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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
}
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