Show the full URL in download error messages

Downloads set req.URL.Opaque to keep "+" percent-encoded in the path.
URL.String() prints "scheme:opaque" and drops the host, so every error
from the http client showed a URL such as "https:/repos/debian/...",
lacking a host name. The request itself was correct, but the message
sent me to look for a malformed URL.

Set req.URL.RawPath instead. The escaping still reaches the wire through
EscapedPath(), and String() keeps the scheme and the host.

The proxy special case is no longer necessary. Requests through a proxy
now get the same "+" escaping as direct requests.

http/download.go

@@ -163,11 +163,10 @@ func (downloader *downloaderImpl) newRequest(ctx context.Context, method, url st
 	req.Close = true
 	req = req.WithContext(ctx)
 
-	proxyURL, _ := downloader.client.Transport.(*http.Transport).Proxy(req)
-	if proxyURL == nil && (req.URL.Scheme == "http" || req.URL.Scheme == "https") {
-		req.URL.Opaque = strings.Replace(req.URL.RequestURI(), "+", "%2b", -1)
-		req.URL.RawQuery = ""
-	}
+	// Some servers (Amazon S3) read a literal "+" in a path as a space, so keep
+	// it percent-encoded. RawPath does this without hiding the host the way
+	// URL.Opaque does, so error messages still show the full URL.
+	req.URL.RawPath = strings.ReplaceAll(req.URL.EscapedPath(), "+", "%2b")
 
 	return req, nil
 }

http/download_test.go

@@ -109,6 +109,16 @@ func (s *DownloaderSuite) TestDownloadWithChecksum(c *C) {
 	c.Check(checksums.SHA512, Equals, "bac18bf4e564856369acc2ed57300fecba3a2c1af5ae8304021e4252488678feb18118466382ee4e1210fe1f065080210e453a80cfb37ccb8752af3269df160e")
 }
 
+func (s *DownloaderSuite) TestNewRequestEscapesPlus(c *C) {
+	d := s.d.(*downloaderImpl)
+
+	req, err := d.newRequest(s.ctx, "GET", s.url+"/test/pkg_1.0+dfsg-1_amd64.deb?a=b")
+	c.Assert(err, IsNil)
+	c.Check(req.URL.RequestURI(), Equals, "/test/pkg_1.0%2bdfsg-1_amd64.deb?a=b")
+	// the host must stay visible, it ends up in error messages
+	c.Check(req.URL.String(), Equals, s.url+"/test/pkg_1.0%2bdfsg-1_amd64.deb?a=b")
+}
+
 func (s *DownloaderSuite) TestDownload404(c *C) {
 	c.Assert(s.d.Download(s.ctx, s.url+"/doesntexist", s.tempfile.Name()),
 		ErrorMatches, "HTTP code 404.*")