Chunks: autogenerate unique ids for sections missing them.

This is needed for TOC generation to work properly. We can't
create TOC links if there are no ids.

This fixes some EPUB validation issues we've been getting since
switching over to Chunks for chunking.

Closes #9383.

src/Text/Pandoc/Chunks.hs

@@ -29,7 +29,7 @@ module Text.Pandoc.Chunks
 
 import Text.Pandoc.Definition
 import Text.Pandoc.Shared (makeSections, stringify, inlineListToIdentifier,
-                           tshow)
+                           tshow, uniqueIdent)
 import Text.Pandoc.Walk (Walkable(..), query)
 import Data.Aeson (FromJSON, ToJSON)
 import Data.Text (Text)
@@ -43,6 +43,8 @@ import Text.HTML.TagSoup (Tag (TagOpen), fromAttrib, parseTags)
 import Data.Tree (Tree(..))
 import Data.Data (Data)
 import Data.Typeable (Typeable)
+import qualified Data.Set as Set
+import Control.Monad.State
 
 -- | Split 'Pandoc' into 'Chunk's, e.g. for conversion into
 -- a set of HTML pages or EPUB chapters.
@@ -63,7 +65,23 @@ splitIntoChunks pathTemplate numberSections mbBaseLevel
  where
   tocTree = fixTOCTreePaths chunks $ toTOCTree sections
   chunks = makeChunks chunklev pathTemplate meta $ sections
-  sections = makeSections numberSections mbBaseLevel $ blocks
+  sections = ensureIds $ makeSections numberSections mbBaseLevel blocks
+
+-- The TOC won't work if we don't have unique identifiers for all sections.
+ensureIds :: [Block] -> [Block]
+ensureIds bs = evalState (walkM go bs) mempty
+ where
+   go :: Block -> State (Set.Set Text) Block
+   go b@(Div (ident,"section":cls,kvs) bs'@(Header _ _ ils : _))
+     | T.null ident
+       = do ids <- get
+            let newid = uniqueIdent mempty ils ids
+            modify $ Set.insert newid
+            pure $ Div (newid,"section":cls,kvs) bs'
+     | otherwise
+       = do modify $ Set.insert ident
+            pure b
+   go b = pure b
 
 -- | Add chunkNext, chunkPrev, chunkUp
 addNav :: ChunkedDoc -> ChunkedDoc