Fix Blackhole implementation for e2e tests

Based on the ideas discussed in the issues [1] and PRs [2][3][6], we
switch from using an L4 reverse proxy to an L7 forward proxy to properly
block peer network traffic, without the need to use external tools.

The design aims to implement only the minimal required features that
satisfy blocking incoming and outgoing peer traffic. Complicated features
such as packet reordering, packet delivery delay, etc. to a future
container-based solution.

A peer will
(a) receive traffic from its peers
(b) initiate connections to its peers (via stream and pipeline).

Thus, the current mechanism of only blocking peer traffic via the peer's
existing reverse proxy is insufficient, since only scenario (a) is
handled, and network traffic in scenario (b) is not blocked at all.

We introduce an L7 forward proxy for each peer, which will be proxying
all the connections initiated from a peer to its peers.

We will remove the current use of the L4 reverse proxy, as the L7
forward proxy holds the information of the destination, we can block all
incoming and outgoing traffic that is initiated from a peer to others,
without having to resort to external tools, such as iptables.

The modified architecture will look something like this:
```
A --- A's forward proxy --- B
   ^ newly introduced
```

The main subtasks are
- redesigned as an L7 forward proxy
- introduce a new environment variable `E2E_TEST_FORWARD_PROXY_IP` to
bypass the limitation of `http.ProxyFromEnvironment`
- implement an L7 forward proxy

Known limitations are
- Doesn't support unix socket, as L7 HTTP transport proxy only supports
HTTP/HTTPS/and socks5 -> Although the e2e test supports unix sockets for
peer communication, only a few of the e2e test cases use unix sockets as
the majority of e2e test cases use HTTP/HTTPS. It's been discussed and
decided that it is ok for now without the unix socket support.
- it's L7 so we need to send a perfectly crafted HTTP request
- doesn’t support reordering, dropping, or manipulating packets on the
fly

- `make gofail-enable && make build && make gofail-disable && go test -timeout 60s -run ^TestBlackholeByMockingPartitionLeader$ go.etcd.io/etcd/tests/v3/e2e -v -count=1`
- `make gofail-enable && make build && make gofail-disable && go test -timeout 60s -run ^TestBlackholeByMockingPartitionFollower$ go.etcd.io/etcd/tests/v3/e2e -v -count=1`
- `go test -timeout 30s -run ^TestServer_ go.etcd.io/etcd/pkg/v3/proxy -v -failfast`

[1] issue etcd-io#17737
[2] PR (V1) https://github.com/henrybear327/etcd/tree/fix/e2e_blackhole
[3] PR (V2) etcd-io#17891
[4] etcd-io#17938 (comment)
[5] etcd-io#17985 (comment)
[6] etcd-io#17938

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

Signed-off-by: Siyuan Zhang <[email protected]>
Signed-off-by: Iván Valdés Castillo <[email protected]>
Signed-off-by: Chun-Hung Tseng <[email protected]>

client/pkg/transport/transport.go

@@ -18,12 +18,29 @@ import (
 	"context"
 	"net"
 	"net/http"
+	"net/url"
+	"os"
 	"strings"
 	"time"
 )
 
 type unixTransport struct{ *http.Transport }
 
+var httpTransportProxyParsingFunc = determineHTTPTransportProxyParsingFunc
+
+func determineHTTPTransportProxyParsingFunc() func(req *http.Request) (*url.URL, error) {
+	// according to the comment of http.ProxyFromEnvironment: if the proxy URL is "localhost"
+	// (with or without a port number), then a nil URL and nil error will be returned.
+	// Thus, we workaround this limitation by manually setting an ENV named E2E_TEST_FORWARD_PROXY_IP
+	// and parse the URL (which is a localhost in our case)
+	if forwardProxy, exists := os.LookupEnv("E2E_TEST_FORWARD_PROXY_IP"); exists {
+		return func(req *http.Request) (*url.URL, error) {
+			return url.Parse(forwardProxy)
+		}
+	}
+	return http.ProxyFromEnvironment
+}
+
 func NewTransport(info TLSInfo, dialtimeoutd time.Duration) (*http.Transport, error) {
 	cfg, err := info.ClientConfig()
 	if err != nil {
@@ -39,7 +56,7 @@ func NewTransport(info TLSInfo, dialtimeoutd time.Duration) (*http.Transport, er
 	}
 
 	t := &http.Transport{
-		Proxy: http.ProxyFromEnvironment,
+		Proxy: httpTransportProxyParsingFunc(),
 		DialContext: (&net.Dialer{
 			Timeout:   dialtimeoutd,
 			LocalAddr: ipAddr,
@@ -60,7 +77,7 @@ func NewTransport(info TLSInfo, dialtimeoutd time.Duration) (*http.Transport, er
 		return dialer.DialContext(ctx, "unix", addr)
 	}
 	tu := &http.Transport{
-		Proxy:               http.ProxyFromEnvironment,
+		Proxy:               httpTransportProxyParsingFunc(),
 		DialContext:         dialContext,
 		TLSHandshakeTimeout: 10 * time.Second,
 		TLSClientConfig:     cfg,