Re: [PATCH 1/5] Rewrite final cleanups

[Lancelot SIX <lsix@lancelotsix.com>]

Hi Tom,

On Fri, Feb 23, 2024 at 02:11:37PM -0700, Tom Tromey wrote:
> This patch rewrites final cleanups to use std::function and otherwise
> be more C++-ish.
> ---
>  gdb/compile/compile.c    |  30 +++++-------
>  gdb/debuginfod-support.c |  14 ++----
>  gdb/python/python.c      |   4 +-
>  gdbsupport/cleanups.cc   | 122 +++++------------------------------------------
>  gdbsupport/cleanups.h    |  17 ++-----
>  5 files changed, 33 insertions(+), 154 deletions(-)
> 
> diff --git a/gdb/compile/compile.c b/gdb/compile/compile.c
> index 8cb2e8ac7f1..27cff2553ee 100644
> --- a/gdb/compile/compile.c
> +++ b/gdb/compile/compile.c
> @@ -427,23 +427,6 @@ compile_print_command (const char *arg, int from_tty)
>      }
>  }
>  
> -/* A cleanup function to remove a directory and all its contents.  */
> -
> -static void
> -do_rmdir (void *arg)
> -{
> -  const char *dir = (const char *) arg;
> -  char *zap;
> -  int wstat;
> -
> -  gdb_assert (startswith (dir, TMP_PREFIX));
> -  zap = concat ("rm -rf ", dir, (char *) NULL);
> -  wstat = system (zap);
> -  if (wstat == -1 || !WIFEXITED (wstat) || WEXITSTATUS (wstat) != 0)
> -    warning (_("Could not remove temporary directory %s"), dir);
> -  XDELETEVEC (zap);
> -}
> -
>  /* Return the name of the temporary directory to use for .o files, and
>     arrange for the directory to be removed at shutdown.  */
>  
> @@ -465,7 +448,18 @@ get_compile_file_tempdir (void)
>      perror_with_name (_("Could not make temporary directory"));
>  
>    tempdir_name = xstrdup (tempdir_name);
> -  make_final_cleanup (do_rmdir, tempdir_name);
> +  add_final_cleanup ([] ()
> +    {
> +      char *zap;
> +      int wstat;
> +
> +      gdb_assert (startswith (tempdir_name, TMP_PREFIX));
> +      zap = concat ("rm -rf ", tempdir_name, (char *) NULL);
> +      wstat = system (zap);
> +      if (wstat == -1 || !WIFEXITED (wstat) || WEXITSTATUS (wstat) != 0)
> +	warning (_("Could not remove temporary directory %s"), tempdir_name);
> +      XDELETEVEC (zap);

I am aware that this is orthogonal to this patch and can be address by
another patch, but in the way to c++ification, this could be replaced
with:

  std::filesystem::remove_all (tempdir_name);

