bin: fix handling for --output & --format with LTL outputs

* bin/common_output.cc: Set the output stream for LTL formats.
* tests/core/serial.test: Add a test case about this issue that also
improve the covering of the previous patch about saving file
descriptors.

NEWS

@@ -35,6 +35,9 @@ New in spot 2.11.5.dev  (not yet released)
     where thousands of different filenames can be created failed with
     "Too many open files".  (Issue #534)
 
+  - Using --format=... on a tool that output formulas would force
+    the output on standard output, even when --output was given.
+
 
 New in spot 2.11.5  (2023-04-20)
 

bin/common_output.cc

@@ -225,7 +225,7 @@ namespace
     }
   };
 
-  class formula_printer final: protected spot::formater
+  class formula_printer final: public spot::formater
   {
   public:
     formula_printer(std::ostream& os, const char* format)
@@ -399,6 +399,7 @@ output_formula(std::ostream& out,
     {
       formula_with_location fl = { f, filename, linenum,
                                    index, prefix, suffix };
+      format->set_output(out);
       format->print(fl, ptimer);
     }
 }

tests/core/serial.test

@@ -78,3 +78,16 @@ test 2000 = `ls -l randaut-*.hoa | wc -l`
 # likewise for LTL formulas
 randltl 2 -n 2000 -o randltl-%l.ltl
 test 2000 = `ls -l randltl-*.ltl | wc -l`
+
+
+# Test the code path that works that has to reopen files
+randltl -n100 --tree-size 1 26 --allow-dups -o '%f'.ltl --format=pass1
+randltl -n100 --tree-size 1 26 --allow-dups -o '>>%f'.ltl --format=pass2
+(uniq -c p1.ltl; uniq -c p20.ltl) | sed 's/^ *\([0-9][0-9]*\)  */\1 /g' >out
+cat >expected <<EOF
+4 pass1
+4 pass2
+5 pass1
+5 pass2
+EOF
+diff out expected