[RFC PATCH 2/2] Allow and evaluate expressions in command arguments

[Keith Seitz <keiths@redhat.com>]

This patch adds support for arbitrary expressions to be passed to
GDB commands, evaluated, and the results substituted into command
arguments.

This allows users to pass arbitrary expressions into commands which
currently do not support any way of doing this. This is especially
useful with, for example, convenience variables and functions.

Example:
$ export MYDIR="tmp"
$ export MYFILE="simple"
$ gdb -nx -q
(gdb) file $($_env("HOME"))/$($_env("MYDIR"))/$($_env("MYFILE"))
Reading symbols from /home/keiths/tmp/simple...
(gdb)

In order to mark the bounds of expressions to be evaluated, I've
introduced the (arbitrary) marker "$()". Everything contained inside
this marker will be passed to the expression parser, evaluated, printed
as a string, and then substituted for the original expression.
---
 gdb/top.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/gdb/top.c b/gdb/top.c
index 205eb360ba3..3f394b5b63c 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -57,6 +57,8 @@
 #include "gdbsupport/pathstuff.h"
 #include "cli/cli-style.h"
 #include "pager.h"
+#include "valprint.h"
+#include "cp-support.h" 	/* for find_toplevel_char  */
 
 /* readline include files.  */
 #include "readline/readline.h"
@@ -567,6 +569,60 @@ set_repeat_arguments (const char *args)
   repeat_arguments = args;
 }
 
+/* Evaluate and expand any expressions in the command line list of arguments
+   given by ORIG_ARGS.  All occurrences of "$(expr)" will be replaced with a
+   string representation of the evaluated EXPR.  */
+
+#define EVAL_START_STRING "$("
+#define EVAL_END_TOKEN ')'
+
+static std::string
+expand_command_arg (const char *orig_arg)
+{
+  std::string new_arg = orig_arg;
+  std::string::size_type n = new_arg.find (EVAL_START_STRING);
+
+  /* These needs adjustment to output "simple" strings.  For example,
+     if a char value is used, we end up with "97 'a'" instead of simply "a".  */
+  struct value_print_options opts;
+  get_user_print_options (&opts);
+
+  string_file stb;
+
+  while (n != std::string::npos)
+    {
+      const char *args = new_arg.c_str ();
+      const char *c = find_toplevel_char (args + n + 2, EVAL_END_TOKEN);
+
+      if (c != nullptr)
+	{
+	  std::string expr = new_arg.substr (n + 2, c - args - n - 2);
+	  struct value *val = parse_and_eval (expr.c_str ());
+
+	  value_print (val, &stb, &opts);
+
+	  /* If value_print returns a quote-enclosed string, remove the
+	     quote characters.  */
+	  std::string r = stb.release ();
+
+	  if (*r.begin () == '\"' && *r.rbegin () == '\"')
+	    {
+	      r.erase (r.begin ());
+	      r.pop_back ();
+	    }
+
+	  new_arg.replace (n, c - args - n + 1, r);
+	  n = new_arg.find (EVAL_START_STRING);
+	}
+    }
+
+#if 0
+  if (new_arg != orig_arg)
+    printf ("NEW_ARG = %s\n", new_arg.c_str ());
+#endif
+  return new_arg;
+}
+
 /* Execute the line P as a command, in the current user context.
    Pass FROM_TTY as second argument to the defining function.  */
 
@@ -657,6 +713,15 @@ execute_command (const char *p, int from_tty)
       if (c->deprecated_warn_user)
 	deprecated_cmd_warning (line, cmdlist);
 
+      /* Do any expression substitutions on ARG.  */
+      std::string expanded_arg;
+
+      if (arg != nullptr)
+	{
+	  expanded_arg = expand_command_arg (arg);
+	  arg = expanded_arg.c_str ();
+	}
+
       /* c->user_commands would be NULL in the case of a python command.  */
       if (c->theclass == class_user && c->user_commands)
 	execute_user_command (c, arg);
-- 
2.38.1