dot: replace large labels by "(label too long)"

Based on a report by Victor Khomenko.

* spot/twaalgos/dot.cc: Here.
* tests/core/readsave.test: Add test case.
* NEWS: Mention it.

NEWS

@@ -1,6 +1,11 @@
 New in spot 2.7.4.dev (not yet released)
 
-  Nothing yet.
+  Library:
+
+  - print_dot will replace labels that have more 2048 characters by a
+    "(label too long)" string.  This works around a limitation of
+    GraphViz that aborts when some label exceeds 16k characters, and
+    also helps making large automata more readable.
 
 New in spot 2.7.4 (2019-04-27)
 

spot/twaalgos/dot.cc

@@ -440,7 +440,15 @@ namespace spot
             print_sclatex_psl(os << '$', f) << '$';
             return os;
           }
-        return escape_for_output(os, str_psl(f));
+        // GraphViz (2.40.1) has a strict limit of 16k for label
+        // length.  The limit we use below is more conservative,
+        // because (1) escaping the html element in the string
+        // (e.g., "&" -> "&amp;") can increase it a lot, and (2)
+        // a label of size 2048 is already unreasonable to display.
+        std::string s = str_psl(f);
+        if (s.size() > 2048)
+          s = "(label too long)";
+        return escape_for_output(os, s);
       }
 
       std::ostream&

tests/core/readsave.test

@@ -1085,3 +1085,8 @@ digraph "a U b" {
 }
 EOF
 diff out expected
+
+f="{{!a;!b}:{{c <-> d} && {e xor f} && {m | {l && {k | {j <-> {i xor {g && h}"
+f="$f}}}}} && {{n && o} | {!n && p}} && {q -> {r <-> s}}}:{[*0..1];t}}[]-> u"
+ltl2tgba -f "$f" --dot=bar > out.dot
+grep 'label too long' out.dot