Fix plpgsql's handling of "simple" expression evaluation.

In general, expression execution state trees aren't re-entrantly usable,
since functions can store private state information in them.
For efficiency reasons, plpgsql tries to cache and reuse state trees for
"simple" expressions.  It can get away with that most of the time, but it
can fail if the state tree is dirty from a previous failed execution (as
in an example from Alvaro) or is being used recursively (as noted by me).

Fix by tracking whether a state tree is in use, and falling back to the
"non-simple" code path if so.  This results in a pretty considerable speed
hit when the non-simple path is taken, but the available alternatives seem
even more unpleasant because they add overhead in the simple path.  Per
idea from Heikki.

Back-patch to all supported branches.

src/test/regress/sql/plpgsql.sql

@@ -3165,6 +3165,48 @@ SELECT nonsimple_expr_test();
 
 DROP FUNCTION nonsimple_expr_test();
 
+--
+-- Test cases involving recursion and error recovery in simple expressions
+-- (bugs in all versions before October 2010).  The problems are most
+-- easily exposed by mutual recursion between plpgsql and sql functions.
+--
+
+create function recurse(float8) returns float8 as
+$$
+begin
+  if ($1 < 10) then
+    return sql_recurse($1 + 1);
+  else
+    return $1;
+  end if;
+end;
+$$ language plpgsql;
+
+-- "limit" is to prevent this from being inlined
+create function sql_recurse(float8) returns float8 as
+$$ select recurse($1) limit 1; $$ language sql;
+
+select recurse(0);
+
+create function error1(text) returns text language sql as
+$$ SELECT relname::text FROM pg_class c WHERE c.oid = $1::regclass $$;
+
+create function error2(p_name_table text) returns text language plpgsql as $$
+begin
+  return error1(p_name_table);
+end$$;
+
+BEGIN;
+create table public.stuffs (stuff text);
+SAVEPOINT a;
+select error2('nonexistent.stuffs');
+ROLLBACK TO a;
+select error2('public.stuffs');
+rollback;
+
+drop function error2(p_name_table text);
+drop function error1(text);
+
 -- Test handling of string literals.
 
 set standard_conforming_strings = off;