fs.go

// Copyright 2009 The Go Authors.

// 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 LLC 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.

// HTTP file system request handler
//
// Upstream: https://cs.opensource.google/go/go/+/refs/tags/go1.23.4:src/net/http/fs.go
// Modifications from upstream:
// * Deleted everything except checkPreconditions and dependent functions
// * Added "http" package prefixes

package pgs

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
	}
	t, err := http.ParseTime(ius)
	if err != nil {
		return condNone
	}

	// The Last-Modified header truncates sub-second precision so
	// the modtime needs to be truncated too.
	modtime = modtime.Truncate(time.Second)
	if ret := modtime.Compare(t); ret <= 0 {
		return condTrue
	}
	return condFalse
}

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:]
			continue
		}
		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 Last-Modified header truncates sub-second precision so
	// the modtime needs to be truncated too.
	modtime = modtime.Truncate(time.Second)
	if ret := modtime.Compare(t); ret <= 0 {
		return condFalse
	}
	return condTrue
}

func checkIfRange(w http.ResponseWriter, r *http.Request, modtime time.Time) condResult {
	if r.Method != "GET" && r.Method != "HEAD" {
		return condNone
	}
	ir := r.Header.Get("If-Range")
	if ir == "" {
		return condNone
	}
	etag, _ := scanETag(ir)
	if etag != "" {
		if etagStrongMatch(etag, w.Header().Get("Etag")) {
			return condTrue
		} else {
			return condFalse
		}
	}
	// The If-Range value is typically the ETag value, but it may also be
	// the modtime date. See golang.org/issue/8367.
	if modtime.IsZero() {
		return condFalse
	}
	t, err := http.ParseTime(ir)
	if err != nil {
		return condFalse
	}
	if t.Unix() == modtime.Unix() {
		return condTrue
	}
	return condFalse
}

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 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 and reports whether a precondition
// resulted in sending http.StatusNotModified or http.StatusPreconditionFailed.
func checkPreconditions(w http.ResponseWriter, r *http.Request, modtime time.Time) (done bool, rangeHeader string) {
	// 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, ""
		}
	}

	rangeHeader = r.Header.Get("Range")
	if rangeHeader != "" && checkIfRange(w, r, modtime) == condFalse {
		rangeHeader = ""
	}
	return false, rangeHeader
}