HTML reader: fix bad handling of empty src attribute in iframe.

- If src is empty, we simply skip the iframe.
- If src is invalid or cannot be fetched, we issue a warning
  and skip instead of failing with an error.
- Closes #7099.

src/Text/Pandoc/Readers/HTML.hs

@@ -26,7 +26,7 @@ module Text.Pandoc.Readers.HTML ( readHtml
 
 import Control.Applicative ((<|>))
 import Control.Monad (guard, msum, mzero, unless, void)
-import Control.Monad.Except (throwError)
+import Control.Monad.Except (throwError, catchError)
 import Control.Monad.Reader (ask, asks, lift, local, runReaderT)
 import Data.ByteString.Base64 (encode)
 import Data.Char (isAlphaNum, isLetter)
@@ -393,11 +393,17 @@ pIframe = try $ do
   tag <- pSatisfy (tagOpen (=="iframe") (isJust . lookup "src"))
   pCloses "iframe" <|> eof
   url <- canonicalizeUrl $ fromAttrib "src" tag
-  (bs, _) <- openURL url
+  if T.null url
+     then ignore $ renderTags' [tag, TagClose "iframe"]
+     else catchError
+       (do (bs, _) <- openURL url
            let inp = UTF8.toText bs
            opts <- readerOpts <$> getState
            Pandoc _ contents <- readHtml opts inp
-  return $ B.divWith ("",["iframe"],[]) $ B.fromList contents
+           return $ B.divWith ("",["iframe"],[]) $ B.fromList contents)
+       (\e -> do
+         logMessage $ CouldNotFetchResource url (renderError e)
+         ignore $ renderTags' [tag, TagClose "iframe"])
 
 pRawHtmlBlock :: PandocMonad m => TagParser m Blocks
 pRawHtmlBlock = do

test/command/7099.md

@@ -2,7 +2,17 @@
 % pandoc -f html -t native --verbose
 <iframe src=""></iframe>
 ^D
-[INFO] Fetching ...
-[INFO] Skipped '<iframe src></iframe>' at input line 1 column 1
+[INFO] Skipped '<iframe src></iframe>' at input line 1 column 16
+[]
+```
+
+```
+% pandoc -f html -t native --verbose
+<iframe src="h:invalid@url"></iframe>
+^D
+[INFO] Fetching h:invalid@url...
+[WARNING] Could not fetch resource 'h:invalid@url': Could not fetch h:invalid@url
+  InvalidUrlException "h:invalid@url" "Invalid scheme"
+[INFO] Skipped '<iframe src="h:invalid@url"></iframe>' at input line 1 column 29
 []
 ```