blob: 6c4073bd4c5b243c3ce2076eb807c374d44da478 [file] [log] [blame]
package remotecontext
import (
// When downloading remote contexts, limit the amount (in bytes)
// to be read from the response body in order to detect its Content-Type
const maxPreambleLength = 100
const acceptableRemoteMIME = `(?:application/(?:(?:x\-)?tar|octet\-stream|((?:x\-)?(?:gzip|bzip2?|xz)))|(?:text/plain))`
var mimeRe = regexp.MustCompile(acceptableRemoteMIME)
// MakeRemoteContext downloads a context from remoteURL and returns it.
// If contentTypeHandlers is non-nil, then the Content-Type header is read along with a maximum of
// maxPreambleLength bytes from the body to help detecting the MIME type.
// Look at acceptableRemoteMIME for more details.
// If a match is found, then the body is sent to the contentType handler and a (potentially compressed) tar stream is expected
// to be returned. If no match is found, it is assumed the body is a tar stream (compressed or not).
// In either case, an (assumed) tar stream is passed to FromArchive whose result is returned.
func MakeRemoteContext(remoteURL string, contentTypeHandlers map[string]func(io.ReadCloser) (io.ReadCloser, error)) (builder.Source, error) {
f, err := GetWithStatusError(remoteURL)
if err != nil {
return nil, fmt.Errorf("error downloading remote context %s: %v", remoteURL, err)
defer f.Body.Close()
var contextReader io.ReadCloser
if contentTypeHandlers != nil {
contentType := f.Header.Get("Content-Type")
clen := f.ContentLength
contentType, contextReader, err = inspectResponse(contentType, f.Body, clen)
if err != nil {
return nil, fmt.Errorf("error detecting content type for remote %s: %v", remoteURL, err)
defer contextReader.Close()
// This loop tries to find a content-type handler for the detected content-type.
// If it could not find one from the caller-supplied map, it tries the empty content-type `""`
// which is interpreted as a fallback handler (usually used for raw tar contexts).
for _, ct := range []string{contentType, ""} {
if fn, ok := contentTypeHandlers[ct]; ok {
defer contextReader.Close()
if contextReader, err = fn(contextReader); err != nil {
return nil, err
// Pass through - this is a pre-packaged context, presumably
// with a Dockerfile with the right name inside it.
return FromArchive(contextReader)
// GetWithStatusError does an http.Get() and returns an error if the
// status code is 4xx or 5xx.
func GetWithStatusError(address string) (resp *http.Response, err error) {
if resp, err = http.Get(address); err != nil {
if uerr, ok := err.(*url.Error); ok {
if derr, ok := uerr.Err.(*net.DNSError); ok && !derr.IsTimeout {
return nil, dnsError{err}
return nil, systemError{err}
if resp.StatusCode < 400 {
return resp, nil
msg := fmt.Sprintf("failed to GET %s with status %s", address, resp.Status)
body, err := ioutil.ReadAll(resp.Body)
if err != nil {
return nil, errors.Wrap(systemError{err}, msg+": error reading body")
msg += ": " + string(bytes.TrimSpace(body))
switch resp.StatusCode {
case http.StatusNotFound:
return nil, notFoundError(msg)
case http.StatusBadRequest:
return nil, requestError(msg)
case http.StatusUnauthorized:
return nil, unauthorizedError(msg)
case http.StatusForbidden:
return nil, forbiddenError(msg)
return nil, unknownError{errors.New(msg)}
// inspectResponse looks into the http response data at r to determine whether its
// content-type is on the list of acceptable content types for remote build contexts.
// This function returns:
// - a string representation of the detected content-type
// - an io.Reader for the response body
// - an error value which will be non-nil either when something goes wrong while
// reading bytes from r or when the detected content-type is not acceptable.
func inspectResponse(ct string, r io.Reader, clen int64) (string, io.ReadCloser, error) {
plen := clen
if plen <= 0 || plen > maxPreambleLength {
plen = maxPreambleLength
preamble := make([]byte, plen)
rlen, err := r.Read(preamble)
if rlen == 0 {
return ct, ioutil.NopCloser(r), errors.New("empty response")
if err != nil && err != io.EOF {
return ct, ioutil.NopCloser(r), err
preambleR := bytes.NewReader(preamble[:rlen])
bodyReader := ioutil.NopCloser(io.MultiReader(preambleR, r))
// Some web servers will use application/octet-stream as the default
// content type for files without an extension (e.g. 'Dockerfile')
// so if we receive this value we better check for text content
contentType := ct
if len(ct) == 0 || ct == mimeTypes.OctetStream {
contentType, _, err = detectContentType(preamble)
if err != nil {
return contentType, bodyReader, err
contentType = selectAcceptableMIME(contentType)
var cterr error
if len(contentType) == 0 {
cterr = fmt.Errorf("unsupported Content-Type %q", ct)
contentType = ct
return contentType, bodyReader, cterr
func selectAcceptableMIME(ct string) string {
return mimeRe.FindString(ct)