fix(transport): prevent memory leak when many clients are instantiate…

…d by multiple goroutines

After the report from @asurak about a potential memory leak within the
Go API client, we've been able to reproduce the issue by starting new
`search.Client` instances in the same program from many goroutines (~50 or
higher) and performing simple search queries to the engine over the
network. Under these conditions, the program was seeing its system memory
increased linearly, without any memory being released to the OS by the
GC. When performing the search queries with hardcoded requests using the Go
standard library's `http.DefaultClient`, the issue disappeared.

After profiling the memory usage (using `pprof`), we've noticed that our top
memory usage was due to many invocations of `bytes.MakeSlice` made by
the `*http.Client.Transport` instance that we create for each new
instance of the Go API client. After some research, those `bytes.MakeSlice`
calls are the consequence of `tls.Conn.readHandshake` calls being performed
when the `http.Transport` needs to establish a new connection or renegociate
one with a server.

The allocated memory of the `bytes.MakeSlice` invocations not being released
when used by multiple goroutines is a known issue, as we can see from
other related discussions:

- https://forum.golangbridge.org/t/bytes-makeslice-in-goroutines-creates-abnormal-memory-usage/2038
- https://topic.alibabacloud.com/a/golang-memory-growing-bytesmakeslice_8_8_31497889.html
- gorilla/websocket#134

After comparing how we instantiate our `http.Transport` instance vs. how
the `http.DefaultClient` instantiates its own `http.Transport`, we've
noticed that the latter uses a singleton instance whereas the Go API
client was creating a new instance of its default `http.Transport` even
though it was the same for all API client instances.

This commit reproduces the behavior of the Go standard library's
`http.DefaultClient` by only instantiating a single default
`http.Transport` instance for all API client instances.

