Re: [PATCH] error handling: Use TFR() macro where applicable

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Montag, 8. August 2022 10:05:56 CEST Markus Armbruster wrote:
> Nikita Ivanov <nivanov@cloudlinux.com> writes:
> > Summing up the discussion above, I suggest the following patch for TFR()
> > macro refactoring. (The patch is sequential to the first one I introduced
> > in the start of the discussion).
> > 
> >>From 6318bee052900aa93bba6620b53c7cb2290e5001 Mon Sep 17 00:00:00 2001
> >>
> > From: Nikita Ivanov <nivanov@cloudlinux.com>
> > Date: Mon, 8 Aug 2022 09:30:34 +0300
> > Subject: [PATCH] Refactoring: rename TFR() to TEMP_FAILURE_RETRY()
> > 
> > glibc's unistd.h header provides the same macro with the
> > subtle difference in type casting. Adjust macro name to the
> > common standard and define conditionally.
> > 
> > Signed-off-by: Nikita Ivanov <nivanov@cloudlinux.com>
> > ---
> > 
> >  chardev/char-fd.c      |  2 +-
> >  chardev/char-pipe.c    | 12 +++++++++---
> >  hw/9pfs/9p-local.c     |  6 ++++--
> >  include/qemu/osdep.h   |  6 ++++--
> >  net/l2tpv3.c           |  8 +++++---
> >  net/tap-linux.c        |  2 +-
> >  net/tap.c              | 10 ++++++----
> >  os-posix.c             |  2 +-
> >  qga/commands-posix.c   |  2 +-
> >  tests/qtest/libqtest.c |  2 +-
> >  util/main-loop.c       |  2 +-
> >  util/osdep.c           |  2 +-
> >  12 files changed, 35 insertions(+), 21 deletions(-)
> > 
> > diff --git a/chardev/char-fd.c b/chardev/char-fd.c
> > index cf78454841..7f5ed9aba3 100644
> > --- a/chardev/char-fd.c
> > +++ b/chardev/char-fd.c
> > @@ -198,7 +198,7 @@ int qmp_chardev_open_file_source(char *src, int flags,
> > Error **errp)
> > 
> >  {
> >  
> >      int fd = -1;
> > 
> > -    TFR(fd = qemu_open_old(src, flags, 0666));
> > +    TEMP_FAILURE_RETRY(fd = qemu_open_old(src, flags, 0666));
> > 
> >      if (fd == -1) {
> >      
> >          error_setg_file_open(errp, errno, src);
> >      
> >      }
> > 
> > diff --git a/chardev/char-pipe.c b/chardev/char-pipe.c
> > index 66d3b85091..aed97e306b 100644
> > --- a/chardev/char-pipe.c
> > +++ b/chardev/char-pipe.c
> > @@ -131,8 +131,12 @@ static void qemu_chr_open_pipe(Chardev *chr,
> > 
> >      filename_in = g_strdup_printf("%s.in", filename);
> >      filename_out = g_strdup_printf("%s.out", filename);
> > 
> > -    TFR(fd_in = qemu_open_old(filename_in, O_RDWR | O_BINARY));
> > -    TFR(fd_out = qemu_open_old(filename_out, O_RDWR | O_BINARY));
> > +    TEMP_FAILURE_RETRY(
> > +        fd_in = qemu_open_old(filename_in, O_RDWR | O_BINARY)
> > +    );
> > +    TEMP_FAILURE_RETRY(
> > +        fd_out = qemu_open_old(filename_out, O_RDWR | O_BINARY)
> > +    );
> 
> Style question: do we want the ");" on its own line?  I think we
> generally don't do that for function and function-like macro calls.

So far scripts/checkpatch.pl doesn't complain, therefore I used this code 
style in QEMU before.