> +    });
>    return tempdir_name;
>  }
>  
> diff --git a/gdb/debuginfod-support.c b/gdb/debuginfod-support.c
> index 7d8ada39e96..9bb3748c8c3 100644
> --- a/gdb/debuginfod-support.c
> +++ b/gdb/debuginfod-support.c
> @@ -188,15 +188,6 @@ progressfn (debuginfod_client *c, long cur, long total)
>    return 0;
>  }
>  
> -/* Cleanup ARG, which is a debuginfod_client pointer.  */
> -
> -static void
> -cleanup_debuginfod_client (void *arg)
> -{
> -  debuginfod_client *client = static_cast<debuginfod_client *> (arg);
> -  debuginfod_end (client);
> -}
> -
>  /* Return a pointer to the single global debuginfod_client, initialising it
>     first if needed.  */
>  
> @@ -221,7 +212,10 @@ get_debuginfod_client ()
>  	     handlers, which is too late.
>  
>  	     So instead, we make use of GDB's final cleanup mechanism.  */
> -	  make_final_cleanup (cleanup_debuginfod_client, global_client);
> +	  add_final_cleanup ([] ()
> +	    {
> +	      debuginfod_end (global_client);
> +	    });
>  	  debuginfod_set_progressfn (global_client, progressfn);
>  	}
>      }
> diff --git a/gdb/python/python.c b/gdb/python/python.c
> index 7b0997c8d52..971fc850dbb 100644
> --- a/gdb/python/python.c
> +++ b/gdb/python/python.c
> @@ -2070,7 +2070,7 @@ static struct cmd_list_element *user_show_python_list;
>     interpreter.  This lets Python's 'atexit' work.  */
>  
>  static void
> -finalize_python (void *ignore)
> +finalize_python ()
>  {
>    struct active_ext_lang_state *previous_active;
>  
> @@ -2310,7 +2310,7 @@ do_start_initialization ()
>    /* Release the GIL while gdb runs.  */
>    PyEval_SaveThread ();
>  
> -  make_final_cleanup (finalize_python, NULL);
> +  add_final_cleanup (finalize_python);
>  
>    /* Only set this when initialization has succeeded.  */
>    gdb_python_initialized = 1;
> diff --git a/gdbsupport/cleanups.cc b/gdbsupport/cleanups.cc
> index 619db023063..cc14523b2d1 100644
> --- a/gdbsupport/cleanups.cc
> +++ b/gdbsupport/cleanups.cc
> @@ -19,126 +19,26 @@
>  
>  #include "common-defs.h"
>  #include "cleanups.h"
> +#include <vector>
>  
> -/* The cleanup list records things that have to be undone
> -   if an error happens (descriptors to be closed, memory to be freed, etc.)
> -   Each link in the chain records a function to call and an
> -   argument to give it.
> +/* All the cleanup functions.  */
>  
> -   Use make_cleanup to add an element to the cleanup chain.
> -   Use do_cleanups to do all cleanup actions back to a given
> -   point in the chain.  Use discard_cleanups to remove cleanups
> -   from the chain back to a given point, not doing them.
> +static std::vector<std::function<void ()>> all_cleanups;
>  
> -   If the argument is pointer to allocated memory, then you need
> -   to additionally set the 'free_arg' member to a function that will
> -   free that memory.  This function will be called both when the cleanup
> -   is executed and when it's discarded.  */
> +/* See cleanups.h.  */
>  
> -struct cleanup
> -{
> -  struct cleanup *next;
> -  void (*function) (void *);
> -  void (*free_arg) (void *);
> -  void *arg;
> -};
> -
> -/* Used to mark the end of a cleanup chain.
> -   The value is chosen so that it:
> -   - is non-NULL so that make_cleanup never returns NULL,
> -   - causes a segv if dereferenced
> -     [though this won't catch errors that a value of, say,
> -     ((struct cleanup *) -1) will]
> -   - displays as something useful when printed in gdb.
> -   This is const for a bit of extra robustness.
> -   It is initialized to coax gcc into putting it into .rodata.
> -   All fields are initialized to survive -Wextra.  */
> -static const struct cleanup sentinel_cleanup = { 0, 0, 0, 0 };
> -
> -/* Handy macro to use when referring to sentinel_cleanup.  */
> -#define SENTINEL_CLEANUP ((struct cleanup *) &sentinel_cleanup)
> -
> -/* Chain of cleanup actions established with make_final_cleanup,
> -   to be executed when gdb exits.  */
> -static struct cleanup *final_cleanup_chain = SENTINEL_CLEANUP;
> -
> -/* Main worker routine to create a cleanup.
> -   PMY_CHAIN is a pointer to either cleanup_chain or final_cleanup_chain.
> -   FUNCTION is the function to call to perform the cleanup.
> -   ARG is passed to FUNCTION when called.
> -   FREE_ARG, if non-NULL, is called after the cleanup is performed.
> -
> -   The result is a pointer to the previous chain pointer
> -   to be passed later to do_cleanups or discard_cleanups.  */
> -
> -static struct cleanup *
> -make_my_cleanup2 (struct cleanup **pmy_chain, make_cleanup_ftype *function,
> -		  void *arg,  void (*free_arg) (void *))
> -{
> -  struct cleanup *newobj = XNEW (struct cleanup);
> -  struct cleanup *old_chain = *pmy_chain;
> -
> -  newobj->next = *pmy_chain;
> -  newobj->function = function;
> -  newobj->free_arg = free_arg;
> -  newobj->arg = arg;
> -  *pmy_chain = newobj;
> -
> -  gdb_assert (old_chain != NULL);
> -  return old_chain;
> -}
> -
> -/* Worker routine to create a cleanup without a destructor.
> -   PMY_CHAIN is a pointer to either cleanup_chain or final_cleanup_chain.
> -   FUNCTION is the function to call to perform the cleanup.
> -   ARG is passed to FUNCTION when called.
> -
> -   The result is a pointer to the previous chain pointer
> -   to be passed later to do_cleanups or discard_cleanups.  */
> -
> -static struct cleanup *
> -make_my_cleanup (struct cleanup **pmy_chain, make_cleanup_ftype *function,
> -		 void *arg)
> -{
> -  return make_my_cleanup2 (pmy_chain, function, arg, NULL);
> -}
> -
> -/* Add a new cleanup to the final cleanup_chain,
> -   and return the previous chain pointer
> -   to be passed later to do_cleanups or discard_cleanups.
> -   Args are FUNCTION to clean up with, and ARG to pass to it.  */
> -
> -struct cleanup *
> -make_final_cleanup (make_cleanup_ftype *function, void *arg)
> -{
> -  return make_my_cleanup (&final_cleanup_chain, function, arg);
> -}
> -
> -/* Worker routine to perform cleanups.
> -   PMY_CHAIN is a pointer to either cleanup_chain or final_cleanup_chain.
> -   OLD_CHAIN is the result of a "make" cleanup routine.
> -   Cleanups are performed until we get back to the old end of the chain.  */
> -
> -static void
> -do_my_cleanups (struct cleanup **pmy_chain,
> -		struct cleanup *old_chain)
> +void
> +add_final_cleanup (std::function<void ()> &&func)
>  {
> -  struct cleanup *ptr;
> -
> -  while ((ptr = *pmy_chain) != old_chain)
> -    {
> -      *pmy_chain = ptr->next;	/* Do this first in case of recursion.  */
> -      (*ptr->function) (ptr->arg);
> -      if (ptr->free_arg)
> -	(*ptr->free_arg) (ptr->arg);
> -      xfree (ptr);
> -    }
> +  all_cleanups.emplace_back (std::move (func));
>  }
>  
> -/* Discard final cleanups and do the actions they describe.  */
> +/* See cleanups.h.  */
>  
>  void
>  do_final_cleanups ()
>  {
> -  do_my_cleanups (&final_cleanup_chain, SENTINEL_CLEANUP);
> +  for (auto &func : all_cleanups)
> +    func ();
> +  all_cleanups.clear ();

