Add new "$_shell(CMD)" internal function

For testing a following patch, I wanted a way to send a SIGINT to GDB
from a breakpoint condition.  And I didn't want to do it from a Python
breakpoint or Python function, as I wanted to exercise non-Python code
paths.  So I thought I'd add a new $_shell internal function, that
runs a command under the shell, and returns the exit code.  With this,
I could write:

  (gdb) b foo if $_shell("kill -SIGINT $gdb_pid") != 0 || <other condition>

I think this is generally useful, hence I'm proposing it here.

Here's the new function in action:

 (gdb) p $_shell("true")
 $1 = 0
 (gdb) p $_shell("false")
 $2 = 1
 (gdb) p $_shell("echo hello")
 hello
 $3 = 0
 (gdb) p $_shell("foobar")
 bash: line 1: foobar: command not found
 $4 = 127
 (gdb) help function _shell
 $_shell - execute a shell command and returns the result.
 Usage: $_shell (command)
 Returns the command's exit code: zero on success, non-zero otherwise.
 (gdb)

NEWS and manual changes included.

Approved-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Eli Zaretskii <eliz@gnu.org>
Change-Id: I7e36d451ee6b428cbf41fded415ae2d6b4efaa4e

gdb/NEWS

@@ -68,6 +68,16 @@ maintenance info frame-unwinders
   List the frame unwinders currently in effect, starting with the highest
   priority.
 
+* New convenience function "$_shell", to execute a shell command and
+  return the result.  This lets you run shell commands in expressions.
+  Some examples:
+
+   (gdb) p $_shell("true")
+   $1 = 0
+   (gdb) p $_shell("false")
+   $2 = 1
+   (gdb) break func if $_shell("some command") == 0
+
 * MI changes
 
 ** mi now reports 'no-history' as a stop reason when hitting the end of the

gdb/cli/cli-cmds.c

@@ -39,6 +39,7 @@
 #include "gdbsupport/filestuff.h"
 #include "location.h"
 #include "block.h"
+#include "valprint.h"
 
 #include "ui-out.h"
 #include "interps.h"
@@ -873,6 +874,9 @@ exit_status_set_internal_vars (int exit_status)
 
   clear_internalvar (var_code);
   clear_internalvar (var_signal);
+
+  /* Keep the logic here in sync with shell_internal_fn.  */
+
   if (WIFEXITED (exit_status))
     set_internalvar_integer (var_code, WEXITSTATUS (exit_status));
 #ifdef __MINGW32__
@@ -893,8 +897,11 @@ exit_status_set_internal_vars (int exit_status)
     warning (_("unexpected shell command exit status %d"), exit_status);
 }
 
-static void
-shell_escape (const char *arg, int from_tty)
+/* Run ARG under the shell, and return the exit status.  If ARG is
+   NULL, run an interactive shell.  */
+
+static int
+run_under_shell (const char *arg, int from_tty)
 {
 #if defined(CANT_FORK) || \
       (!defined(HAVE_WORKING_VFORK) && !defined(HAVE_WORKING_FORK))
@@ -915,7 +922,7 @@ shell_escape (const char *arg, int from_tty)
      the shell command we just ran changed it.  */
   chdir (current_directory);
 #endif
-  exit_status_set_internal_vars (rc);
+  return rc;
 #else /* Can fork.  */
   int status, pid;
 
@@ -942,10 +949,21 @@ shell_escape (const char *arg, int from_tty)
     waitpid (pid, &status, 0);
   else
     error (_("Fork failed"));
-  exit_status_set_internal_vars (status);
+  return status;
 #endif /* Can fork.  */
 }
 
+/* Escape out to the shell to run ARG.  If ARG is NULL, launch and
+   interactive shell.  Sets $_shell_exitcode and $_shell_exitsignal
+   convenience variables based on the exits status.  */
+
+static void
+shell_escape (const char *arg, int from_tty)
+{
+  int status = run_under_shell (arg, from_tty);
+  exit_status_set_internal_vars (status);
+}
+
 /* Implementation of the "shell" command.  */
 
 static void
@@ -2417,6 +2435,63 @@ gdb_maint_setting_str_internal_fn (struct gdbarch *gdbarch,
   return str_value_from_setting (*show_cmd->var, gdbarch);
 }
 
