Honor X-Forwarded-Port and Forwarded headers

Prefer non-standard headers like X-Forwarded-Proto, X-Forwarded-Host and
X-Forwarded-Port over the standard Forwarded header to maintain
backwards compatibility.

If a port is not specified neither in Host nor in forwarded headers but
it is specified just with X-Forwarded-Port, use its value in base urls
for redirects.

Forwarded header is defined in rfc7239.

X-Forwarded-Port is a non-standard header. Here's a description copied
from "HTTP Headers and Elastic Load Balancing" of AWS ELB docs:

> The X-Forwarded-Port request header helps you identify the port that
> an HTTP or HTTPS load balancer uses to connect to the client.

Signed-off-by: Michal Minář <miminar@redhat.com>

registry/api/v2/headerparser_test.go

@@ -0,0 +1,161 @@
+package v2
+
+import (
+	"testing"
+)
+
+func TestParseForwardedHeader(t *testing.T) {
+	for _, tc := range []struct {
+		name          string
+		raw           string
+		expected      map[string]string
+		expectedRest  string
+		expectedError bool
+	}{
+		{
+			name: "empty",
+			raw:  "",
+		},
+		{
+			name:     "one pair",
+			raw:      " key = value ",
+			expected: map[string]string{"key": "value"},
+		},
+		{
+			name:     "two pairs",
+			raw:      " key1 = value1; key2=value2",
+			expected: map[string]string{"key1": "value1", "key2": "value2"},
+		},
+		{
+			name:     "uppercase parameter",
+			raw:      "KeY=VaL",
+			expected: map[string]string{"key": "VaL"},
+		},
+		{
+			name:     "missing key=value pair - be tolerant",
+			raw:      "key=val;",
+			expected: map[string]string{"key": "val"},
+		},
+		{
+			name:     "quoted values",
+			raw:      `key="val";param = "[[ $((1 + 1)) == 3 ]] && echo panic!;" ; p=" abcd "`,
+			expected: map[string]string{"key": "val", "param": "[[ $((1 + 1)) == 3 ]] && echo panic!;", "p": " abcd "},
+		},
+		{
+			name:     "empty quoted value",
+			raw:      `key=""`,
+			expected: map[string]string{"key": ""},
+		},
+		{
+			name:     "quoted double quotes",
+			raw:      `key="\"value\""`,
+			expected: map[string]string{"key": `"value"`},
+		},
+		{
+			name:     "quoted backslash",
+			raw:      `key="\"\\\""`,
+			expected: map[string]string{"key": `"\"`},
+		},
+		{
+			name:         "ignore subsequent elements",
+			raw:          "key=a, param= b",
+			expected:     map[string]string{"key": "a"},
+			expectedRest: " param= b",
+		},
+		{
+			name:         "empty element - be tolerant",
+			raw:          " , key=val",
+			expectedRest: " key=val",
+		},
+		{
+			name:     "obscure key",
+			raw:      `ob₷C&r€ = value`,
+			expected: map[string]string{`ob₷c&r€`: "value"},
+		},
+		{
+			name:          "duplicate parameter",
+			raw:           "key=a; p=b; key=c",
+			expectedError: true,
+		},
+		{
+			name:          "empty parameter",
+			raw:           "=value",
+			expectedError: true,
+		},
+		{
+			name:          "empty value",
+			raw:           "key= ",
+			expectedError: true,
+		},
+		{
+			name:          "empty value before a new element ",
+			raw:           "key=,",
+			expectedError: true,
+		},
+		{
+			name:          "empty value before a new pair",
+			raw:           "key=;",
+			expectedError: true,
+		},
+		{
+			name:          "just parameter",
+			raw:           "key",
+			expectedError: true,
+		},
+		{
+			name:          "missing key-value",
+			raw:           "a=b;;",
+			expectedError: true,
+		},
+		{
+			name:          "unclosed quoted value",
+			raw:           `key="value`,
+			expectedError: true,
+		},
+		{
+			name:          "escaped terminating dquote",
+			raw:           `key="value\"`,
+			expectedError: true,
+		},
+		{
+			name:          "just a quoted value",
+			raw:           `"key=val"`,
+			expectedError: true,
+		},
+		{
+			name:          "quoted key",
+			raw:           `"key"=val`,
+			expectedError: true,
+		},
+	} {
+		parsed, rest, err := parseForwardedHeader(tc.raw)
+		if err != nil && !tc.expectedError {
+			t.Errorf("[%s] got unexpected error: %v", tc.name, err)
+		}
+		if err == nil && tc.expectedError {
+			t.Errorf("[%s] got unexpected non-error", tc.name)
+		}
+		if err != nil || tc.expectedError {
+			continue
+		}
+		for key, value := range tc.expected {
+			v, exists := parsed[key]
+			if !exists {
+				t.Errorf("[%s] missing expected parameter %q", tc.name, key)
+				continue
+			}
+			if v != value {
+				t.Errorf("[%s] got unexpected value for parameter %q: %q != %q", tc.name, key, v, value)
+			}
+		}
+		for key, value := range parsed {
+			if _, exists := tc.expected[key]; !exists {
+				t.Errorf("[%s] got unexpected key/value pair: %q=%q", tc.name, key, value)
+			}
+		}
+
+		if rest != tc.expectedRest {
+			t.Errorf("[%s] got unexpected unparsed string: %q != %q", tc.name, rest, tc.expectedRest)
+		}
+	}
+}