228 lines
7.5 KiB
Go
228 lines
7.5 KiB
Go
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// Package noxssrw (No XSS ResponseWriter) behaves like the Go standard
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// library's ResponseWriter by detecting the Content-Type of a response if it
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// has not been explicitly set. However, unlike the standard library's
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// implementation, this implementation will never return the "text/html"
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// Content-Type and instead return "text/plain".
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package noxssrw
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import (
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"net/http"
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"strings"
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)
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var (
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// DefaultUnsafeTypes are Content-Types that browsers will render as hypertext.
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// Any Content-Types that allow Javascript or remote resource fetching must be
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// converted to a Content-Type that prevents evaluation.
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//
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// Types are prefix matched to avoid comparing against specific
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// character sets (eg "text/html; charset=utf-8") which may be user
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// controlled.
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DefaultUnsafeTypes = map[string]string{
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"text/html": "text/plain",
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"text/xhtml": "text/plain",
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"text/xhtml+xml": "text/plain",
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}
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// DefaultHeaders contain CORS headers meant to prevent the execution
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// of Javascript in compliant browsers.
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DefaultHeaders = map[string]string{
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"Content-Security-Policy": "default-src 'none'; style-src 'unsafe-inline'; sandbox",
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"X-Content-Type-Options": "nosniff",
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"X-XSS-Protection": "1; mode=block",
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}
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)
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// NoXSSResponseWriter implements http.ResponseWriter but prevents renderable
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// Content-Types from being automatically detected. Create with
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// NewResponseWriter.
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type NoXSSResponseWriter struct {
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// TypeMap maps types unsafe for untrusted content to their safe
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// version; may be replaced but not mutated.
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TypeMap map[string]string
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// DefaultHeaders to set on first write if they are not already
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// explicitly set.
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DefaultHeaders map[string]string
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// buffer up to 512 bytes before detecting Content-Type and writing
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// response.
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buf []byte
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// subsequentWrite is true after the first Write is called
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subsequentWrite bool
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// flushed is true if Content-Type has been set and Writes may be
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// passed through.
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flushed bool
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// original ResponseWriter being wrapped
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orig http.ResponseWriter
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}
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// Header returns the header map that will be sent by
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// WriteHeader. The Header map also is the mechanism with which
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// Handlers can set HTTP trailers.
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//
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// Changing the header map after a call to WriteHeader (or
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// Write) has no effect unless the modified headers are
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// trailers.
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//
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// There are two ways to set Trailers. The preferred way is to
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// predeclare in the headers which trailers you will later
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// send by setting the "Trailer" header to the names of the
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// trailer keys which will come later. In this case, those
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// keys of the Header map are treated as if they were
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// trailers. See the example. The second way, for trailer
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// keys not known to the Handler until after the first Write,
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// is to prefix the Header map keys with the TrailerPrefix
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// constant value. See TrailerPrefix.
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//
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// To suppress automatic response headers (such as "Date"), set
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// their value to nil.
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func (w *NoXSSResponseWriter) Header() http.Header {
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return w.orig.Header()
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}
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// Write writes the data to the connection as part of an HTTP reply.
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//
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// If WriteHeader has not yet been called, Write calls
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// WriteHeader(http.StatusOK) before writing the data. If the Header
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// does not contain a Content-Type line, Write adds a Content-Type set
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// to the result of passing the initial 512 bytes of written data to
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// DetectContentType. Additionally, if the total size of all written
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// data is under a few KB and there are no Flush calls, the
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// Content-Length header is added automatically.
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//
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// Depending on the HTTP protocol version and the client, calling
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// Write or WriteHeader may prevent future reads on the
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// Request.Body. For HTTP/1.x requests, handlers should read any
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// needed request body data before writing the response. Once the
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// headers have been flushed (due to either an explicit Flusher.Flush
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// call or writing enough data to trigger a flush), the request body
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// may be unavailable. For HTTP/2 requests, the Go HTTP server permits
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// handlers to continue to read the request body while concurrently
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// writing the response. However, such behavior may not be supported
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// by all HTTP/2 clients. Handlers should read before writing if
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// possible to maximize compatibility.
