From 56e7391087ebcfa65a6b26b1b6fd55091b9b212a Mon Sep 17 00:00:00 2001
From: Mikhail Mazurskiy <mmazurskiy@gitlab.com>
Date: Tue, 19 Oct 2021 16:17:25 +1100
Subject: [PATCH] HTTP client factory for per-request clients

---
 client.go | 87 +++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 66 insertions(+), 21 deletions(-)

diff --git a/client.go b/client.go
index f40d241..ae3e9d0 100644
--- a/client.go
+++ b/client.go
@@ -368,11 +368,24 @@ type Backoff func(min, max time.Duration, attemptNum int, resp *http.Response) t
 // attempted. If overriding this, be sure to close the body if needed.
 type ErrorHandler func(resp *http.Response, err error, numTries int) (*http.Response, error)
 
+type HTTPClient interface {
+	// Do performs an HTTP request and returns an HTTP response.
+	Do(*http.Request) (*http.Response, error)
+	// Done is called when the client is no longer needed.
+	Done()
+}
+
+type HTTPClientFactory interface {
+	// New returns an HTTP client to use for a request, including retries.
+	New() HTTPClient
+}
+
 // Client is used to make HTTP requests. It adds additional functionality
 // like automatic retries to tolerate minor outages.
 type Client struct {
-	HTTPClient *http.Client // Internal HTTP client.
-	Logger     interface{}  // Customer logger instance. Can be either Logger or LeveledLogger
+	HTTPClient        *http.Client // Internal HTTP client. This field is used if set, otherwise HTTPClientFactory is used.
+	HTTPClientFactory HTTPClientFactory
+	Logger            interface{} // Customer logger instance. Can be either Logger or LeveledLogger
 
 	RetryWaitMin time.Duration // Minimum time to wait
 	RetryWaitMax time.Duration // Maximum time to wait
@@ -397,19 +410,18 @@ type Client struct {
 	ErrorHandler ErrorHandler
 
 	loggerInit sync.Once
-	clientInit sync.Once
 }
 
 // NewClient creates a new Client with default settings.
 func NewClient() *Client {
 	return &Client{
-		HTTPClient:   cleanhttp.DefaultPooledClient(),
-		Logger:       defaultLogger,
-		RetryWaitMin: defaultRetryWaitMin,
-		RetryWaitMax: defaultRetryWaitMax,
-		RetryMax:     defaultRetryMax,
-		CheckRetry:   DefaultRetryPolicy,
-		Backoff:      DefaultBackoff,
+		HTTPClientFactory: &CleanPooledClientFactory{},
+		Logger:            defaultLogger,
+		RetryWaitMin:      defaultRetryWaitMin,
+		RetryWaitMax:      defaultRetryWaitMax,
+		RetryMax:          defaultRetryMax,
+		CheckRetry:        DefaultRetryPolicy,
+		Backoff:           DefaultBackoff,
 	}
 }
 
@@ -573,12 +585,6 @@ func PassthroughErrorHandler(resp *http.Response, err error, _ int) (*http.Respo
 
 // Do wraps calling an HTTP method with retries.
 func (c *Client) Do(req *Request) (*http.Response, error) {
-	c.clientInit.Do(func() {
-		if c.HTTPClient == nil {
-			c.HTTPClient = cleanhttp.DefaultPooledClient()
-		}
-	})
-
 	logger := c.logger()
 
 	if logger != nil {
@@ -590,6 +596,9 @@ func (c *Client) Do(req *Request) (*http.Response, error) {
 		}
 	}
 
+	httpClient := c.getHTTPClient()
+	defer httpClient.Done()
+
 	var resp *http.Response
 	var attempt int
 	var shouldRetry bool
@@ -603,7 +612,6 @@ func (c *Client) Do(req *Request) (*http.Response, error) {
 		if req.body != nil {
 			body, err := req.body()
 			if err != nil {
-				c.HTTPClient.CloseIdleConnections()
 				return resp, err
 			}
 			if c, ok := body.(io.ReadCloser); ok {
@@ -625,7 +633,8 @@ func (c *Client) Do(req *Request) (*http.Response, error) {
 		}
 
 		// Attempt the request
-		resp, doErr = c.HTTPClient.Do(req.Request)
+
+		resp, doErr = httpClient.Do(req.Request)
 
 		// Check if we should continue with retries.
 		shouldRetry, checkErr = c.CheckRetry(req.Context(), resp, doErr)
@@ -694,7 +703,6 @@ func (c *Client) Do(req *Request) (*http.Response, error) {
 		select {
 		case <-req.Context().Done():
 			timer.Stop()
-			c.HTTPClient.CloseIdleConnections()
 			return nil, req.Context().Err()
 		case <-timer.C:
 		}
@@ -710,8 +718,6 @@ func (c *Client) Do(req *Request) (*http.Response, error) {
 		return resp, nil
 	}
 
-	defer c.HTTPClient.CloseIdleConnections()
-
 	var err error
 	if checkErr != nil {
 		err = checkErr
@@ -758,6 +764,19 @@ func (c *Client) drainBody(body io.ReadCloser) {
 	}
 }
 
+func (c *Client) getHTTPClient() HTTPClient {
+	if c.HTTPClient != nil {
+		return &idleConnectionsClosingClient{
+			httpClient: c.HTTPClient,
+		}
+	}
+	clientFactory := c.HTTPClientFactory
+	if clientFactory == nil {
+		clientFactory = &CleanPooledClientFactory{}
+	}
+	return clientFactory.New()
+}
+
 // Get is a shortcut for doing a GET request without making a new client.
 func Get(url string) (*http.Response, error) {
 	return defaultClient.Get(url)
@@ -820,3 +839,29 @@ func (c *Client) StandardClient() *http.Client {
 		Transport: &RoundTripper{Client: c},
 	}
 }
+
+var (
+	_ HTTPClientFactory = &CleanPooledClientFactory{}
+	_ HTTPClient        = &idleConnectionsClosingClient{}
+)
+
+type CleanPooledClientFactory struct {
+}
+
+func (f *CleanPooledClientFactory) New() HTTPClient {
+	return &idleConnectionsClosingClient{
+		httpClient: cleanhttp.DefaultPooledClient(),
+	}
+}
+
+type idleConnectionsClosingClient struct {
+	httpClient *http.Client
+}
+
+func (c *idleConnectionsClosingClient) Do(req *http.Request) (*http.Response, error) {
+	return c.httpClient.Do(req)
+}
+
+func (c *idleConnectionsClosingClient) Done() {
+	c.httpClient.CloseIdleConnections()
+}