Re: [PATCH v2] gdb::handle_eintr, remove need to specify return type

[Kamil Rytarowski <kamil@netbsd.org>]

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On 13.10.2020 16:17, Pedro Alves wrote:
> On 10/13/20 2:43 PM, Kamil Rytarowski wrote:
> 
>> This patch applied on trunk gives me:
>>
>> gmake[2]: Wejście do katalogu '/public/binutils-gdb-netbsd/build/gdbserver'
>>   CXX    netbsd-low.o
>> ../../gdbserver/netbsd-low.cc: In function ‘bool elf_64_file_p(const
>> char*)’:
>> ../../gdbserver/netbsd-low.cc:1152:57: error: no matching function for
>> call to ‘handle_eintr(int, int (&)(const char*, int, ...), const char*&,
>> int)’
>>  1152 |   int fd = gdb::handle_eintr (-1, ::open, file, O_RDONLY);
>>       |                                                         ^
>> In file included from ../../gdbserver/netbsd-low.cc:37:
>> ../../gdbserver/../gdbsupport/eintr.h:58:1: note: candidate:
>> ‘template<class ErrorValType, class Fun, class ... Args> typename
>> gdb::return_type<Fun>::type gdb::handle_eintr(ErrorValType, const Fun&,
>> const Args& ...)’
>>    58 | handle_eintr (ErrorValType errval, const Fun &f, const Args &...
>> args)
>>       | ^~~~~~~~~~~~
>> ../../gdbserver/../gdbsupport/eintr.h:58:1: note:   template argument
>> deduction/substitution failed:
>> ../../gdbserver/../gdbsupport/eintr.h: In substitution of
>> ‘template<class ErrorValType, class Fun, class ... Args> typename
>> gdb::return_type<Fun>::type gdb::handle_eintr(ErrorValType, const Fun&,
>> const Args& ...) [with ErrorValType = int; Fun = int(const char*, int,
>> ...); Args = {const char*, int}]’:
>> ../../gdbserver/netbsd-low.cc:1152:57:   required from here
>> ../../gdbserver/../gdbsupport/eintr.h:58:1: error: invalid use of
>> incomplete type ‘struct gdb::return_type<int(const char*, int, ...)>’
>> In file included from ../../gdbserver/../gdbsupport/poison.h:23,
>>                  from ../../gdbserver/../gdbsupport/common-utils.h:26,
>>                  from ../../gdbserver/../gdbsupport/common-defs.h:125,
>>                  from ../../gdbserver/server.h:22,
>>                  from ../../gdbserver/netbsd-low.cc:18:
>> ../../gdbserver/../gdbsupport/traits.h:49:8: note: declaration of
>> ‘struct gdb::return_type<int(const char*, int, ...)>’
>>    49 | struct return_type;
>>       |        ^~~~~~~~~~~
> 
> Huh, Fun is clearly a function, yet the compiler doesn't pick
> the partial specialization.
> 
> I could reproduce it here, seems to be related to open being varargs.
> 
> Anyhow, we don't really need the new trait.  We can just 
> use decltype and a trailing return.  I don't know why I didn't
> think of it in the first place.
> 
> Here's an updated patch.  I believe this one should work for you too.
> 

This version works for me.

