Fix leaked client transport on copy (#3051)

Fixes: #3043.

github/github.go

@@ -450,15 +450,18 @@ func (c *Client) copy() *Client {
 	c.clientMu.Lock()
 	// can't use *c here because that would copy mutexes by value.
 	clone := Client{
-		client:                  c.client,
+		client:                  &http.Client{},
 		UserAgent:               c.UserAgent,
 		BaseURL:                 c.BaseURL,
 		UploadURL:               c.UploadURL,
 		secondaryRateLimitReset: c.secondaryRateLimitReset,
 	}
 	c.clientMu.Unlock()
-	if clone.client == nil {
-		clone.client = &http.Client{}
+	if c.client != nil {
+		clone.client.Transport = c.client.Transport
+		clone.client.CheckRedirect = c.client.CheckRedirect
+		clone.client.Jar = c.client.Jar
+		clone.client.Timeout = c.client.Timeout
 	}
 	c.rateMu.Lock()
 	copy(clone.rateLimits[:], c.rateLimits[:])

github/github_test.go

@@ -2599,3 +2599,32 @@ func TestParseTokenExpiration(t *testing.T) {
 		}
 	}
 }
+
+func TestClientCopy_leak_transport(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		accessToken := r.Header.Get("Authorization")
+		_, _ = fmt.Fprintf(w, `{"login": "%s"}`, accessToken)
+	}))
+	clientPreconfiguredWithURLs, err := NewClient(nil).WithEnterpriseURLs(srv.URL, srv.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	aliceClient := clientPreconfiguredWithURLs.WithAuthToken("alice")
+	bobClient := clientPreconfiguredWithURLs.WithAuthToken("bob")
+
+	alice, _, err := aliceClient.Users.Get(context.Background(), "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	assertNoDiff(t, "Bearer alice", alice.GetLogin())
+
+	bob, _, err := bobClient.Users.Get(context.Background(), "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	assertNoDiff(t, "Bearer bob", bob.GetLogin())
+}