make curl show http errors to stderr

* Run curl with -S, so HTTP errors are displayed, even when
  it's otherwise silent.
* When downloading in --json or --quiet mode, use curl in preference
  to wget, since curl is able to display only errors to stderr, unlike
  wget.

This does mean that downloadQuiet is only silent on stdout, not necessarily
on stderr, which affects a couple other calls of it. For example,
downloading the .git/config of a http remote may show an error message now,
perhaps with slightly suboptimal formatting due to other output.

CHANGELOG

@@ -27,6 +27,11 @@ git-annex (6.20170215) UNRELEASED; urgency=medium
   * adjust: Fix behavior when used in a repository that contains
     submodules.
   * Run wget with -nv instead of -q, so it will display HTTP errors.
+  * Run curl with -S, so HTTP errors are displayed, even when
+    it's otherwise silent.
+  * When downloading in --json or --quiet mode, use curl in preference
+    to wget, since curl is able to display only errors to stderr, unlike
+    wget.
 
  -- Joey Hess <id@joeyh.name>  Tue, 14 Feb 2017 15:54:25 -0400
 

Utility/Url.hs

@@ -246,17 +246,16 @@ headRequest r = r
 		(requestHeaders r)
 	}
 
-{- Used to download large files, such as the contents of keys.
+{- Download a perhaps large file, with auto-resume of incomplete downloads.
  -
- - Uses wget or curl program for its progress bar. (Wget has a better one,
- - so is preferred.) Which program to use is determined at run time; it
- - would not be appropriate to test at configure time and build support
- - for only one in.
+ - Uses wget or curl program for its progress bar and resuming support. 
+ - Which program to use is determined at run time depending on which is
+ - in path and which works best in a particular situation.
  -}
 download :: URLString -> FilePath -> UrlOptions -> IO Bool
 download = download' False
 
-{- No output, even on error. -}
+{- No output to stdout. -}
 downloadQuiet :: URLString -> FilePath -> UrlOptions -> IO Bool
 downloadQuiet = download' True
 
@@ -265,6 +264,12 @@ download' quiet url file uo = do
 	case parseURIRelaxed url of
 		Just u
 			| uriScheme u == "file:" -> curl
+			-- curl is preferred in quiet mode, because
+			-- it displays http errors to stderr, while wget
+			-- does not display them in quiet mode
+			| quiet -> ifM (inPath "curl") (curl, wget)
+			-- wget is preferred mostly because it has a better
+			-- progress bar
 			| otherwise -> ifM (inPath "wget") (wget , curl)
 		_ -> return False
   where
@@ -275,7 +280,8 @@ download' quiet url file uo = do
 	 - support, or need that option.
 	 -
 	 - When the wget version is new enough, pass options for
-	 - a less cluttered download display.
+	 - a less cluttered download display. Using -nv rather than -q
+	 - avoids most clutter while still displaying http errors.
 	 -}
 #ifndef __ANDROID__
 	wgetparams = concat
@@ -296,7 +302,7 @@ download' quiet url file uo = do
 		-- curl does not create destination file
 		-- if the url happens to be empty, so pre-create.
 		writeFile file ""
-		go "curl" $ headerparams ++ quietopt "-s" ++
+		go "curl" $ headerparams ++ quietopt "-sS" ++
 			[ Param "-f"
 			, Param "-L"
 			, Param "-C", Param "-"

doc/todo/more_of_diagnostic_information_in_case_of_failures_into_returned_json.mdwn

@@ -18,3 +18,5 @@ ATM I am experiencing sporadic failures of the batched  git annex addurl call --
 besides me blindly trying to re-run it e.g. 3 times and only then declare total failure, I wondered if json output could provide more information (if any known) about the failure... e.g. if a custom remote crashed/errorred (I guess the case here due to "from datalad") -- what was stderr/exit code for that process if crashed/ERROR msg... if wget -- what was stderr there
 
 [[!meta name=yoh]]
+
+> Switched to curl with -sS in --json mode. [[done]] I suppose. --[[Joey]]

doc/todo/more_of_diagnostic_information_in_case_of_failures_into_returned_json/comment_2_82b851629c695084cbf62e2b636bcc91._comment

@@ -3,13 +3,14 @@
  subject="""comment 2"""
  date="2017-02-20T19:15:18Z"
  content="""
+It seems there is no way to get only errors on
+stderr with wget; the choice is between no output, and a mixture of errors
+and informational messages on stderr. In --json or --quiet mode, only
+errors should be output to stderr.
+
 In general, the --json output does include a "note" with any
 available message about why an operation failed. 
 
-Since wget outputs HTTP errors to stdout, there's no way to capture
-the actual message for json, and so swiching to wget -nv won't improve
-the json, since in json mode wget is run with -q.
-
 It would not be hard to use a HTTP library and propagate the HTTP errors
 into the json "note", but it might be hard to get resumption of partial
 downloads to work as well with a HTTP library as it works with wget/curl.