change TestNewAPIClientFromFlagsWithHttpProxyEnv to an e2e test

cli/command/cli_test.go

@@ -6,7 +6,6 @@ import (
 	"crypto/x509"
 	"fmt"
 	"io/ioutil"
-	"net/http"
 	"os"
 	"runtime"
 	"testing"
@@ -80,24 +79,6 @@ func TestNewAPIClientFromFlagsWithAPIVersionFromEnv(t *testing.T) {
 	assert.Check(t, is.Equal(customVersion, apiclient.ClientVersion()))
 }
 
-func TestNewAPIClientFromFlagsWithHttpProxyEnv(t *testing.T) {
-	defer env.Patch(t, "HTTP_PROXY", "http://proxy.acme.example.com:1234")()
-	defer env.Patch(t, "DOCKER_HOST", "tcp://docker.acme.example.com:2376")()
-
-	opts := &flags.CommonOptions{}
-	configFile := &configfile.ConfigFile{}
-	apiclient, err := NewAPIClientFromFlags(opts, configFile)
-	assert.NilError(t, err)
-	transport, ok := apiclient.HTTPClient().Transport.(*http.Transport)
-	assert.Assert(t, ok)
-	assert.Assert(t, transport.Proxy != nil)
-	request, err := http.NewRequest(http.MethodGet, "tcp://docker.acme.example.com:2376", nil)
-	assert.NilError(t, err)
-	url, err := transport.Proxy(request)
-	assert.NilError(t, err)
-	assert.Check(t, is.Equal("http://proxy.acme.example.com:1234", url.String()))
-}
-
 type fakeClient struct {
 	client.Client
 	pingFunc   func() (types.Ping, error)

e2e/global/cli_test.go

@@ -1,9 +1,13 @@
 package global
 
 import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"github.com/docker/cli/internal/test/environment"
+	"gotest.tools/v3/assert"
 	"gotest.tools/v3/icmd"
 	"gotest.tools/v3/skip"
 )
@@ -22,3 +26,42 @@ func TestTLSVerify(t *testing.T) {
 	result = icmd.RunCmd(icmd.Command("docker", "--tlsverify=true", "ps"))
 	result.Assert(t, icmd.Expected{ExitCode: 1, Err: "ca.pem"})
 }
+
+// TestTCPSchemeUsesHTTPProxyEnv verifies that the cli uses HTTP_PROXY if
+// DOCKER_HOST is set to use the 'tcp://' scheme.
+//
+// Prior to go1.16, https:// schemes would use HTTPS_PROXY, and any other
+// scheme would use HTTP_PROXY. However, golang/net@7b1cca2 (per a request in
+// golang/go#40909) changed this behavior to only use HTTP_PROXY for http://
+// schemes, no longer using a proxy for any other scheme.
+//
+// Docker uses the tcp:// scheme as a default for API connections, to indicate
+// that the API is not "purely" HTTP. Various parts in the code also *require*
+// this scheme to be used. While we could change the default and allow http(s)
+// schemes to be used, doing so will take time, taking into account that there
+// are many installs in existence that have tcp:// configured as DOCKER_HOST.
+//
+// Note that due to Golang's use of sync.Once for proxy-detection, this test
+// cannot be done as a unit-test, hence it being an e2e test.
+func TestTCPSchemeUsesHTTPProxyEnv(t *testing.T) {
+	const responseJSON = `{"Version": "99.99.9", "ApiVersion": "1.41", "MinAPIVersion": "1.12"}`
+	var received string
+	proxyServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		received = r.Host
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(responseJSON))
+	}))
+	defer proxyServer.Close()
+
+	// Configure the CLI to use our proxyServer. DOCKER_HOST can point to any
+	// address (as it won't be connected to), but must use tcp:// for this test,
+	// to verify it's using HTTP_PROXY.
+	result := icmd.RunCmd(
+		icmd.Command("docker", "version", "--format", "{{ .Server.Version }}"),
+		icmd.WithEnv("HTTP_PROXY="+proxyServer.URL, "DOCKER_HOST=tcp://docker.acme.example.com:2376"),
+	)
+	// Verify the command ran successfully, and that it connected to the proxyServer
+	result.Assert(t, icmd.Success)
+	assert.Equal(t, strings.TrimSpace(result.Stdout()), "99.99.9")
+	assert.Equal(t, received, "docker.acme.example.com:2376")
+}