[obv] Remove some superfluous code in corelow.c

  On 05/04/2017 10:09 AM, Andreas Arnez wrote:
> Pushed this as obvious.
> 
> 
> In corelow.c I stumbled upon an extra semicolon and an xfree of a NULL
> pointer.  Remove them.

> @@ -129,7 +129,7 @@ sniff_core_bfd (bfd *abfd)
>  {
>    struct core_fns *cf;
>    struct core_fns *yummy = NULL;
> -  int matches = 0;;
> +  int matches = 0;
>  
>    /* Don't sniff if we have support for register sets in
>       CORE_GDBARCH.  */
> @@ -511,7 +511,7 @@ get_core_register_section (struct regcache *regcache,
>  			   const char *human_name,
>  			   int required)
>  {
> -  static char *section_name = NULL;
> +  static char *section_name;

But this variable is static..

>    struct bfd_section *section;
>    bfd_size_type size;
>    char *contents;
> @@ -519,8 +519,6 @@ get_core_register_section (struct regcache *regcache,
>  				&& regset->flags & REGSET_VARIABLE_SIZE);
>    ptid_t ptid = regcache_get_ptid (regcache);
>  
> -  xfree (section_name);
> -

..so this was releasing the memory allocated in a previous invocation of
get_core_register_section, AFAICS.  So, this wasn't superfluous,
and we now leak.

I'm not sure why the variable is static but from [1] I suspect 
that the reason was just "laziness" to avoid having to think
about freeing it on every function return path:

 https://sourceware.org/ml/gdb-patches/2005-03/msg00237.html

The easiest to fix that nowadays is to use a std::string.

I don't see a need to xstrdup the section name in the single-threaded
case though, and also there's more than one place that computes
a multi-threaded section name in the same way, so how about the
patch below?

From 7c91ec5530e157c33d6e5edc3dc040d9fba1818b Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Thu, 4 May 2017 10:47:04 +0100
Subject: [PATCH] fix leak, introduce thread_section_name

---
 gdb/corelow.c | 76 +++++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 53 insertions(+), 23 deletions(-)
  

On Thu, May 04 2017, Pedro Alves wrote:

[...]

 But this variable is static..
>
>>    struct bfd_section *section;
>>    bfd_size_type size;
>>    char *contents;
>> @@ -519,8 +519,6 @@ get_core_register_section (struct regcache *regcache,
>>  				&& regset->flags & REGSET_VARIABLE_SIZE);
>>    ptid_t ptid = regcache_get_ptid (regcache);
>>  
>> -  xfree (section_name);
>> -
>
> ..so this was releasing the memory allocated in a previous invocation of
> get_core_register_section, AFAICS.  So, this wasn't superfluous,
> and we now leak.
>
> I'm not sure why the variable is static but from [1] I suspect 
> that the reason was just "laziness" to avoid having to think
> about freeing it on every function return path:
>
>  https://sourceware.org/ml/gdb-patches/2005-03/msg00237.html

Oh my, you're right, of course.  Sorry for that.

> The easiest to fix that nowadays is to use a std::string.
>
> I don't see a need to xstrdup the section name in the single-threaded
> case though, and also there's more than one place that computes
> a multi-threaded section name in the same way, so how about the
> patch below?

Yeah, looks OK to me.

