Simplify interp::exec / interp_exec - let exceptions propagate

This patch implements a simplication that I suggested here:

  https://sourceware.org/pipermail/gdb-patches/2022-March/186320.html

Currently, the interp::exec virtual method interface is such that
subclass implementations must catch exceptions and then return them
via normal function return.

However, higher up the in chain, for the CLI we get to
interpreter_exec_cmd, which does:

  for (i = 1; i < nrules; i++)
    {
      struct gdb_exception e = interp_exec (interp_to_use, prules[i]);

      if (e.reason < 0)
	{
	  interp_set (old_interp, 0);
	  error (_("error in command: \"%s\"."), prules[i]);
	}
    }

and for MI we get to mi_cmd_interpreter_exec, which has:

  void
  mi_cmd_interpreter_exec (const char *command, char **argv, int argc)
  {
  ...
    for (i = 1; i < argc; i++)
      {
	struct gdb_exception e = interp_exec (interp_to_use, argv[i]);

	if (e.reason < 0)
	  error ("%s", e.what ());
      }
  }

Note that if those errors are reached, we lose the original
exception's error code.  I can't see why we'd want that.

And, I can't see why we need to have interp_exec catch the exception
and return it via the normal return path.  That's normally needed when
we need to handle propagating exceptions across C code, like across
readline or ncurses, but that's not the case here.

It seems to me that we can simplify things by removing some
try/catch-ing and just letting exceptions propagate normally.

Note, the "error in command" error shown above, which only exists in
the CLI interpreter-exec command, is only ever printed AFAICS if you
run "interpreter-exec console" when the top level interpreter is
already the console/tui.  Like:

 (gdb) interpreter-exec console "foobar"
 Undefined command: "foobar".  Try "help".
 error in command: "foobar".

You won't see it with MI's "-interpreter-exec console" from a top
level MI interpreter:

 (gdb)
 -interpreter-exec console "foobar"
 &"Undefined command: \"foobar\".  Try \"help\".\n"
 ^error,msg="Undefined command: \"foobar\".  Try \"help\"."
 (gdb)

nor with MI's "-interpreter-exec mi" from a top level MI interpreter:

 (gdb)
 -interpreter-exec mi "-foobar"
 ^error,msg="Undefined MI command: foobar",code="undefined-command"
 ^done
 (gdb)

in both these cases because MI's -interpreter-exec just does:

  error ("%s", e.what ());

You won't see it either when running an MI command with the CLI's
"interpreter-exec mi":

 (gdb) interpreter-exec mi "-foobar"
 ^error,msg="Undefined MI command: foobar",code="undefined-command"
 (gdb)

This last case is because MI's interp::exec implementation never
returns an error:

 gdb_exception
 mi_interp::exec (const char *command)
 {
   mi_execute_command_wrapper (command);
   return gdb_exception ();
 }

Thus I think that "error in command" error is pretty pointless, and
since it simplifies things to not have it, the patch just removes it.

The patch also ends up addressing an old FIXME.

Change-Id: I5a6432a80496934ac7127594c53bf5221622e393
Approved-By: Tom Tromey <tromey@adacore.com>
Approved-By: Kevin Buettner <kevinb@redhat.com>

gdb/cli/cli-interp.c

@@ -48,7 +48,7 @@ class cli_interp final : public cli_interp_base
   void init (bool top_level) override;
   void resume () override;
   void suspend () override;
-  gdb_exception exec (const char *command_str) override;
+  void exec (const char *command_str) override;
   ui_out *interp_ui_out () override;
 
 private:
@@ -75,11 +75,6 @@ as_cli_interp_base (interp *interp)
   return dynamic_cast<cli_interp_base *> (interp);
 }
 
-/* Longjmp-safe wrapper for "execute_command".  */
-static struct gdb_exception safe_execute_command (struct ui_out *uiout,
-						  const char *command, 
-						  int from_tty);
-
 /* See cli-interp.h.
 
    Breakpoint hits should always be mirrored to a console.  Deciding
@@ -314,12 +309,9 @@ cli_interp::suspend ()
   gdb_disable_readline ();
 }
 
-gdb_exception
+void
 cli_interp::exec (const char *command_str)
 {
-  struct ui_file *old_stream;
-  struct gdb_exception result;
-
   /* gdb_stdout could change between the time m_cli_uiout was
      initialized and now.  Since we're probably using a different
      interpreter which has a new ui_file for gdb_stdout, use that one
@@ -327,10 +319,22 @@ cli_interp::exec (const char *command_str)
 
      It is important that it gets reset everytime, since the user
      could set gdb to use a different interpreter.  */
-  old_stream = m_cli_uiout->set_stream (gdb_stdout);
+  ui_file *old_stream = m_cli_uiout->set_stream (gdb_stdout);
-  result = safe_execute_command (m_cli_uiout.get (), command_str, 1);
+  SCOPE_EXIT { m_cli_uiout->set_stream (old_stream); };
-  m_cli_uiout->set_stream (old_stream);
+
-  return result;
+  /* Save and override the global ``struct ui_out'' builder.  */
+  scoped_restore saved_uiout = make_scoped_restore (&current_uiout,
+						    m_cli_uiout.get ());
+
+  try
+    {
+      execute_command (command_str, 1);
+    }
+  catch (const gdb_exception_error &ex)
+    {
+      exception_print (gdb_stderr, ex);
+      throw;
+    }
 }
 
 bool
@@ -339,31 +343,6 @@ cli_interp_base::supports_command_editing ()
   return true;
 }
 
