{false, true} {
+		if setCA {
+			t.Setenv(APIHostCert, caFile.Name())
+		}
+
+		client := NewClient(nil)
+
+		transport, err := client.resty.Transport()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if setCA && (transport.TLSClientConfig == nil || transport.TLSClientConfig.RootCAs == nil) {
+			t.Error("expected root CAs to be set")
+		}
+		if !setCA && transport.TLSClientConfig != nil {
+			t.Errorf("didn't set a custom CA, but client TLS config is not nil: %#v", transport.TLSClientConfig)
+		}
+	}
+}