> 
> From 37d06f47a379e44a895f68a22cbb0f745da33ec5 Mon Sep 17 00:00:00 2001
> From: Pedro Alves <pedro@palves.net>
> Date: Tue, 8 Sep 2020 17:34:41 +0100
> Subject: [PATCH v2] gdb::handle_eintr, remove need to specify return type
> 
> This eliminates the need to specify the return type when using
> handle_eintr.  We let the compiler deduce it for us.
> 
> Also, use lowercase for function parameter names.  Uppercase should
> only be used on template parameters.
> 
> gdb/ChangeLog:
> 
> 	* nat/linux-waitpid.c: Include "gdbsupport/eintr.h".
> 	(my_waitpid): Use gdb::handle_eintr.
> 
> gdbserver/ChangeLog:
> 
> 	* netbsd-low.cc (netbsd_waitpid, netbsd_process_target::kill)
> 	(netbsd_qxfer_libraries_svr4): Use gdb::handle_eintr without
> 	explicit type.
> 
> gdbsupport/ChangeLog:
> 
> 	* eintr.h (handle_eintr): Replace Ret template parameter with
> 	ErrorValType.  Use it as type of the failure value.  Deduce the
> 	function's return type using decltype.  Use lowercase for function
> 	parameter names.
> ---
>  gdb/nat/linux-waitpid.c | 11 ++---------
>  gdbserver/netbsd-low.cc | 10 +++++-----
>  gdbsupport/eintr.h      | 32 +++++++++++++++++++-------------
>  3 files changed, 26 insertions(+), 27 deletions(-)
> 
> diff --git a/gdb/nat/linux-waitpid.c b/gdb/nat/linux-waitpid.c
> index f50e0c7bcff..d066239220a 100644
> --- a/gdb/nat/linux-waitpid.c
> +++ b/gdb/nat/linux-waitpid.c
> @@ -22,6 +22,7 @@
>  #include "linux-nat.h"
>  #include "linux-waitpid.h"
>  #include "gdbsupport/gdb_wait.h"
> +#include "gdbsupport/eintr.h"
>  
>  /* Convert wait status STATUS to a string.  Used for printing debug
>     messages only.  */
> @@ -54,13 +55,5 @@ status_to_str (int status)
>  int
>  my_waitpid (int pid, int *status, int flags)
>  {
> -  int ret;
> -
> -  do
> -    {
> -      ret = waitpid (pid, status, flags);
> -    }
> -  while (ret == -1 && errno == EINTR);
> -
> -  return ret;
> +  return gdb::handle_eintr (-1, ::waitpid, pid, status, flags);
>  }
> diff --git a/gdbserver/netbsd-low.cc b/gdbserver/netbsd-low.cc
> index 30028d3a384..519614d0473 100644
> --- a/gdbserver/netbsd-low.cc
> +++ b/gdbserver/netbsd-low.cc
> @@ -245,7 +245,7 @@ netbsd_waitpid (ptid_t ptid, struct target_waitstatus *ourstatus,
>    int options = (target_options & TARGET_WNOHANG) ? WNOHANG : 0;
>  
>    pid_t pid
> -    = gdb::handle_eintr<int> (-1, ::waitpid, ptid.pid (), &status, options);
> +    = gdb::handle_eintr (-1, ::waitpid, ptid.pid (), &status, options);
>  
>    if (pid == -1)
>      perror_with_name (_("Child process unexpectedly missing"));
> @@ -456,7 +456,7 @@ netbsd_process_target::kill (process_info *process)
>      return -1;
>  
>    int status;
> -  if (gdb::handle_eintr<int> (-1, ::waitpid, pid, &status, 0) == -1)
> +  if (gdb::handle_eintr (-1, ::waitpid, pid, &status, 0) == -1)
>      return -1;
>    mourn (process);
>    return 0;
> @@ -1149,15 +1149,15 @@ netbsd_qxfer_libraries_svr4 (const pid_t pid, const char *annex,
>  static bool
>  elf_64_file_p (const char *file)
>  {
> -  int fd = gdb::handle_eintr<int> (-1, ::open, file, O_RDONLY);
> +  int fd = gdb::handle_eintr (-1, ::open, file, O_RDONLY);
>    if (fd < 0)
>      perror_with_name (("open"));
>  
>    Elf64_Ehdr header;
> -  ssize_t ret = gdb::handle_eintr<ssize_t> (-1, ::read, fd, &header, sizeof (header));
> +  ssize_t ret = gdb::handle_eintr (-1, ::read, fd, &header, sizeof (header));
>    if (ret == -1)
>      perror_with_name (("read"));
> -  gdb::handle_eintr<int> (-1, ::close, fd);
> +  gdb::handle_eintr (-1, ::close, fd);
>    if (ret != sizeof (header))
>      error ("Cannot read ELF file header: %s", file);
>  
> diff --git a/gdbsupport/eintr.h b/gdbsupport/eintr.h
> index 64ff5940b75..02c26e4def1 100644
> --- a/gdbsupport/eintr.h
> +++ b/gdbsupport/eintr.h
> @@ -43,25 +43,31 @@ namespace gdb
>  
>     You could wrap it by writing the wrapped form:
>  
> -   ssize_t ret = gdb::handle_eintr<ssize_t> (-1, ::write, pipe[1], "+", 1);
> -
> -   The RET typename specifies the return type of the wrapped system call, which
> -   is typically int or ssize_t.  The R argument specifies the failure value
> -   indicating the interrupted syscall when calling the F function with
> -   the A... arguments.  */
> -
> -template <typename Ret, typename Fun, typename... Args>
> -inline Ret handle_eintr (const Ret &R, const Fun &F, const Args &... A)
> +   ssize_t ret = gdb::handle_eintr (-1, ::write, pipe[1], "+", 1);
> +
> +   ERRVAL specifies the failure value indicating that the call to the
> +   F function with ARGS... arguments was possibly interrupted with a
> +   signal.  */
> +
> +template <typename ErrorValType,
> +	  typename Fun,
> +	  typename... Args>
> +inline auto
> +handle_eintr (ErrorValType errval, const Fun &f, const Args &... args)
> +  -> decltype (f(args...))
>  {
> -  Ret ret;
> +  decltype (f(args...)) ret;
> +
>    do
>      {
>        errno = 0;
> -      ret = F (A...);
> +      ret = f (args...);
>      }
> -  while (ret == R && errno == EINTR);
> +  while (ret == errval && errno == EINTR);
> +
>    return ret;
>  }
> -}
> +
> +} /* namespace gdb */
>  
>  #endif /* GDBSUPPORT_EINTR_H */
> 
> base-commit: 87a37e5e078f506fa9905b74e9238593c537fcd5
> 



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