Add support for conditional requests (#162)

This change pulls in some code copied from net/http's fs.go so that we
can support If-Match/If-None-Match requests. This will make it easy to
put a caching proxy in front of linx-server instances. Request
validation will still happen as long as the proxy can contact the
origin, so expiration and deletion will still work as expected under
normal circumstances.

fileserve.go

@@ -1,14 +1,17 @@
 package main
 
 import (
+	"fmt"
 	"io"
 	"net/http"
 	"net/url"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/andreimarcu/linx-server/backends"
 	"github.com/andreimarcu/linx-server/expiry"
+	"github.com/andreimarcu/linx-server/httputil"
 	"github.com/zenazn/goji/web"
 )
 
@@ -37,21 +40,22 @@ func fileServeHandler(c web.C, w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Security-Policy", Config.fileContentSecurityPolicy)
 	w.Header().Set("Referrer-Policy", Config.fileReferrerPolicy)
 
-	_, reader, err := storageBackend.Get(fileName)
-	if err == backends.NotFoundErr {
-		notFoundHandler(c, w, r)
-		return
-	} else if err != nil {
-		oopsHandler(c, w, r, RespAUTO, "Unable to open file.")
-		return
-	}
-
 	w.Header().Set("Content-Type", metadata.Mimetype)
 	w.Header().Set("Content-Length", strconv.FormatInt(metadata.Size, 10))
-	w.Header().Set("Etag", metadata.Sha256sum)
-	w.Header().Set("Cache-Control", "max-age=0")
+	w.Header().Set("Etag", fmt.Sprintf("\"%s\"", metadata.Sha256sum))
+	w.Header().Set("Cache-Control", "public, no-cache")
+
+	modtime := time.Unix(0, 0)
+	if done := httputil.CheckPreconditions(w, r, modtime); done == true {
+		return
+	}
 
 	if r.Method != "HEAD" {
+		_, reader, err := storageBackend.Get(fileName)
+		if err != nil {
+			oopsHandler(c, w, r, RespAUTO, "Unable to open file.")
+			return
+		}
 		defer reader.Close()
 
 		if _, err = io.CopyN(w, reader, metadata.Size); err != nil {
@@ -77,8 +81,8 @@ func staticHandler(c web.C, w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
-		w.Header().Set("Etag", timeStartedStr)
-		w.Header().Set("Cache-Control", "max-age=86400")
+		w.Header().Set("Etag", fmt.Sprintf("\"%s\"", timeStartedStr))
+		w.Header().Set("Cache-Control", "public, max-age=86400")
 		http.ServeContent(w, r, filePath, timeStarted, file)
 		return
 	}

