[libvirt] [PATCH 1/6] util: file: introduce VIR_AUTOCLOSE macro to close fd of the file automatically

[Michal Privoznik]

On 09/12/2018 09:18 AM, Erik Skultety wrote:
> On Tue, Sep 11, 2018 at 10:31:18PM +0800, Shi Lei wrote:
>> On 2018-09-11 at 20:22, Michal Privoznik wrote:
>>> On 09/10/2018 05:47 AM, Shi Lei wrote:
>>>> By making use of GNU C's cleanup attribute handled by the VIR_AUTOCLOSE macro,
>>>> many of the VIR_FORCE_CLOSE calls can be dropped, which in turn leads to
>>>> getting rid of many of our cleanup sections.
>>>>
>>>> Signed-off-by: Shi Lei <shi_lei at massclouds.com>
>>>> ---
>>>>   src/util/virfile.h | 20 ++++++++++++++++++--
>>>>   1 file changed, 18 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/src/util/virfile.h b/src/util/virfile.h
>>>> index b30a1d3..70e7203 100644
>>>> --- a/src/util/virfile.h
>>>> +++ b/src/util/virfile.h
>>>> @@ -54,6 +54,11 @@ int virFileClose(int *fdptr, virFileCloseFlags flags)
>>>>   int virFileFclose(FILE **file, bool preserve_errno) ATTRIBUTE_RETURN_CHECK;
>>>>   FILE *virFileFdopen(int *fdptr, const char *mode) ATTRIBUTE_RETURN_CHECK;
>>>>
>>>> +static inline void virForceCloseHelper(int *_fd)
>>>
>>> No need for this argument to have underscore in its name.
>>
>> Okay.
>>
>>>
>>>> +{
>>>> +    ignore_value(virFileClose(_fd, VIR_FILE_CLOSE_PRESERVE_ERRNO));
>>>> +}
>>>> +
>>>>   /* For use on normal paths; caller must check return value,
>>>>      and failure sets errno per close. */
>>>>   # define VIR_CLOSE(FD) virFileClose(&(FD), 0)
>>>> @@ -64,8 +69,7 @@ FILE *virFileFdopen(int *fdptr, const char *mode) ATTRIBUTE_RETURN_CHECK;
>>>>
>>>>   /* For use on cleanup paths; errno is unaffected by close,
>>>>      and no return value to worry about. */
>>>> -# define VIR_FORCE_CLOSE(FD) \
>>>> -    ignore_value(virFileClose(&(FD), VIR_FILE_CLOSE_PRESERVE_ERRNO))
>>>> +# define VIR_FORCE_CLOSE(FD) virForceCloseHelper(&(FD))
>>>>   # define VIR_FORCE_FCLOSE(FILE) ignore_value(virFileFclose(&(FILE), true))
>>>>
>>>>   /* Similar VIR_FORCE_CLOSE() but ignores EBADF errors since they are expected
>>>> @@ -80,6 +84,18 @@ FILE *virFileFdopen(int *fdptr, const char *mode) ATTRIBUTE_RETURN_CHECK;
>>>>                    VIR_FILE_CLOSE_PRESERVE_ERRNO | \
>>>>                    VIR_FILE_CLOSE_DONT_LOG))
>>>>
>>>> +/**
>>>> + * VIR_AUTOCLOSE:
>>>> + * @fd: fd of the file to be closed automatically
>>>> + *
>>>> + * Macro to automatically force close the fd by calling virForceCloseHelper
>>>> + * when the fd goes out of scope. It's used to eliminate VIR_FORCE_CLOSE
>>>> + * in cleanup sections.
>>>> + */
>>>> +# define VIR_AUTOCLOSE(fd) \
>>>> +    __attribute__((cleanup(virForceCloseHelper))) int fd = -1
>>>
>>> While this may helps us to initialize variables correctly, I think we
>>> should do that explicitly. Not only it follows what VIR_AUTOFREE is
>>> doing, it also is more visible when used. For instance, in 2/6 when the
>>> macro is used for the first time, it's not visible what is @fd
>>> initialized to.
>>
>> Okay. So I think the macro could be like:
>>
>> # define VIR_AUTOCLOSE \
>>    __attribute__((cleanup(virForceCloseHelper))) int
> 
> Actually, I'd prefer if we stayed consistent with the existing AUTO_ macros,
> IOW, the form of VIR_AUTOCLOSE(fd) = <rvalue> is IMHO desired.

I don't think this is correct. Sure, we do have parentheses in
VIR_AUTOFREE but only for type, not variable itself:

  VIR_AUTOFREE(char *) str = NULL;

Therefore, in order to be consistent the autoclose macro should look
like I'm suggesting actually:

  VIR_AUTOCLOSE fd = -1;

To extend this idea further, we can then have VIR_AUTOFCLOSE and
VIR_AUTODIRCLOSE macros to call fclose() and dirclose() respectively (or
our wrappers around those functions).

Michal