I am wondering if we want special handling if one of the cleanup
function ever throws.  It is probably acceptable to expect callbacks to
never throw (unfortunately, we can’t have use std::function<void ()
noexcept> to have this in the type system).  If we accept that callbacks
can throw, is it OK to skip pending cleanup functions?

Best,
Lancelot.

>  }
> diff --git a/gdbsupport/cleanups.h b/gdbsupport/cleanups.h
> index 3e64f7d1684..985cf81ff7d 100644
> --- a/gdbsupport/cleanups.h
> +++ b/gdbsupport/cleanups.h
> @@ -19,21 +19,12 @@
>  #ifndef COMMON_CLEANUPS_H
>  #define COMMON_CLEANUPS_H
>  
> -/* Outside of cleanups.c, this is an opaque type.  */
> -struct cleanup;
> +#include <functional>
>  
> -/* NOTE: cagney/2000-03-04: This typedef is strictly for the
> -   make_cleanup function declarations below.  Do not use this typedef
> -   as a cast when passing functions into the make_cleanup() code.
> -   Instead either use a bounce function or add a wrapper function.
> -   Calling a f(char*) function with f(void*) is non-portable.  */
> -typedef void (make_cleanup_ftype) (void *);
> -
> -/* Function type for the dtor in make_cleanup_dtor.  */
> -typedef void (make_cleanup_dtor_ftype) (void *);
> -
> -extern struct cleanup *make_final_cleanup (make_cleanup_ftype *, void *);
> +/* Register a function that will be called on exit.  */
> +extern void add_final_cleanup (std::function<void ()> &&func);
>  
> +/* Run all the registered functions.  */
>  extern void do_final_cleanups ();
>  
>  #endif /* COMMON_CLEANUPS_H */
> 
> -- 
> 2.43.0
>