Re: [RFA 3/3] Remove cleanups from break-catch-syscall.c

[Simon Marchi <simon.marchi@ericsson.com>]

On 2017-10-18 12:06 AM, Tom Tromey wrote:
> This removes the remaining cleanups from break-catch-syscall.c by
> storing temporary strings in a vector.
> 
> ChangeLog
> 2017-10-17  Tom Tromey  <tom@tromey.com>
> 
> 	* break-catch-syscall.c (catch_syscall_completer): Use
> 	std::string, gdb::unique_xmalloc_ptr.
> ---
>  gdb/ChangeLog             |  5 +++++
>  gdb/break-catch-syscall.c | 35 ++++++++++++++++++-----------------
>  2 files changed, 23 insertions(+), 17 deletions(-)
> 
> diff --git a/gdb/break-catch-syscall.c b/gdb/break-catch-syscall.c
> index 01e761ce37..2bbfee0ac4 100644
> --- a/gdb/break-catch-syscall.c
> +++ b/gdb/break-catch-syscall.c
> @@ -560,9 +560,7 @@ catch_syscall_completer (struct cmd_list_element *cmd,
>                           const char *text, const char *word)
>  {
>    struct gdbarch *gdbarch = get_current_arch ();
> -  struct cleanup *cleanups = make_cleanup (null_cleanup, NULL);
> -  const char **group_list = NULL;
> -  const char **syscall_list = NULL;
> +  gdb::unique_xmalloc_ptr<const char *> group_list;
>    const char *prefix;
>    int i;
>  
> @@ -575,34 +573,37 @@ catch_syscall_completer (struct cmd_list_element *cmd,
>    if (startswith (prefix, "g:") || startswith (prefix, "group:"))
>      {
>        /* Perform completion inside 'group:' namespace only.  */
> -      group_list = get_syscall_group_names (gdbarch);
> +      group_list.reset (get_syscall_group_names (gdbarch));
>        if (group_list != NULL)
> -	complete_on_enum (tracker, group_list, word, word);
> +	complete_on_enum (tracker, group_list.get (), word, word);
>      }
>    else
>      {
>        /* Complete with both, syscall names and groups.  */
> -      syscall_list = get_syscall_names (gdbarch);
> -      group_list = get_syscall_group_names (gdbarch);
> +      gdb::unique_xmalloc_ptr<const char *> syscall_list
> +	(get_syscall_names (gdbarch));
> +      group_list.reset (get_syscall_group_names (gdbarch));
> +
> +      const char **group_ptr = group_list.get ();
> +
> +      /* Hold on to strings while we're using them.  */
> +      std::vector<std::string> holders;
>  
>        /* Append "group:" prefix to syscall groups.  */
> -      for (i = 0; group_list[i] != NULL; i++)
> +      for (i = 0; group_ptr[i] != NULL; i++)
>  	{
> -	  char *prefixed_group = xstrprintf ("group:%s", group_list[i]);
> +	  std::string prefixed_group = string_printf ("group:%s",
> +						      group_ptr[i]);
>  
> -	  group_list[i] = prefixed_group;
> -	  make_cleanup (xfree, prefixed_group);
> +	  group_ptr[i] = prefixed_group.c_str ();
> +	  holders.push_back (prefixed_group);
>  	}

Err, I think there's something that doesn't make sense here actually.  We
record in group_ptr[i] a pointer to the buffer of the temporary std::string,
that gets deleted when we go out of scope (end of the iteration).  That
causes this fail:

Running /home/emaisin/src/binutils-gdb/gdb/testsuite/gdb.base/catch-syscall.exp ...
FAIL: gdb.base/catch-syscall.exp: complete catch syscall group suggests 'group:' prefix (pattern 2)

By hand, you can do

(gdb) catch syscall g<TAB>

There should be many entries starting with group:, in the failing case there's only
one.  Presumably because in group_ptr all the pointers point to the same location,
that contains the last group added.  The completion mechanism then removes duplicates.

It is not enough to assign holders.back ().c_str () (after having pushed the string in
the vector), because when the vector gets reallocated it can now point to stale memory.
I think we have to do it in two pass, prepare the vector of std::string, and then get
pointers to the strings.  Something like this:


commit f9cab673480425a130b73394c3a63d256eadf314
Author: Simon Marchi <simon.marchi@ericsson.com>
Date:   Fri Oct 20 16:22:40 2017 -0400

    Fix syscall group completion

diff --git a/gdb/break-catch-syscall.c b/gdb/break-catch-syscall.c
index 82d3e36..095284c 100644
--- a/gdb/break-catch-syscall.c
+++ b/gdb/break-catch-syscall.c
@@ -562,7 +562,6 @@ catch_syscall_completer (struct cmd_list_element *cmd,
   struct gdbarch *gdbarch = get_current_arch ();
   gdb::unique_xmalloc_ptr<const char *> group_list;
   const char *prefix;
-  int i;

   /* Completion considers ':' to be a word separator, so we use this to
      verify whether the previous word was a group prefix.  If so, we
@@ -590,14 +589,11 @@ catch_syscall_completer (struct cmd_list_element *cmd,
       std::vector<std::string> holders;

       /* Append "group:" prefix to syscall groups.  */
-      for (i = 0; group_ptr[i] != NULL; i++)
-	{
-	  std::string prefixed_group = string_printf ("group:%s",
-						      group_ptr[i]);
+      for (int i = 0; group_ptr[i] != NULL; i++)
+	holders.push_back (string_printf ("group:%s", group_ptr[i]));

-	  group_ptr[i] = prefixed_group.c_str ();
-	  holders.push_back (std::move (prefixed_group));
-	}
+      for (int i = 0; group_ptr[i] != NULL; i++)
+	group_ptr[i] = holders[i].c_str ();

       if (syscall_list != NULL)
 	complete_on_enum (tracker, syscall_list.get (), word, word);