go mod init httpmocktest && go test -v

x_test.go

package httpmocktest_test

import (
	"context"
	"net/http"
	"testing"
	"time"

	"github.com/jarcoal/httpmock"
)

func TestTimeout(t *testing.T) {
	httpmock.Activate()
	defer httpmock.DeactivateAndReset()

	httpmock.RegisterResponder("POST", "https://example.com/",
		func(req *http.Request) (*http.Response, error) {
			resp, err := httpmock.NewJsonResponse(200, nil)
			if err != nil {
				return httpmock.NewStringResponse(400, ""), nil
			}
			time.Sleep(1000 * time.Millisecond)
			return resp, nil
		},
	)

	req, err := http.NewRequest("POST", "https://example.com/", nil)
	if err != nil {
		t.Fatal(err)
	}

	client := &http.Client{Timeout: 1 * time.Millisecond}
	_, err = client.Do(req)
	if err == nil {
		t.Fatal("Want a timeout exceeded error but got none.")
	} else {
		t.Logf("An error occurred: %s", err)
	}
}

func TestTimeout2(t *testing.T) {
	httpmock.Activate()
	defer httpmock.DeactivateAndReset()

	httpmock.RegisterResponder("POST", "https://example.com/",
		func(req *http.Request) (*http.Response, error) {
			resp, err := httpmock.NewJsonResponse(200, nil)
			if err != nil {
				return httpmock.NewStringResponse(400, ""), nil
			}
			time.Sleep(1000 * time.Millisecond)
			return resp, nil
		},
	)

	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
	defer cancel()
	req, err := http.NewRequestWithContext(ctx, "POST", "https://example.com/", nil)
	if err != nil {
		t.Fatal(err)
	}

	client := &http.Client{}
	_, err = client.Do(req)
	if err == nil {
		t.Fatal("Want a timeout exceeded error but got none.")
	} else {
		t.Logf("An error occurred: %s", err)
	}
}