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Add precondition.go.

This commit is contained in:
Juliusz Chroboczek 2024-04-09 14:56:10 +02:00
parent 1666abf2f3
commit eee6650336
2 changed files with 224 additions and 0 deletions

110
webserver/precondition.go Normal file
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package webserver
import (
"net/http"
"strings"
)
// This is partly based on the Go standard library file net/http/fs.go.
// scanETag determines if a syntactically valid ETag is present at s. If so,
// the ETag and remaining text after consuming ETag is returned. Otherwise,
// it returns "", "".
func scanETag(s string) (etag string, remain string) {
s = strings.TrimLeft(s, " \t\n\r")
start := 0
if strings.HasPrefix(s, "W/") {
start = 2
}
if len(s[start:]) < 2 || s[start] != '"' {
return "", ""
}
// ETag is either W/"text" or "text".
// See RFC 7232 2.3.
for i := start + 1; i < len(s); i++ {
c := s[i]
switch {
// Character values allowed in ETags.
case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
case c == '"':
return s[:i+1], s[i+1:]
default:
return "", ""
}
}
return "", ""
}
// etagMatch returns whether the given eTag matches an IM/INM header value.
// The empty string in etag is interpreted as a non-existent object.
func etagMatch(etag, header string) bool {
if header == "" {
return false
}
if header == etag {
return true
}
for {
header = strings.TrimLeft(header, " \t\n\r")
if len(header) == 0 {
break
}
if header[0] == ',' {
header = header[1:]
continue
}
if header[0] == '*' {
return etag != ""
}
e, remain := scanETag(header)
if e == "" {
break
}
if e == etag {
return true
}
header = remain
}
return false
}
func writeNotModified(w http.ResponseWriter) {
// RFC 7232 section 4.1:
// a sender SHOULD NOT generate representation metadata other than the
// above listed fields unless said metadata exists for the purpose of
// guiding cache updates (e.g., Last-Modified might be useful if the
// response does not have an ETag field).
h := w.Header()
delete(h, "Content-Type")
delete(h, "Content-Length")
delete(h, "Content-Encoding")
if h.Get("Etag") != "" {
delete(h, "Last-Modified")
}
w.WriteHeader(http.StatusNotModified)
}
// checkPreconditions evaluates request preconditions.
// It interprets an empty etag as a non-existent object.
func checkPreconditions(w http.ResponseWriter, r *http.Request, etag string) (done bool) {
// RFC 7232 section 6.
im := r.Header.Get("If-Match")
if im != "" && !etagMatch(etag, im) {
w.WriteHeader(http.StatusPreconditionFailed)
return true
}
inm := r.Header.Get("If-None-Match")
if inm != "" && etagMatch(etag, inm) {
if r.Method == "GET" || r.Method == "HEAD" {
writeNotModified(w)
return true
} else {
w.WriteHeader(http.StatusPreconditionFailed)
return true
}
}
return false
}

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package webserver
import (
"net/http"
"testing"
)
type testWriter struct {
statusCode int
}
func (w *testWriter) Header() http.Header {
return nil
}
func (w *testWriter) Write(buf []byte) (int, error) {
return len(buf), nil
}
func (w *testWriter) WriteHeader(statusCode int) {
if w.statusCode != 0 {
panic("WriteHeader called twice")
}
w.statusCode = statusCode
}
func TestEtagMatch(t *testing.T) {
type tst struct {
etag, header string
}
var match = []tst{
{`"foo"`, `"foo"`},
{`"foo"`, ` "foo"`},
{`"foo"`, `"foo" `},
{`"foo"`, ` "foo" `},
{`"foo"`, `"foo", "bar"`},
{`"foo"`, `"bar", "foo"`},
{`W/"foo"`, `W/"foo"`},
}
var mismatch = []tst{
{``, ``},
{``, `*`},
{``, `"foo"`},
{`"foo"`, ``},
{`"foo"`, `"bar"`},
{`"foo"`, `"bar", "baz"`},
{`"foo"`, `"baz", "bar"`},
{`"foo"`, `W/"foo"`},
{`W/"foo"`, `"foo"`},
}
for _, tst := range match {
m := etagMatch(tst.etag, tst.header)
if !m {
t.Errorf("%#v %#v: got %v, expected true",
tst.etag, tst.header, m,
)
}
}
for _, tst := range mismatch {
m := etagMatch(tst.etag, tst.header)
if m {
t.Errorf("%#v %#v: got %v, expected false",
tst.etag, tst.header, m,
)
}
}
}
func TestCheckPreconditions(t *testing.T) {
var tests = []struct {
method, etag, im, inm string
result int
}{
{"GET", ``, ``, ``, 0},
{"GET", ``, `*`, ``, 412},
{"GET", ``, ``, `*`, 0},
{"POST", ``, `*`, ``, 412},
{"POST", ``, ``, `*`, 0},
{"GET", `"123"`, ``, ``, 0},
{"GET", `"123"`, `"123"`, ``, 0},
{"GET", `"123"`, `"124"`, ``, 412},
{"POST", `"123"`, `"124"`, ``, 412},
{"GET", `"123"`, `*`, ``, 0},
{"GET", `"123"`, ``, `"123"`, 304},
{"POST", `"123"`, ``, `"123"`, 412},
{"GET", `"123"`, ``, `"124"`, 0},
{"GET", `"123"`, ``, `*`, 304},
}
for _, tst := range tests {
var w testWriter
h := make(http.Header)
if tst.im != "" {
h.Set("If-Match", tst.im)
}
if tst.inm != "" {
h.Set("If-None-Match", tst.inm)
}
r := http.Request{
Method: tst.method,
Header: h,
}
done := checkPreconditions(&w, &r, tst.etag)
if done != (tst.result != 0) || w.statusCode != tst.result {
t.Errorf("%#v %#v %#v: got %v, expected %v",
tst.etag, tst.im, tst.inm,
w.statusCode, tst.result)
}
}
}