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func (w *NoXSSResponseWriter) Write(p []byte) (int, error) {
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headers := w.Header()
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// If first write, set any unset default headers. Do this on first write
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// to allow overriding the default set of headers.
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if !w.subsequentWrite {
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for k, v := range w.DefaultHeaders {
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if headers.Get(k) == "" {
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headers.Set(k, v)
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}
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}
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w.subsequentWrite = true
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}
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// If already flushed, write-through and short-circuit
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if w.flushed {
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return w.orig.Write(p)
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}
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// < 512 bytes available, buffer and wait for closing or a subsequent
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// request
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if len(w.buf)+len(p) < 512 {
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w.buf = append(w.buf, p...)
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return len(p), nil
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}
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// >= 512 bytes available, set the Content-Type and flush.
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all := append(w.buf, p...)
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contentType := http.DetectContentType(all)
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// Prefix match to exclude the character set which may be user
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// controlled.
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for prefix, safe := range w.TypeMap {
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if strings.HasPrefix(contentType, prefix) {
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contentType = safe
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break
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}
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}
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// Set the Content-Type iff it was not already explicitly set
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if headers.Get("Content-Type") == "" {
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headers.Set("Content-Type", contentType)
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}
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// Write the buffer
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n, err := w.orig.Write(w.buf)
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if err != nil {
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// Throw away part of buffer written successfully and
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// inform caller p was not written at all
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w.buf = w.buf[:n]
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return 0, err
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}
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// Headers and buffer were written, this writer has been
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// flushed and can be a passthrough
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w.flushed = true
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// Write p
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return w.orig.Write(p)
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}
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// Close and flush the writer. Necessary for responses that never reached 512
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// bytes.
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func (w *NoXSSResponseWriter) Close() (int, error) {
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// If the buffer was already flushed this is a noop
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if w.flushed {
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return 0, nil
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}
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// Prefix match to exclude the character set which may be user
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// controlled.
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contentType := http.DetectContentType(w.buf)
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for prefix, safe := range w.TypeMap {
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if strings.HasPrefix(contentType, prefix) {
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contentType = safe
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break
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}
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}
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// Set the Content-Type iff it was not already explicitly set
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if headers := w.Header(); headers.Get("Content-Type") == "" {
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headers.Set("Content-Type", contentType)
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}
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// Write the buffer
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return w.orig.Write(w.buf)
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}
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// WriteHeader sends an HTTP response header with the provided
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// status code.
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//
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// If WriteHeader is not called explicitly, the first call to Write
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// will trigger an implicit WriteHeader(http.StatusOK).
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// Thus explicit calls to WriteHeader are mainly used to
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// send error codes.
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//
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// The provided code must be a valid HTTP 1xx-5xx status code.
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// Only one header may be written. Go does not currently
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// support sending user-defined 1xx informational headers,
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// with the exception of 100-continue response header that the
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// Server sends automatically when the Request.Body is read.
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func (w *NoXSSResponseWriter) WriteHeader(statusCode int) {
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w.orig.WriteHeader(statusCode)
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}
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// NewResponseWriter creates a new ResponseWriter and Close func which will
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// prevent Go's http.ResponseWriter default behavior of detecting the
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// Content-Type.
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//
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// The Close func must be called to ensure that responses < 512 bytes are
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// flushed as up to 512 bytes are buffered without flushing.
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func NewResponseWriter(orig http.ResponseWriter) (http.ResponseWriter, func() (int, error)) {
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w := &NoXSSResponseWriter{
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TypeMap: DefaultUnsafeTypes,
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DefaultHeaders: DefaultHeaders,
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buf: make([]byte, 0, 512),
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orig: orig,
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}
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return w, w.Close
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}
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