importfeed: When there's a problem parsing the feed, --debug will output the feed content that was downloaded.

And let the user know about it in the failure messages.

CHANGELOG

@@ -10,6 +10,8 @@ git-annex (7.20190616) UNRELEASED; urgency=medium
     avoid using http range to resume, since a range "bytes=0-" is an unusual
     edge case that it's best to avoid relying on working. This is known to
     fix a case where importfeed downloaded a partial feed from such a server.
+  * importfeed: When there's a problem parsing the feed, --debug will
+    output the feed content that was downloaded.
 
  -- Joey Hess <id@joeyh.name>  Sat, 15 Jun 2019 12:38:25 -0400
 

Command/ImportFeed.hs

@@ -21,6 +21,7 @@ import Data.Time.Format
 import System.Locale
 #endif
 import qualified Data.Text as T
+import System.Log.Logger
 
 import Command
 import qualified Annex
@@ -75,11 +76,9 @@ getFeed opts cache url = do
 		Nothing -> showEndResult =<< feedProblem url
 			"downloading the feed failed"
 		Just feedcontent -> case parseFeedString feedcontent of
-			Nothing -> showEndResult =<< feedProblem url
+			Nothing -> debugfeedcontent feedcontent "parsing the feed failed"
-				"parsing the feed failed"
 			Just f -> case findDownloads url f of
-				[] -> showEndResult =<< feedProblem url
+				[] -> debugfeedcontent feedcontent "bad feed content; no enclosures to download"
-					"bad feed content; no enclosures to download"
 				l -> do
 					showEndOk
 					ifM (and <$> mapM (performDownload opts cache) l)
@@ -87,6 +86,15 @@ getFeed opts cache url = do
 						, void $ feedProblem url 
 							"problem downloading some item(s) from feed"
 						)
+  where
+	debugfeedcontent feedcontent msg = do
+		liftIO $ debugM "feed content" $ unlines
+			[ "start of feed content"
+			, feedcontent
+			, "end of feed content"
+			]
+		showEndResult =<< feedProblem url
+			(msg ++ " (use --debug to see the feed content that was downloaded)")
 
 data ToDownload = ToDownload
 	{ feed :: Feed