httputil/LICENSE

@@ -0,0 +1,27 @@
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

httputil/conditional.go

@@ -0,0 +1,218 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// HTTP file system request handler
+
+package httputil
+
+import (
+	"net/http"
+	"net/textproto"
+	"strings"
+	"time"
+)
+
+// 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 = textproto.TrimString(s)
+	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 "", ""
+}
+
+// etagStrongMatch reports whether a and b match using strong ETag comparison.
+// Assumes a and b are valid ETags.
+func etagStrongMatch(a, b string) bool {
+	return a == b && a != "" && a[0] == '"'
+}
+
+// etagWeakMatch reports whether a and b match using weak ETag comparison.
+// Assumes a and b are valid ETags.
+func etagWeakMatch(a, b string) bool {
+	return strings.TrimPrefix(a, "W/") == strings.TrimPrefix(b, "W/")
+}
+
+// condResult is the result of an HTTP request precondition check.
+// See https://tools.ietf.org/html/rfc7232 section 3.
+type condResult int
+
+const (
+	condNone condResult = iota
+	condTrue
+	condFalse
+)
+
+func checkIfMatch(w http.ResponseWriter, r *http.Request) condResult {
+	im := r.Header.Get("If-Match")
+	if im == "" {
+		return condNone
+	}
+	for {
+		im = textproto.TrimString(im)
+		if len(im) == 0 {
+			break
+		}
+		if im[0] == ',' {
+			im = im[1:]
+			continue
+		}
+		if im[0] == '*' {
+			return condTrue
+		}
+		etag, remain := scanETag(im)
+		if etag == "" {
+			break
+		}
+		if etagStrongMatch(etag, w.Header().Get("Etag")) {
+			return condTrue
+		}
+		im = remain
+	}
+
+	return condFalse
+}
+
+func checkIfUnmodifiedSince(r *http.Request, modtime time.Time) condResult {
+	ius := r.Header.Get("If-Unmodified-Since")
+	if ius == "" || isZeroTime(modtime) {
+		return condNone
+	}
+	if t, err := http.ParseTime(ius); err == nil {
+		// The Date-Modified header truncates sub-second precision, so
+		// use mtime < t+1s instead of mtime <= t to check for unmodified.
+		if modtime.Before(t.Add(1 * time.Second)) {
+			return condTrue
+		}
+		return condFalse
+	}
+	return condNone
+}
+
+func checkIfNoneMatch(w http.ResponseWriter, r *http.Request) condResult {
+	inm := r.Header.Get("If-None-Match")
+	if inm == "" {
+		return condNone
+	}
+	buf := inm
+	for {
+		buf = textproto.TrimString(buf)
+		if len(buf) == 0 {
+			break
+		}
+		if buf[0] == ',' {
+			buf = buf[1:]
+		}
+		if buf[0] == '*' {
+			return condFalse
+		}
+		etag, remain := scanETag(buf)
+		if etag == "" {
+			break
+		}
+		if etagWeakMatch(etag, w.Header().Get("Etag")) {
+			return condFalse
+		}
+		buf = remain
+	}
+	return condTrue
+}
+
+func checkIfModifiedSince(r *http.Request, modtime time.Time) condResult {
+	if r.Method != "GET" && r.Method != "HEAD" {
+		return condNone
+	}
+	ims := r.Header.Get("If-Modified-Since")
+	if ims == "" || isZeroTime(modtime) {
+		return condNone
+	}
+	t, err := http.ParseTime(ims)
+	if err != nil {
+		return condNone
+	}
+	// The Date-Modified header truncates sub-second precision, so
+	// use mtime < t+1s instead of mtime <= t to check for unmodified.
+	if modtime.Before(t.Add(1 * time.Second)) {
+		return condFalse
+	}
+	return condTrue
+}
+
+var unixEpochTime = time.Unix(0, 0)
+
+// isZeroTime reports whether t is obviously unspecified (either zero or Unix()=0).
+func isZeroTime(t time.Time) bool {
+	return t.IsZero() || t.Equal(unixEpochTime)
+}
+
+func setLastModified(w http.ResponseWriter, modtime time.Time) {
+	if !isZeroTime(modtime) {
+		w.Header().Set("Last-Modified", modtime.UTC().Format(http.TimeFormat))
+	}
+}
+
+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")
+	if h.Get("Etag") != "" {
+		delete(h, "Last-Modified")
+	}
+	w.WriteHeader(http.StatusNotModified)
+}
+
+// CheckPreconditions evaluates request preconditions and reports whether a precondition
+// resulted in sending StatusNotModified or StatusPreconditionFailed.
+func CheckPreconditions(w http.ResponseWriter, r *http.Request, modtime time.Time) (done bool) {
+	// This function carefully follows RFC 7232 section 6.
+	ch := checkIfMatch(w, r)
+	if ch == condNone {
+		ch = checkIfUnmodifiedSince(r, modtime)
+	}
+	if ch == condFalse {
+		w.WriteHeader(http.StatusPreconditionFailed)
+		return true
+	}
+	switch checkIfNoneMatch(w, r) {
+	case condFalse:
+		if r.Method == "GET" || r.Method == "HEAD" {
+			writeNotModified(w)
+			return true
+		} else {
+			w.WriteHeader(http.StatusPreconditionFailed)
+			return true
+		}
+	case condNone:
+		if checkIfModifiedSince(r, modtime) == condFalse {
+			writeNotModified(w)
+			return true
+		}
+	}
+
+	return false
+}