refactor(proxy): make proxy configuration explicit

- introduce `SLING_PROXY` environment variable for proxy configuration
- allow setting proxy URL directly on connection via `PROXY_URL` property
- prioritize `PROXY_URL` over `SLING_PROXY` environment variable
- pass proxy URL directly to `OpenTunnelProxy` for improved clarity and testability
- remove dependency on generic `ALL_PROXY` environment variable

Author: flarco

core/dbio/database/database.go

@@ -680,8 +680,12 @@ func (conn *BaseConn) Connect(timeOut ...int) (err error) {
 		conn.SetProp("ssh_url", connURL) // set ssh url for 3rd party bulk loading
 	}
 
-	// start SOCKS5 proxy tunnel with ALL_PROXY env var
-	if os.Getenv("ALL_PROXY") != "" {
+	// start SOCKS5 proxy tunnel with SLING_PROXY env var
+	proxyURL := conn.GetProp("PROXY_URL")
+	if proxyURL == "" {
+		proxyURL = os.Getenv("SLING_PROXY")
+	}
+	if proxyURL != "" {
 		connU, err := url.Parse(connURL)
 		if err != nil {
 			return g.Error(err, "could not parse connection URL for proxy forwarding")
@@ -698,7 +702,7 @@ func (conn *BaseConn) Connect(timeOut ...int) (err error) {
 				)
 			}
 
-			localPort, err := iop.OpenTunnelProxy(connHost, connPort)
+			localPort, err := iop.OpenTunnelProxy(proxyURL, connHost, connPort)
 			if err != nil {
 				return g.Error(err, "could not establish SOCKS5 proxy tunnel")
 			}

core/dbio/iop/proxy.go

@@ -4,29 +4,27 @@ import (
 	"io"
 	"net"
 	"net/url"
-	"os"
 	"strings"
 	"sync"
 
 	"github.com/flarco/g"
 	"golang.org/x/net/proxy"
 )
 
-// OpenTunnelProxy forwards traffic through the SOCKS5 proxy in ALL_PROXY
-func OpenTunnelProxy(tgtHost string, tgtPort int) (localPort int, err error) {
-	proxyURL := os.Getenv("ALL_PROXY")
+// OpenTunnelProxy forwards traffic through the SOCKS5 proxy
+func OpenTunnelProxy(proxyURL, tgtHost string, tgtPort int) (localPort int, err error) {
 	if proxyURL == "" {
-		return 0, g.Error("no proxy configured in environment (ALL_PROXY)")
+		return 0, g.Error("no proxy URL provided")
 	}
 
 	u, err := url.Parse(proxyURL)
 	if err != nil {
-		return 0, g.Error(err, "could not parse ALL_PROXY URL")
+		return 0, g.Error(err, "could not parse proxy URL")
 	}
 
 	dialer, err := proxy.FromURL(u, proxy.Direct)
 	if err != nil {
-		return 0, g.Error(err, "could not create dialer from ALL_PROXY URL")
+		return 0, g.Error(err, "could not create dialer from proxy URL")
 	}
 
 	localPort, err = g.GetPort("localhost:0")

core/dbio/iop/proxy_test.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"io"
 	"net"
-	"os"
 	"testing"
 	"time"
 
@@ -89,23 +88,20 @@ func startEchoServer(t *testing.T) net.Listener {
 }
 
 func TestOpenTunnelProxy_NoProxyConfigured(t *testing.T) {
-	os.Unsetenv("ALL_PROXY")
-
-	_, err := OpenTunnelProxy("example.com", 5432)
+	_, err := OpenTunnelProxy("", "example.com", 5432)
 	assert.Error(t, err)
 	assert.Contains(t, err.Error(), "no proxy configured")
 }
 
 func TestOpenTunnelProxy_UnreachableProxy(t *testing.T) {
-	// Point ALL_PROXY at a port with nothing listening
+	// supply proxyURL at a port with nothing listening
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	g.AssertNoError(t, err)
 	deadPort := ln.Addr().(*net.TCPAddr).Port
 	ln.Close() // close immediately so the port is unreachable
 
-	t.Setenv("ALL_PROXY", fmt.Sprintf("socks5://127.0.0.1:%d", deadPort))
-
-	localPort, err := OpenTunnelProxy("127.0.0.1", 5432)
+	proxyURL := fmt.Sprintf("socks5://127.0.0.1:%d", deadPort)
+	localPort, err := OpenTunnelProxy(proxyURL, "127.0.0.1", 5432)
 	// tunnel setup succeeds (listener created), but connecting through it should fail
 	g.AssertNoError(t, err)
 
@@ -133,9 +129,8 @@ func TestOpenTunnelProxy_ForwardsTraffic(t *testing.T) {
 	defer socksLn.Close()
 	go minimalSOCKS5(t, socksLn)
 
-	t.Setenv("ALL_PROXY", fmt.Sprintf("socks5://127.0.0.1:%d", socksLn.Addr().(*net.TCPAddr).Port))
-
-	localPort, err := OpenTunnelProxy("127.0.0.1", echoAddr.Port)
+	proxyURL := fmt.Sprintf("socks5://127.0.0.1:%d", socksLn.Addr().(*net.TCPAddr).Port)
+	localPort, err := OpenTunnelProxy(proxyURL, "127.0.0.1", echoAddr.Port)
 	g.AssertNoError(t, err)
 	assert.Greater(t, localPort, 0)