net: support blank host and port as specified by curl connect-to flag

Implement the following

"HOST1" and "PORT1" may be the empty string, meaning "any host/port".
"HOST2" and "PORT2" may also be the empty string, meaning "use the request's original host/port".

net.go

@@ -11,6 +11,7 @@ import (
 	"crypto/tls"
 	"fmt"
 	"net"
+	"slices"
 	"sync"
 	"sync/atomic"
 	"syscall"
@@ -23,15 +24,27 @@ import (
 type DialRedirectFunc func(network, address string) (targetNetwork, targetAddress string)
 
 func DialRedirectFromHostPortPairs(subs []HostPortPair) DialRedirectFunc {
-	m := make(map[string]string, len(subs))
-	for _, h := range subs {
-		m[net.JoinHostPort(h.Src.Host, h.Src.Port)] = net.JoinHostPort(h.Dst.Host, h.Dst.Port)
-	}
-
+	subs = slices.Clone(subs)
 	return func(network, address string) (string, string) {
-		if target, ok := m[address]; ok {
-			return network, target
+		host, port, err := net.SplitHostPort(address)
+		if err != nil {
+			return network, address
 		}
+
+		for _, s := range subs {
+			if (s.Src.Host == "" || s.Src.Host == host) && (s.Src.Port == "" || s.Src.Port == port) { //nolint:gocritic // nestingReduce: invert if cond, replace body with `continue`, move old body after the statement
+				h := s.Dst.Host
+				if h == "" {
+					h = host
+				}
+				p := s.Dst.Port
+				if p == "" {
+					p = port
+				}
+				return network, net.JoinHostPort(h, p)
+			}
+		}
+
 		return network, address
 	}
 }

net_test.go

@@ -22,6 +22,71 @@ import (
 	"github.com/saucelabs/forwarder/utils/golden"
 )
 
+func TestDialRedirectFromHostPortPairs(t *testing.T) {
+	tests := []struct {
+		name  string
+		hp    HostPortPair
+		input string
+		want  string
+	}{
+		{
+			name: "basic",
+			hp: HostPortPair{
+				Src: HostPort{"a", "80"},
+				Dst: HostPort{"b", "443"},
+			},
+			input: "a:80",
+			want:  "b:443",
+		},
+		{
+			name: "blank src host",
+			hp: HostPortPair{
+				Src: HostPort{"", "80"},
+				Dst: HostPort{"b", "443"},
+			},
+			input: "a:80",
+			want:  "b:443",
+		},
+		{
+			name: "blank src port",
+			hp: HostPortPair{
+				Src: HostPort{"a", ""},
+				Dst: HostPort{"b", "443"},
+			},
+			input: "a:80",
+			want:  "b:443",
+		},
+		{
+			name: "blank dst host",
+			hp: HostPortPair{
+				Src: HostPort{"a", "80"},
+				Dst: HostPort{"", "443"},
+			},
+			input: "a:80",
+			want:  "a:443",
+		},
+		{
+			name: "blank dst port",
+			hp: HostPortPair{
+				Src: HostPort{"a", "80"},
+				Dst: HostPort{"b", ""},
+			},
+			input: "a:80",
+			want:  "b:80",
+		},
+	}
+
+	for i := range tests {
+		tc := tests[i]
+		t.Run(tc.name, func(t *testing.T) {
+			_, got := DialRedirectFromHostPortPairs([]HostPortPair{tc.hp})("tcp", tc.input)
+			if got != tc.want {
+				t.Fatalf("DialRedirectFromHostPortPairs(): got %v, want %v", got, tc.want)
+			}
+		})
+	}
+}
+
 func TestDialerRedirect(t *testing.T) {
 	l := Listener{
 		Address: "localhost:0",