+/* Implementation of the convenience function $_shell.  */
+
+static struct value *
+shell_internal_fn (struct gdbarch *gdbarch,
+		   const struct language_defn *language,
+		   void *cookie, int argc, struct value **argv)
+{
+  if (argc != 1)
+    error (_("You must provide one argument for $_shell."));
+
+  value *val = argv[0];
+  struct type *type = check_typedef (val->type ());
+
+  if (!language->is_string_type_p (type))
+    error (_("Argument must be a string."));
+
+  value_print_options opts;
+  get_no_prettyformat_print_options (&opts);
+
+  string_file stream;
+  value_print (val, &stream, &opts);
+
+  /* We should always have two quote chars, which we'll strip.  */
+  gdb_assert (stream.size () >= 2);
+
+  /* Now strip them.  We don't need the original string, so it's
+     cheaper to do it in place, avoiding a string allocation.  */
+  std::string str = stream.release ();
+  str[str.size () - 1] = 0;
+  const char *command = str.c_str () + 1;
+
+  int exit_status = run_under_shell (command, 0);
+
+  struct type *int_type = builtin_type (gdbarch)->builtin_int;
+
+  /* Keep the logic here in sync with
+     exit_status_set_internal_vars.  */
+
+  if (WIFEXITED (exit_status))
+    return value_from_longest (int_type, WEXITSTATUS (exit_status));
+#ifdef __MINGW32__
+  else if (WIFSIGNALED (exit_status) && WTERMSIG (exit_status) == -1)
+    {
+      /* See exit_status_set_internal_vars.  */
+      return value_from_longest (int_type, exit_status);
+    }
+#endif
+  else if (WIFSIGNALED (exit_status))
+    {
+      /* (0x80 | SIGNO) is what most (all?) POSIX-like shells set as
+	 exit code on fatal signal termination.  */
+      return value_from_longest (int_type, 0x80 | WTERMSIG (exit_status));
+    }
+  else
+    return value::allocate_optimized_out (int_type);
+}
+
 void _initialize_cli_cmds ();
 void
 _initialize_cli_cmds ()
@@ -2606,6 +2681,19 @@ Some integer settings accept an unlimited value, returned\n\
 as 0 or -1 depending on the setting."),
 			 gdb_maint_setting_internal_fn, NULL);
 
+  add_internal_function ("_shell", _("\
+$_shell - execute a shell command and return the result.\n\
+\n\
+    Usage: $_shell (COMMAND)\n\
+\n\
+    Arguments:\n\
+\n\
+      COMMAND: The command to execute.  Must be a string.\n\
+\n\
+    Returns:\n\
+      The command's exit code: zero on success, non-zero otherwise."),
+			 shell_internal_fn, NULL);
+
   add_cmd ("commands", no_set_class, show_commands, _("\
 Show the history of commands you typed.\n\
 You can supply a command number to start with, or a `+' to start after\n\

gdb/doc/gdb.texinfo

@@ -1621,7 +1621,7 @@ just use the @code{shell} command.
 @cindex shell escape
 @item shell @var{command-string}
 @itemx !@var{command-string}
-Invoke a standard shell to execute @var{command-string}.
+Invoke a shell to execute @var{command-string}.
 Note that no space is needed between @code{!} and @var{command-string}.
 On GNU and Unix systems, the environment variable @env{SHELL}, if it
 exists, determines which shell to run.  Otherwise @value{GDBN} uses
@@ -1629,6 +1629,10 @@ the default shell (@file{/bin/sh} on GNU and Unix systems,
 @file{cmd.exe} on MS-Windows, @file{COMMAND.COM} on MS-DOS, etc.).
 @end table
 
+You may also invoke shell commands from expressions, using the
+@code{$_shell} convenience function.  @xref{$_shell convenience
+function}.
+
 The utility @code{make} is often needed in development environments.
 You do not have to use the @code{shell} command for this purpose in
 @value{GDBN}:
@@ -12977,6 +12981,60 @@ Like the @code{$_gdb_setting_str} function, but works with
 Like the @code{$_gdb_setting} function, but works with
 @code{maintenance set} variables.
 