-static struct gdb_exception
-safe_execute_command (struct ui_out *command_uiout, const char *command,
-		      int from_tty)
-{
-  struct gdb_exception e;
-
-  /* Save and override the global ``struct ui_out'' builder.  */
-  scoped_restore saved_uiout = make_scoped_restore (&current_uiout,
-						    command_uiout);
-
-  try
-    {
-      execute_command (command, from_tty);
-    }
-  catch (gdb_exception &exception)
-    {
-      e = std::move (exception);
-    }
-
-  /* FIXME: cagney/2005-01-13: This shouldn't be needed.  Instead the
-     caller should print the exception.  */
-  exception_print (gdb_stderr, e);
-  return e;
-}
-
 ui_out *
 cli_interp::interp_ui_out ()
 {

gdb/interps.c

@@ -39,6 +39,7 @@
 #include "top.h"		/* For command_loop.  */
 #include "main.h"
 #include "gdbsupport/buildargv.h"
+#include "gdbsupport/scope-exit.h"
 
 /* Each UI has its own independent set of interpreters.  */
 
@@ -332,7 +333,7 @@ interp_supports_command_editing (struct interp *interp)
 /* interp_exec - This executes COMMAND_STR in the current 
    interpreter.  */
 
-struct gdb_exception
+void
 interp_exec (struct interp *interp, const char *command_str)
 {
   struct ui_interp_info *ui_interp = get_current_interp_info ();
@@ -341,7 +342,7 @@ interp_exec (struct interp *interp, const char *command_str)
   scoped_restore save_command_interp
     = make_scoped_restore (&ui_interp->command_interpreter, interp);
 
-  return interp->exec (command_str);
+  interp->exec (command_str);
 }
 
 /* A convenience routine that nulls out all the common command hooks.
@@ -393,19 +394,13 @@ interpreter_exec_cmd (const char *args, int from_tty)
     error (_("Could not find interpreter \"%s\"."), prules[0]);
 
   interp_set (interp_to_use, false);
+  SCOPE_EXIT
+    {
+      interp_set (old_interp, false);
+    };
 
   for (i = 1; i < nrules; i++)
-    {
+    interp_exec (interp_to_use, prules[i]);
-      struct gdb_exception e = interp_exec (interp_to_use, prules[i]);
-
-      if (e.reason < 0)
-	{
-	  interp_set (old_interp, 0);
-	  error (_("error in command: \"%s\"."), prules[i]);
-	}
-    }
-
-  interp_set (old_interp, 0);
 }
 
 /* See interps.h.  */

gdb/interps.h

@@ -36,8 +36,7 @@ typedef struct interp *(*interp_factory_func) (const char *name);
 extern void interp_factory_register (const char *name,
 				     interp_factory_func func);
 
-extern struct gdb_exception interp_exec (struct interp *interp,
+extern void interp_exec (struct interp *interp, const char *command);
-					 const char *command);
 
 class interp
 {
@@ -51,7 +50,7 @@ public:
   virtual void resume () = 0;
   virtual void suspend () = 0;
 
-  virtual gdb_exception exec (const char *command) = 0;
+  virtual void exec (const char *command) = 0;
 
   /* Returns the ui_out currently used to collect results for this
      interpreter.  It can be a formatter for stdout, as is the case

gdb/mi/mi-interp.c

@@ -199,11 +199,10 @@ mi_interp::suspend ()
   gdb_disable_readline ();
 }
 
-gdb_exception
+void
 mi_interp::exec (const char *command)
 {
   mi_execute_command_wrapper (command);
-  return gdb_exception ();
 }
 
 void
@@ -240,12 +239,7 @@ mi_cmd_interpreter_exec (const char *command, char **argv, int argc)
     };
 
   for (i = 1; i < argc; i++)
-    {
+    interp_exec (interp_to_use, argv[i]);
-      struct gdb_exception e = interp_exec (interp_to_use, argv[i]);
-
-      if (e.reason < 0)
-	error ("%s", e.what ());
-    }
 }
 
 /* This inserts a number of hooks that are meant to produce

gdb/mi/mi-interp.h

@@ -36,7 +36,7 @@ public:
   void init (bool top_level) override;
   void resume () override;
   void suspend () override;
-  gdb_exception exec (const char *command_str) override;
+  void exec (const char *command_str) override;
   ui_out *interp_ui_out () override;
   void set_logging (ui_file_up logfile, bool logging_redirect,
 		    bool debug_redirect) override;

gdb/python/py-dap.c

@@ -45,10 +45,9 @@ public:
   {
   }
 
-  gdb_exception exec (const char *command) override
+  void exec (const char *command) override
   {
     /* Just ignore it.  */
-    return {};
   }
 
   void set_logging (ui_file_up logfile, bool logging_redirect,

gdb/tui/tui-interp.c

@@ -49,7 +49,7 @@ public:
   void init (bool top_level) override;
   void resume () override;
   void suspend () override;
-  gdb_exception exec (const char *command_str) override;
+  void exec (const char *command_str) override;
   ui_out *interp_ui_out () override;
 };
 
@@ -149,7 +149,7 @@ tui_interp::interp_ui_out ()
     return tui_old_uiout;
 }
 
-gdb_exception
+void
 tui_interp::exec (const char *command_str)
 {
   internal_error (_("tui_exec called"));