Fix accidentally quadratic behavior in _modify

We need to be careful to not grab an extra reference when mutating data
structures because that means we make extra copies.  Doing that every
time in `_modify` is really painful, as that function implements `|=`
and all modify-assign operators.

`jv_setpath()` also grabs additional references, so this is not the only
fix needed for the modify-assign operators to not be accidentally
quadratic.

We have to use `LOADVN` in order to make this fix possible, so we should
really byte-code `_modify` rather than jq-code it.  However, as a
stop-gap to make this fix easier, I'm adding syntax for referring to a
`$binding` such that you get `LOADVN` instead of `LOADV`.

This syntax is not meant to be used outside jq's internals!  Therefore
it is not documented.  I promise to break it later, so don't use it!

TBD, but later:

 - Optimize `_modify` for the case where `update` outputs more than one
   value.

   This is trivial: add a `setpath($p; null)` in the middle of `_modify`
   before calling `update`.  But unfortunately the VM retains a
   reference to `value_at_path` for path expression checking, and fixing
   that will require more work.

src/builtin.jq

@@ -11,7 +11,25 @@ def min_by(f): _min_by_impl(map([f]));
 def add: reduce .[] as $x (null; . + $x);
 def del(f): delpaths([path(f)]);
 def _assign(paths; $value): reduce path(paths) as $p (.; setpath($p; $value));
-def _modify(paths; update): reduce path(paths) as $p (.; label $out | (setpath($p; getpath($p) | update) | ., break $out), delpaths([$p]));
+def _modify(paths; update):
+    reduce path(paths) as $p (.;
+        . as $dot
+      | null
+      | label $out
+      | ($dot | getpath($p)) as $v
+      | (
+          (   $$$$v
+            | update
+            | (., break $out) as $v
+            | $$$$dot
+            | setpath($p; $v)
+          ),
+          (
+              $$$$dot
+            | delpaths([$p])
+          )
+        )
+    );
 def map_values(f): .[] |= f;
 
 # recurse

src/parser.h

@@ -1,9 +1,8 @@
-/* A Bison parser, made by GNU Bison 3.3.2.  */
+/* A Bison parser, made by GNU Bison 3.0.4.  */
 
 /* Bison interface for Yacc-like parsers in C
 
-   Copyright (C) 1984, 1989-1990, 2000-2015, 2018-2019 Free Software Foundation,
-   Inc.
+   Copyright (C) 1984, 1989-1990, 2000-2015 Free Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -31,9 +30,6 @@
    This special exception was added by the Free Software Foundation in
    version 2.2 of Bison.  */
 
-/* Undocumented macros, especially those whose name start with YY_,
-   are private implementation details.  Do not rely on them.  */
-
 #ifndef YY_YY_SRC_PARSER_H_INCLUDED
 # define YY_YY_SRC_PARSER_H_INCLUDED
 /* Debug traces.  */
@@ -44,7 +40,7 @@
 extern int yydebug;
 #endif
 /* "%code requires" blocks.  */
-#line 11 "src/parser.y" /* yacc.c:1927  */
+#line 11 "src/parser.y" /* yacc.c:1909  */
 
 #include "locfile.h"
 struct lexer_param;
@@ -61,7 +57,7 @@ struct lexer_param;
     }                                           \
   } while (0)
 
-#line 65 "src/parser.h" /* yacc.c:1927  */
+#line 61 "src/parser.h" /* yacc.c:1909  */
 
 /* Token type.  */
 #ifndef YYTOKENTYPE
@@ -167,12 +163,12 @@ struct lexer_param;
 
 union YYSTYPE
 {
-#line 31 "src/parser.y" /* yacc.c:1927  */
+#line 31 "src/parser.y" /* yacc.c:1909  */
 
   jv literal;
   block blk;
 
-#line 176 "src/parser.h" /* yacc.c:1927  */
+#line 172 "src/parser.h" /* yacc.c:1909  */
 };
 
 typedef union YYSTYPE YYSTYPE;

src/parser.y

@@ -749,6 +749,25 @@ FORMAT {
   else
     $$ = BLOCK(gen_subexp(gen_const(jv_object())), $2, gen_op_simple(POP));
 } |
+/*
+ * This `$$$$varname` hack is strictly private to jq builtins.  DO NOT USE!!
+ *
+ * This is used in `_modify`, in src/builtin.jq, to avoid holding on to a
+ * reference to `.`.
+ *
+ * We could just have the compiler emit bytecode for `_modify` so it can use
+ * LOADVN w/o needing jq syntax for LOADVN.
+ *
+ * This syntax, `$$$$varname`, violates referential transparency: it has
+ * side-effects that are surprising.
+ *
+ * DO NOT USE!!  I will break your jq code if you do use this outside
+ * src/builtin.jq.
+ */
+'$' '$' '$' '$' IDENT {
+  $$ = gen_location(@$, locations, gen_op_unbound(LOADVN, jv_string_value($5)));
+  jv_free($5);
+} |
 '$' IDENT {
   $$ = gen_location(@$, locations, gen_op_unbound(LOADV, jv_string_value($2)));
   jv_free($2);