+@anchor{$_shell convenience function}
+@findex $_shell@r{, convenience function}
+@item $_shell (@var{command-string})
+
+Invoke a shell to execute @var{command-string}.  @var{command-string}
+must be a string.  The shell runs on the host machine, the machine
+@value{GDBN} is running on.  Returns the command's exit status.  On
+Unix systems, a command which exits with a zero exit status has
+succeeded, and non-zero exit status indicates failure.  When a command
+terminates on a fatal signal whose number is @var{N}, @value{GDBN}
+uses the value 128+@var{N} as the exit status, as is standard in Unix
+shells.  Note that @var{N} is a host signal number, not a target
+signal number.  If you're native debugging, they will be the same, but
+if cross debugging, the host vs target signal numbers may be
+completely unrelated.  Please consult your host operating system's
+documentation for the mapping between host signal numbers and signal
+names.  The shell to run is determined in the same way as for the
+@code{shell} command.  @xref{Shell Commands, ,Shell Commands}.
+
+@smallexample
+(@value{GDBP}) print $_shell("true")
+$1 = 0
+(@value{GDBP}) print $_shell("false")
+$2 = 1
+(@value{GDBP}) p $_shell("echo hello")
+hello
+$3 = 0
+(@value{GDBP}) p $_shell("foobar")
+bash: line 1: foobar: command not found
+$4 = 127
+@end smallexample
+
+This may also be useful in breakpoint conditions.  For example:
+
+@smallexample
+(@value{GDBP}) break function if $_shell("some command") == 0
+@end smallexample
+
+In this scenario, you'll want to make sure that the shell command you
+run in the breakpoint condition takes the least amount of time
+possible.  For example, avoid running a command that may block
+indefinitely, or that sleeps for a while before exiting.  Prefer a
+command or script which analyzes some state and exits immediately.
+This is important because the debugged program stops for the
+breakpoint every time, and then @value{GDBN} evaluates the breakpoint
+condition.  If the condition is false, the program is re-resumed
+transparently, without informing you of the stop.  A quick shell
+command thus avoids significantly slowing down the debugged program
+unnecessarily.
+
+Note: unlike the @code{shell} command, the @code{$_shell} convenience
+function does not affect the @code{$_shell_exitcode} and
+@code{$_shell_exitsignal} convenience variables.
+
 @end table
 
 The following functions require @value{GDBN} to be configured with

gdb/testsuite/gdb.base/default.exp

@@ -606,6 +606,7 @@ set show_conv_list \
 	{$_cimag = <internal function _cimag>} \
 	{$_creal = <internal function _creal>} \
 	{$_isvoid = <internal function _isvoid>} \
+	{$_shell = <internal function _shell>} \
 	{$_gdb_maint_setting_str = <internal function _gdb_maint_setting_str>} \
 	{$_gdb_maint_setting = <internal function _gdb_maint_setting>} \
 	{$_gdb_setting_str = <internal function _gdb_setting_str>} \

gdb/testsuite/gdb.base/shell.exp

@@ -41,6 +41,42 @@ if { ! [ishost *-*-mingw*] } {
     gdb_test "p \$_shell_exitsignal" " = 2" "shell interrupt exitsignal"
 }
 
+# Test the $_shell convenience function.
+
+with_test_prefix "\$_shell convenience function" {
+    # Simple commands, check the result code.
+    gdb_test "p \$_shell(\"true\")" " = 0"
+    gdb_test "p \$_shell(\"false\")" " = 1"
+
+    # Test command with arguments.
+    gdb_test "p \$_shell(\"echo foo\")" "foo\r\n\\$${decimal} = 0"
+
+    # Check the type of the result.
+    gdb_test "ptype \$_shell(\"true\")" "type = int"
+
+    # Test passing a non-literal string as command name.
+    gdb_test "p \$cmd = \"echo bar\"" " = \"echo bar\""
+    gdb_test "p \$_shell(\$cmd)" "bar\r\n\\$${decimal} = 0"
+
+    # Test executing a non-existing command.  The result is
+    # shell-dependent, but most (all?) POSIX-like shells return 127 in
+    # this case.
+    gdb_test "p \$_shell(\"non-existing-command-foo-bar-qux\")" " = 127"
+
+    gdb_test "p \$_shell" \
+	" = <internal function _shell>"
+    gdb_test "ptype \$_shell" \
+	"type = <internal function>"
+
+    # Test error scenarios.
+    gdb_test "p \$_shell()" \
+	"You must provide one argument for \\\$_shell\\\."
+    gdb_test "p \$_shell(\"a\", \"b\")" \
+	"You must provide one argument for \\\$_shell\\\."
+    gdb_test "p \$_shell(1)" \
+	"Argument must be a string\\\."
+}
+
 # Define the user command "foo", used to test "pipe" command.
 gdb_test_multiple "define foo" "define foo" {
     -re "End with"  {