package builder
 
 import (
 	"bytes"
 	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"regexp"
 
 	"github.com/docker/docker/pkg/httputils"
 )
 
 // 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 MakeTarSumContext whose result is returned.
 func MakeRemoteContext(remoteURL string, contentTypeHandlers map[string]func(io.ReadCloser) (io.ReadCloser, error)) (ModifiableContext, error) {
 	f, err := httputils.Download(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
 				}
 				break
 			}
 		}
 	}
 
 	// Pass through - this is a pre-packaged context, presumably
 	// with a Dockerfile with the right name inside it.
 	return MakeTarSumContext(contextReader)
 }
 
 // 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.ReadCloser, clen int64) (string, io.ReadCloser, error) {
 	plen := clen
 	if plen <= 0 || plen > maxPreambleLength {
 		plen = maxPreambleLength
 	}
 
 	preamble := make([]byte, plen, plen)
 	rlen, err := r.Read(preamble)
 	if rlen == 0 {
 		return ct, r, errors.New("Empty response")
 	}
 	if err != nil && err != io.EOF {
 		return ct, r, err
 	}
 
 	preambleR := bytes.NewReader(preamble)
 	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 == httputils.MimeTypes.OctetStream {
 		contentType, _, err = httputils.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)
 }