>
> From 7c91ec5530e157c33d6e5edc3dc040d9fba1818b Mon Sep 17 00:00:00 2001
> From: Pedro Alves <palves@redhat.com>
> Date: Thu, 4 May 2017 10:47:04 +0100
> Subject: [PATCH] fix leak, introduce thread_section_name
>
> ---
>  gdb/corelow.c | 76 +++++++++++++++++++++++++++++++++++++++++------------------
>  1 file changed, 53 insertions(+), 23 deletions(-)
>
> diff --git a/gdb/corelow.c b/gdb/corelow.c
> index 2266f24..0aff02d 100644
> --- a/gdb/corelow.c
> +++ b/gdb/corelow.c
> @@ -485,6 +485,51 @@ core_detach (struct target_ops *ops, const char *args, int from_tty)
>      printf_filtered (_("No core file now.\n"));
>  }
>
> +/* Build either a single-thread or multi-threaded section name for
> +   PTID.
> +
> +   If ptid's lwp member is zero, we want to do the single-threaded
> +   thing: look for a section named NAME (as passed to the
> +   constructor).  If ptid's lwp member is non-zero, we'll want do the
> +   multi-threaded thing: look for a section named "NAME/LWP", where
> +   LWP is the shortest ASCII decimal representation of ptid's lwp
> +   member.  */
> +
> +class thread_section_name
> +{
> +public:
> +  /* NAME is the single-threaded section name.  If PTID represents an
> +     LWP, then the build section name is "NAME/LWP", otherwise it's
> +     just "NAME" unmodified.  */
> +  thread_section_name (const char *name, ptid_t ptid)
> +  {
> +    if (ptid.lwp_p ())
> +      {
> +	m_storage = string_printf ("%s/%ld", name, ptid.lwp ());
> +	m_section_name = m_storage.c_str ();
> +      }
> +    else
> +      m_section_name = name;
> +  }
> +
> +  /* Return the computed section name.  The result is valid as long as
> +     this thread_section_name object is live.  */
> +  const char *c_str () const
> +  { return m_section_name; }
> +
> +  /* Disable copy.  */
> +  thread_section_name (const thread_section_name &) = delete;
> +  void operator= (const thread_section_name &) = delete;
> +
> +private:
> +  /* Either a pointer into M_STORAGE, or a pointer to the name passed
> +     as parameter to the constructor.  */
> +  const char *m_section_name;
> +  /* If we need to build a new section name, this is where we store
> +     it.  */
> +  std::string m_storage;
> +};
> +
>  /* Try to retrieve registers from a section in core_bfd, and supply
>     them to core_vec->core_read_registers, as the register set numbered
>     WHICH.
> @@ -511,21 +556,15 @@ get_core_register_section (struct regcache *regcache,
>  			   const char *human_name,
>  			   int required)
>  {
> -  static char *section_name;
>    struct bfd_section *section;
>    bfd_size_type size;
>    char *contents;
>    bool variable_size_section = (regset != NULL
>  				&& regset->flags & REGSET_VARIABLE_SIZE);
> -  ptid_t ptid = regcache_get_ptid (regcache);
>
> -  if (ptid_get_lwp (ptid))
> -    section_name = xstrprintf ("%s/%ld", name,
> -			       ptid_get_lwp (ptid));
> -  else
> -    section_name = xstrdup (name);
> +  thread_section_name section_name (name, regcache->ptid ());
>
> -  section = bfd_get_section_by_name (core_bfd, section_name);
> +  section = bfd_get_section_by_name (core_bfd, section_name.c_str ());
>    if (! section)
>      {
>        if (required)
> @@ -537,13 +576,14 @@ get_core_register_section (struct regcache *regcache,
>    size = bfd_section_size (core_bfd, section);
>    if (size < min_size)
>      {
> -      warning (_("Section `%s' in core file too small."), section_name);
> +      warning (_("Section `%s' in core file too small."),
> +	       section_name.c_str ());
>        return;
>      }
>    if (size != min_size && !variable_size_section)
>      {
>        warning (_("Unexpected size of section `%s' in core file."),
> -	       section_name);
> +	       section_name.c_str ());
>      }
>
>    contents = (char *) alloca (size);
> @@ -551,7 +591,7 @@ get_core_register_section (struct regcache *regcache,
>  				  (file_ptr) 0, size))
>      {
>        warning (_("Couldn't read %s registers from `%s' section in core file."),
> -	       human_name, name);
> +	       human_name, section_name.c_str ());
>        return;
>      }
>
> @@ -681,18 +721,8 @@ add_to_spuid_list (bfd *abfd, asection *asect, void *list_p)
>  static LONGEST
>  get_core_siginfo (bfd *abfd, gdb_byte *readbuf, ULONGEST offset, ULONGEST len)
>  {
> -  asection *section;
> -  char *section_name;
> -  const char *name = ".note.linuxcore.siginfo";
> -
> -  if (ptid_get_lwp (inferior_ptid))
> -    section_name = xstrprintf ("%s/%ld", name,
> -			       ptid_get_lwp (inferior_ptid));
> -  else
> -    section_name = xstrdup (name);
> -
> -  section = bfd_get_section_by_name (abfd, section_name);
> -  xfree (section_name);
> +  thread_section_name section_name (".note.linuxcore.siginfo", inferior_ptid);
> +  asection *section = bfd_get_section_by_name (abfd, section_name.c_str ());
>    if (section == NULL)
>      return -1;