Add support for GCP extensions

The support is modeled on azure.go under tlvparse. The GCP spec is similar but
encodes uint64. There's no subtype. Since zero value of uint64 is technically
a valid header value, I've opted for an extra boolean to indicate if it was
found (same as with Azure).

tlvparse/gcp.go

@@ -0,0 +1,47 @@
+package tlvparse
+
+import (
+	"encoding/binary"
+
+	"github.com/pires/go-proxyproto"
+)
+
+const (
+	// PP2_TYPE_GCP indicates a Google Cloud Platform header
+	PP2_TYPE_GCP proxyproto.PP2Type = 0xE0
+)
+
+// ExtractPSCConnectionID returns the first PSC Connection ID in the TLV if it exists and is well-formed and
+// a bool indicating one was found.
+func ExtractPSCConnectionID(tlvs []proxyproto.TLV) (uint64, bool) {
+	for _, tlv := range tlvs {
+		if linkID, err := pscConnectionID(tlv); err == nil {
+			return linkID, true
+		}
+	}
+	return 0, false
+}
+
+// pscConnectionID returns the ID of a GCP PSC extension TLV or errors with ErrIncompatibleTLV or
+// ErrMalformedTLV if it's the wrong TLV type or is malformed.
+//
+//	Field	Length (bytes)	Description
+//	Type	1	PP2_TYPE_GCP (0xE0)
+//	Length	2	Length of value (always 0x0008)
+//	Value	8	The 8-byte PSC Connection ID (decode to uint64; big endian)
+//
+// For example proxyproto.TLV{Type:0xea, Length:8, Value:[]byte{0xff, 0xff, 0xff, 0xff, 0xc0, 0xa8, 0x64, 0x02}}
+// will be decoded as 18446744072646845442.
+//
+// See https://cloud.google.com/vpc/docs/configure-private-service-connect-producer
+func pscConnectionID(t proxyproto.TLV) (uint64, error) {
+	if !isPSCConnectionID(t) {
+		return 0, proxyproto.ErrIncompatibleTLV
+	}
+	linkID := binary.BigEndian.Uint64(t.Value)
+	return linkID, nil
+}
+
+func isPSCConnectionID(t proxyproto.TLV) bool {
+	return t.Type == PP2_TYPE_GCP && len(t.Value) == 8
+}

tlvparse/gcp_test.go

@@ -0,0 +1,82 @@
+package tlvparse
+
+import (
+	"testing"
+
+	"github.com/pires/go-proxyproto"
+)
+
+func TestExtractPSCConnectionID(t *testing.T) {
+	tests := []struct {
+		name                string
+		tlvs                []proxyproto.TLV
+		wantPSCConnectionID uint64
+		wantFound           bool
+	}{
+		{
+			name:      "nil TLVs",
+			tlvs:      nil,
+			wantFound: false,
+		},
+		{
+			name:      "empty TLVs",
+			tlvs:      []proxyproto.TLV{},
+			wantFound: false,
+		},
+		{
+			name: "AWS VPC endpoint ID",
+			tlvs: []proxyproto.TLV{
+				{
+					Type:  0xEA,
+					Value: []byte{0x01, 0x76, 0x70, 0x63, 0x65, 0x2d, 0x61, 0x62, 0x63, 0x31, 0x32, 0x33},
+				},
+			},
+			wantFound: false,
+		},
+		{
+			name: "GCP link ID",
+			tlvs: []proxyproto.TLV{
+				{
+					Type:  PP2_TYPE_GCP,
+					Value: []byte{'\xff', '\xff', '\xff', '\xff', '\xc0', '\xa8', '\x64', '\x02'},
+				},
+			},
+			wantPSCConnectionID: 18446744072646845442,
+			wantFound:           true,
+		},
+		{
+			name: "Multiple TLVs",
+			tlvs: []proxyproto.TLV{
+				{ // AWS
+					Type:  0xEA,
+					Value: []byte{0x01, 0x76, 0x70, 0x63, 0x65, 0x2d, 0x61, 0x62, 0x63, 0x31, 0x32, 0x33},
+				},
+				{ // Azure
+					Type:  0xEE,
+					Value: []byte{0x02, 0x01, 0x01, 0x01, 0x01},
+				},
+				{ // GCP but wrong length
+					Type:  0xE0,
+					Value: []byte{0xff, 0xff, 0xff},
+				},
+				{ // Correct
+					Type:  0xE0,
+					Value: []byte{'\xff', '\xff', '\xff', '\xff', '\xc0', '\xa8', '\x64', '\x02'},
+				},
+			},
+			wantPSCConnectionID: 18446744072646845442,
+			wantFound:           true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			linkID, hasLinkID := ExtractPSCConnectionID(tt.tlvs)
+			if hasLinkID != tt.wantFound {
+				t.Errorf("ExtractPSCConnectionID() got1 = %v, want %v", hasLinkID, tt.wantFound)
+			}
+			if linkID != tt.wantPSCConnectionID {
+				t.Errorf("ExtractPSCConnectionID() got = %v, want %v", linkID, tt.wantPSCConnectionID)
+			}
+		})
+	}
+}