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.go

@@ -0,0 +1,161 @@
+package v2
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+	"unicode"
+)
+
+var (
+	// according to rfc7230
+	reToken            = regexp.MustCompile(`^[^"(),/:;<=>?@[\]{}[:space:][:cntrl:]]+`)
+	reQuotedValue      = regexp.MustCompile(`^[^\\"]+`)
+	reEscapedCharacter = regexp.MustCompile(`^[[:blank:][:graph:]]`)
+)
+
+// parseForwardedHeader is a benevolent parser of Forwarded header defined in rfc7239. The header contains
+// a comma-separated list of forwarding key-value pairs. Each list element is set by single proxy. The
+// function parses only the first element of the list, which is set by the very first proxy. It returns a map
+// of corresponding key-value pairs and an unparsed slice of the input string.
+//
+// Examples of Forwarded header values:
+//
+//  1. Forwarded: For=192.0.2.43; Proto=https,For="[2001:db8:cafe::17]",For=unknown
+//  2. Forwarded: for="192.0.2.43:443"; host="registry.example.org", for="10.10.05.40:80"
+//
+// The first will be parsed into {"for": "192.0.2.43", "proto": "https"} while the second into
+// {"for": "192.0.2.43:443", "host": "registry.example.org"}.
+func parseForwardedHeader(forwarded string) (map[string]string, string, error) {
+	// Following are states of forwarded header parser. Any state could transition to a failure.
+	const (
+		// terminating state; can transition to Parameter
+		stateElement = iota
+		// terminating state; can transition to KeyValueDelimiter
+		stateParameter
+		// can transition to Value
+		stateKeyValueDelimiter
+		// can transition to one of { QuotedValue, PairEnd }
+		stateValue
+		// can transition to one of { EscapedCharacter, PairEnd }
+		stateQuotedValue
+		// can transition to one of { QuotedValue }
+		stateEscapedCharacter
+		// terminating state; can transition to one of { Parameter, Element }
+		statePairEnd
+	)
+
+	var (
+		parameter string
+		value     string
+		parse     = forwarded[:]
+		res       = map[string]string{}
+		state     = stateElement
+	)
+
+Loop:
+	for {
+		// skip spaces unless in quoted value
+		if state != stateQuotedValue && state != stateEscapedCharacter {
+			parse = strings.TrimLeftFunc(parse, unicode.IsSpace)
+		}
+
+		if len(parse) == 0 {
+			if state != stateElement && state != statePairEnd && state != stateParameter {
+				return nil, parse, fmt.Errorf("unexpected end of input")
+			}
+			// terminating
+			break
+		}
+
+		switch state {
+		// terminate at list element delimiter
+		case stateElement:
+			if parse[0] == ',' {
+				parse = parse[1:]
+				break Loop
+			}
+			state = stateParameter
+
+		// parse parameter (the key of key-value pair)
+		case stateParameter:
+			match := reToken.FindString(parse)
+			if len(match) == 0 {
+				return nil, parse, fmt.Errorf("failed to parse token at position %d", len(forwarded)-len(parse))
+			}
+			parameter = strings.ToLower(match)
+			parse = parse[len(match):]
+			state = stateKeyValueDelimiter
+
+		// parse '='
+		case stateKeyValueDelimiter:
+			if parse[0] != '=' {
+				return nil, parse, fmt.Errorf("expected '=', not '%c' at position %d", parse[0], len(forwarded)-len(parse))
+			}
+			parse = parse[1:]
+			state = stateValue
+
+		// parse value or quoted value
+		case stateValue:
+			if parse[0] == '"' {
+				parse = parse[1:]
+				state = stateQuotedValue
+			} else {
+				value = reToken.FindString(parse)
+				if len(value) == 0 {
+					return nil, parse, fmt.Errorf("failed to parse value at position %d", len(forwarded)-len(parse))
+				}
+				if _, exists := res[parameter]; exists {
+					return nil, parse, fmt.Errorf("duplicate parameter %q at position %d", parameter, len(forwarded)-len(parse))
+				}
+				res[parameter] = value
+				parse = parse[len(value):]
+				value = ""
+				state = statePairEnd
+			}
+
+		// parse a part of quoted value until the first backslash
+		case stateQuotedValue:
+			match := reQuotedValue.FindString(parse)
+			value += match
+			parse = parse[len(match):]
+			switch {
+			case len(parse) == 0:
+				return nil, parse, fmt.Errorf("unterminated quoted string")
+			case parse[0] == '"':
+				res[parameter] = value
+				value = ""
+				parse = parse[1:]
+				state = statePairEnd
+			case parse[0] == '\\':
+				parse = parse[1:]
+				state = stateEscapedCharacter
+			}
+
+		// parse escaped character in a quoted string, ignore the backslash
+		// transition back to QuotedValue state
+		case stateEscapedCharacter:
+			c := reEscapedCharacter.FindString(parse)
+			if len(c) == 0 {
+				return nil, parse, fmt.Errorf("invalid escape sequence at position %d", len(forwarded)-len(parse)-1)
+			}
+			value += c
+			parse = parse[1:]
+			state = stateQuotedValue
+
+		// expect either a new key-value pair, new list or end of input
+		case statePairEnd:
+			switch parse[0] {
+			case ';':
+				parse = parse[1:]
+				state = stateParameter
+			case ',':
+				state = stateElement
+			default:
+				return nil, parse, fmt.Errorf("expected ',' or ';', not %c at position %d", parse[0], len(forwarded)-len(parse))
+			}
+		}
+	}
+
+	return res, parse, nil
+}