BTW, another exotic function call code style (not being compalained about yet) 
in approach:
https://lore.kernel.org/qemu-devel/E1oDQqv-0003d4-Hm@lizzy.crudebyte.com/

> >      g_free(filename_in);
> >      g_free(filename_out);
> >      if (fd_in < 0 || fd_out < 0) {
> > 
> > @@ -142,7 +146,9 @@ static void qemu_chr_open_pipe(Chardev *chr,
> > 
> >          if (fd_out >= 0) {
> >          
> >              close(fd_out);
> >          
> >          }
> > 
> > -        TFR(fd_in = fd_out = qemu_open_old(filename, O_RDWR | O_BINARY));
> > +        TEMP_FAILURE_RETRY(
> > +            fd_in = fd_out = qemu_open_old(filename, O_RDWR | O_BINARY)
> > +        );
> > 
> >          if (fd_in < 0) {
> >          
> >              error_setg_file_open(errp, errno, filename);
> >              return;
> > 
> > diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
> > index c90ab947ba..e803c05d0c 100644
> > --- a/hw/9pfs/9p-local.c
> > +++ b/hw/9pfs/9p-local.c
> > @@ -470,7 +470,7 @@ static ssize_t local_readlink(FsContext *fs_ctx,
> > V9fsPath *fs_path,
> > 
> >          if (fd == -1) {
> >          
> >              return -1;
> >          
> >          }
> > 
> > -        TFR(tsize = read(fd, (void *)buf, bufsz));
> > +        TEMP_FAILURE_RETRY(tsize = read(fd, (void *)buf, bufsz));
> > 
> >          close_preserve_errno(fd);
> >      
> >      } else if ((fs_ctx->export_flags & V9FS_SM_PASSTHROUGH) ||
> >      
> >                 (fs_ctx->export_flags & V9FS_SM_NONE)) {
> > 
> > @@ -906,7 +906,9 @@ static int local_symlink(FsContext *fs_ctx, const char
> > *oldpath,
> > 
> >          }
> >          /* Write the oldpath (target) to the file. */
> >          oldpath_size = strlen(oldpath);
> > 
> > -        TFR(write_size = write(fd, (void *)oldpath, oldpath_size));
> > +        TEMP_FAILURE_RETRY(
> > +            write_size = write(fd, (void *)oldpath, oldpath_size)
> > +        );
> > 
> >          close_preserve_errno(fd);
> >          
> >          if (write_size != oldpath_size) {
> > 
> > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > index b1c161c035..55f2927d8b 100644
> > --- a/include/qemu/osdep.h
> > +++ b/include/qemu/osdep.h
> > @@ -242,8 +242,10 @@ void QEMU_ERROR("code path is reachable")
> > 
> >  #if !defined(ESHUTDOWN)
> >  #define ESHUTDOWN 4099
> >  #endif
> > 
> > -
> > -#define TFR(expr) do { if ((expr) != -1) break; } while (errno == EINTR)
> > +#if !defined(TEMP_FAILURE_RETRY)
> > +#define TEMP_FAILURE_RETRY(expr) \
> > +    do { if ((expr) != -1) break; } while (errno == EINTR)
> > +#endif
> 
> GLibc's version is
> 
>    # define TEMP_FAILURE_RETRY(expression) \
>      (__extension__						
	      \
>        ({ long int __result;					
	      \
>           do __result = (long int) (expression);			
	      \
>           while (__result == -1L && errno == EINTR);			
      \
>           __result; }))
> 
> The difference isn't just "type casting", it's also statement
> vs. expression.
> 
> Is it a good idea to have the macro expand into a statement on some
> hosts, and into an expression on others?  Sure, CI should catch any uses
> as expression, but delaying compile errors to CI wastes developer time.

For consistency and simplicity, I would define exactly one version (no ifdefs) 
of the macro with a different macro name than glibc's TEMP_FAILURE_RETRY(), 
and use that QEMU specific macro name in QEMU code everywhere.

As for statement vs. expression: The only advantage of the statement version 
is if you'd need __result as an rvalue, which is not needed ATM, right? So I 
would go for the expression version (with cast) for now.

The glibc history does not reveal why they chose the statement version.

Best regards,
Christian Schoenebeck