Finally, here are some before/after comparison benchmarks as reported by
runtime.MemStats:

```
// Memory consumption using `http.DefaultClient`
Alloc = 3 MiB   TotalAlloc = 865 MiB    Sys = 71 MiB    NumGC = 470
Alloc = 2 MiB   TotalAlloc = 1807 MiB   Sys = 71 MiB    NumGC = 977
Alloc = 3 MiB   TotalAlloc = 2787 MiB   Sys = 71 MiB    NumGC = 1514
Alloc = 3 MiB   TotalAlloc = 3722 MiB   Sys = 71 MiB    NumGC = 2008
Alloc = 4 MiB   TotalAlloc = 4611 MiB   Sys = 71 MiB    NumGC = 2453
Alloc = 4 MiB   TotalAlloc = 5548 MiB   Sys = 71 MiB    NumGC = 2959
Alloc = 3 MiB   TotalAlloc = 6395 MiB   Sys = 71 MiB    NumGC = 3387
Alloc = 2 MiB   TotalAlloc = 7269 MiB   Sys = 71 MiB    NumGC = 3867
Alloc = 3 MiB   TotalAlloc = 8198 MiB   Sys = 71 MiB    NumGC = 4356
Alloc = 2 MiB   TotalAlloc = 9087 MiB   Sys = 71 MiB    NumGC = 4806
Alloc = 6 MiB   TotalAlloc = 10006 MiB  Sys = 71 MiB    NumGC = 5271
Alloc = 3 MiB   TotalAlloc = 10887 MiB  Sys = 71 MiB    NumGC = 5743
Alloc = 4 MiB   TotalAlloc = 11740 MiB  Sys = 71 MiB    NumGC = 6182
Alloc = 3 MiB   TotalAlloc = 12601 MiB  Sys = 71 MiB    NumGC = 6644
Alloc = 3 MiB   TotalAlloc = 13414 MiB  Sys = 71 MiB    NumGC = 7061
Alloc = 4 MiB   TotalAlloc = 14343 MiB  Sys = 71 MiB    NumGC = 7528
Alloc = 3 MiB   TotalAlloc = 15245 MiB  Sys = 71 MiB    NumGC = 8002
Alloc = 3 MiB   TotalAlloc = 16144 MiB  Sys = 71 MiB    NumGC = 8481
Alloc = 3 MiB   TotalAlloc = 17046 MiB  Sys = 71 MiB    NumGC = 8941
Alloc = 4 MiB   TotalAlloc = 17980 MiB  Sys = 71 MiB    NumGC = 9441

// Memory consumption using Go API client before the fix
Alloc = 27 MiB  TotalAlloc = 1077 MiB   Sys = 71 MiB    NumGC = 136
Alloc = 59 MiB  TotalAlloc = 2171 MiB   Sys = 137 MiB   NumGC = 173
Alloc = 95 MiB  TotalAlloc = 3265 MiB   Sys = 205 MiB   NumGC = 195
Alloc = 113 MiB TotalAlloc = 4363 MiB   Sys = 206 MiB   NumGC = 211
Alloc = 181 MiB TotalAlloc = 5476 MiB   Sys = 272 MiB   NumGC = 223
Alloc = 216 MiB TotalAlloc = 6578 MiB   Sys = 340 MiB   NumGC = 233
Alloc = 162 MiB TotalAlloc = 7667 MiB   Sys = 340 MiB   NumGC = 242
Alloc = 231 MiB TotalAlloc = 8753 MiB   Sys = 408 MiB   NumGC = 249
Alloc = 340 MiB TotalAlloc = 9849 MiB   Sys = 474 MiB   NumGC = 255
Alloc = 350 MiB TotalAlloc = 10960 MiB  Sys = 475 MiB   NumGC = 261
Alloc = 433 MiB TotalAlloc = 12051 MiB  Sys = 542 MiB   NumGC = 266
Alloc = 417 MiB TotalAlloc = 13148 MiB  Sys = 610 MiB   NumGC = 271
Alloc = 289 MiB TotalAlloc = 14230 MiB  Sys = 610 MiB   NumGC = 276
Alloc = 324 MiB TotalAlloc = 15312 MiB  Sys = 677 MiB   NumGC = 280
Alloc = 570 MiB TotalAlloc = 16400 MiB  Sys = 744 MiB   NumGC = 283
Alloc = 465 MiB TotalAlloc = 17499 MiB  Sys = 811 MiB   NumGC = 287
Alloc = 613 MiB TotalAlloc = 18601 MiB  Sys = 812 MiB   NumGC = 290
Alloc = 707 MiB TotalAlloc = 19710 MiB  Sys = 879 MiB   NumGC = 293
Alloc = 713 MiB TotalAlloc = 20789 MiB  Sys = 880 MiB   NumGC = 296
Alloc = 708 MiB TotalAlloc = 21872 MiB  Sys = 880 MiB   NumGC = 299

// Memory consumption using Go API client after the fix
Alloc = 5 MiB   TotalAlloc = 143 MiB    Sys = 71 MiB    NumGC = 40
Alloc = 5 MiB   TotalAlloc = 275 MiB    Sys = 71 MiB    NumGC = 75
Alloc = 5 MiB   TotalAlloc = 408 MiB    Sys = 71 MiB    NumGC = 109
Alloc = 4 MiB   TotalAlloc = 541 MiB    Sys = 71 MiB    NumGC = 146
Alloc = 5 MiB   TotalAlloc = 660 MiB    Sys = 71 MiB    NumGC = 179
Alloc = 6 MiB   TotalAlloc = 793 MiB    Sys = 71 MiB    NumGC = 213
Alloc = 7 MiB   TotalAlloc = 925 MiB    Sys = 71 MiB    NumGC = 249
Alloc = 4 MiB   TotalAlloc = 1058 MiB   Sys = 71 MiB    NumGC = 285
Alloc = 6 MiB   TotalAlloc = 1191 MiB   Sys = 71 MiB    NumGC = 320
Alloc = 5 MiB   TotalAlloc = 1323 MiB   Sys = 71 MiB    NumGC = 355
Alloc = 6 MiB   TotalAlloc = 1456 MiB   Sys = 71 MiB    NumGC = 390
Alloc = 6 MiB   TotalAlloc = 1576 MiB   Sys = 71 MiB    NumGC = 422
Alloc = 5 MiB   TotalAlloc = 1708 MiB   Sys = 71 MiB    NumGC = 459
Alloc = 5 MiB   TotalAlloc = 1841 MiB   Sys = 71 MiB    NumGC = 495
Alloc = 6 MiB   TotalAlloc = 1973 MiB   Sys = 71 MiB    NumGC = 531
Alloc = 6 MiB   TotalAlloc = 2106 MiB   Sys = 71 MiB    NumGC = 567
Alloc = 7 MiB   TotalAlloc = 2239 MiB   Sys = 71 MiB    NumGC = 602
Alloc = 4 MiB   TotalAlloc = 2358 MiB   Sys = 71 MiB    NumGC = 636
Alloc = 4 MiB   TotalAlloc = 2491 MiB   Sys = 71 MiB    NumGC = 673
Alloc = 5 MiB   TotalAlloc = 2624 MiB   Sys = 71 MiB    NumGC = 708
```

algolia/transport/requester.go

@@ -24,19 +24,21 @@ const (
 // pass it to an HTTP interceptor.
 func DefaultHTTPClient() *http.Client {
 	return &http.Client{
-		Transport: &http.Transport{
-			Dial: (&net.Dialer{
-				KeepAlive: DefaultKeepAliveDuration,
-				Timeout:   DefaultConnectTimeout,
-			}).Dial,
-			DisableKeepAlives:   false,
-			MaxIdleConnsPerHost: DefaultMaxIdleConnsPerHost,
-			Proxy:               http.ProxyFromEnvironment,
-			TLSHandshakeTimeout: DefaultTLSHandshakeTimeout,
-		},
+		Transport: defaultTransport,
 	}
 }
 
+var defaultTransport http.RoundTripper = &http.Transport{
+	Dial: (&net.Dialer{
+		KeepAlive: DefaultKeepAliveDuration,
+		Timeout:   DefaultConnectTimeout,
+	}).Dial,
+	DisableKeepAlives:   false,
+	MaxIdleConnsPerHost: DefaultMaxIdleConnsPerHost,
+	Proxy:               http.ProxyFromEnvironment,
+	TLSHandshakeTimeout: DefaultTLSHandshakeTimeout,
+}
+
 type Requester interface {
 	Request(req *http.Request) (*http.Response, error)
 }