Re: [PATCH v2] tests/9pfs: Use g_autofree and g_autoptr where possible

[Greg Kurz <groug@kaod.org>]

On Mon, 31 Jan 2022 18:43:22 +0100
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Montag, 31. Januar 2022 17:39:30 CET Greg Kurz wrote:
> > It is recommended to use g_autofree or g_autoptr as it reduces
> > the odds of introducing memory leaks in future changes. Let's do
> > that everywhere a glib allocation is performed.
> > 
> > The virtio_9p_create_local_test_dir() function needs some extra
> > care though : the template pointer is eventually cached into the
> > local_test_path global variable for the duration of the tests and
> > should not be freed. Add the g_autofree annotation but negate it
> > with g_steal_pointer() to make it clear that the pointer ownership
> > is dropped on purpose.
> > 
> > Based-on:
> > <f6602123c6f7d0d593466231b04fba087817abbd.1642879848.git.qemu_oss@crudebyte
> > .com> Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> > v2: - fix crash with local_test_path
> > ---
> >  tests/qtest/libqos/virtio-9p.c | 30 +++++++++++++++---------------
> >  1 file changed, 15 insertions(+), 15 deletions(-)
> > 
> > diff --git a/tests/qtest/libqos/virtio-9p.c b/tests/qtest/libqos/virtio-9p.c
> > index ef96ef006adc..f0ffbc583492 100644
> > --- a/tests/qtest/libqos/virtio-9p.c
> > +++ b/tests/qtest/libqos/virtio-9p.c
> > @@ -37,18 +37,22 @@ static char *concat_path(const char* a, const char* b)
> >      return g_build_filename(a, b, NULL);
> >  }
> > 
> > -void virtio_9p_create_local_test_dir(void)
> > +static char *make_temp_dir(char *template)
> >  {
> > -    struct stat st;
> > -    char *pwd = g_get_current_dir();
> > -    char *template = concat_path(pwd, "qtest-9p-local-XXXXXX");
> > -
> > -    local_test_path = mkdtemp(template);
> > -    if (!local_test_path) {
> > +    char *path = mkdtemp(template);
> > +    if (!path) {
> >          g_test_message("mkdtemp('%s') failed: %s", template,
> > strerror(errno)); }
> > -    g_free(pwd);
> > +    return path;
> > +}
> > +
> > +void virtio_9p_create_local_test_dir(void)
> > +{
> > +    g_autofree char *pwd = g_get_current_dir();
> > +    g_autofree char *template = concat_path(pwd, "qtest-9p-local-XXXXXX");
> > +    struct stat st;
> > 
> > +    local_test_path = make_temp_dir(g_steal_pointer(&template));
> 
> Quite noisy diff and under the bottom line, it does not fix what it originally 
> supposed to: addressing the leak of the global variable 'local_test_path'. 
> g_steal_pointer() simply makes the previous g_autofree prefix pointless.
> 

The g_autofree and g_steal_pointer() dance is just an idiom that
says "I'm not freeing this pointer on purpose, it will be handled
somewhere else". No big deal. I'll split this in two patches : one
for the leak of 'local_test_path' with a clear comment instead of
g_autofree and one for the remaining g_auto* conversions.

> Why not just moving g_autofree from 'template' to the global variable 
> 'local_test_path' and avoid all that noise? 
> 

Because the 'cleanup' attribute cannot be set on a static variable as
documented in GCC:

cleanup (cleanup_function)

    The cleanup attribute runs a function when the variable goes out of
    scope. This attribute can only be applied to auto function scope
    variables; it may not be applied to parameters or variables with
    static storage duration. The function must take one parameter, a
    pointer to a type compatible with the variable. The return value
    of the function (if any) is ignored. 

and trying to do so produces:

../../tests/qtest/libqos/virtio-9p.c:32:1: error: ‘cleanup’ attribute ignored [-Werror=attributes]
   32 | static g_autofree char *local_test_path;
      | ^~~~~~
cc1: all warnings being treated as errors

The leak of 'local_test_path' can only be addressed with a g_free()
in its last user, i.e. virtio_9p_remove_local_test_dir().

> >      g_assert(local_test_path != NULL);
> > 
> >      /* ensure test directory exists now ... */
> > @@ -60,12 +64,11 @@ void virtio_9p_create_local_test_dir(void)
> >  void virtio_9p_remove_local_test_dir(void)
> >  {
> >      g_assert(local_test_path != NULL);
> > -    char *cmd = g_strdup_printf("rm -fr '%s'\n", local_test_path);
> > +    g_autofree char *cmd = g_strdup_printf("rm -fr '%s'\n",
> > local_test_path); int res = system(cmd);
> >      if (res < 0) {
> >          /* ignore error, dummy check to prevent compiler error */
> >      }
> > -    g_free(cmd);
> >  }
> > 
> >  char *virtio_9p_test_path(const char *path)
> > @@ -209,8 +212,8 @@ static void *virtio_9p_pci_create(void *pci_bus,
> > QGuestAllocator *t_alloc, static void regex_replace(GString *haystack,
> > const char *pattern, const char *replace_fmt, ...)
> >  {
> > -    GRegex *regex;
> > -    char *replace, *s;
> > +    g_autoptr(GRegex) regex = NULL;
> > +    g_autofree char *replace = NULL, *s = NULL;
> >      va_list argp;
> > 
> >      va_start(argp, replace_fmt);
> > @@ -220,9 +223,6 @@ static void regex_replace(GString *haystack, const char
> > *pattern, regex = g_regex_new(pattern, 0, 0, NULL);
> >      s = g_regex_replace(regex, haystack->str, -1, 0, replace, 0, NULL);
> >      g_string_assign(haystack, s);
> > -    g_free(s);
> > -    g_regex_unref(regex);
> > -    g_free(replace);
> >  }
> > 
> >  void virtio_9p_assign_local_driver(GString *cmd_line, const char *args)
> 
>