Subject: [PATCH v2] async_die_is_recursing: work around GCC v11.x issue on Fedora

From: "Victoria Dye via GitGitGadget" <[email-masked]>
Date: Thu, 04 Nov 2021 04:01:03 +0000

From: Victoria Dye <[email-masked]>

This fix corrects an issue found in the `dockerized(pedantic, fedora)` CI
build, first appearing after the introduction of a new version of the Fedora
docker image version. This image includes a version of `glibc` with the
attribute `__attr_access_none` added to `pthread_setspecific` [1], the
implementation of which only exists for GCC 11.X - the version included in
the Fedora image. The attribute requires that the pointer provided in the
second argument of `pthread_getspecific` must, if not NULL, be a pointer to
a valid object. In the usage in `async_die_is_recursing`, `(void *)1` is not
valid, causing the error.

This fix imitates a workaround added in SELinux [2] by using the pointer to
the static `async_die_counter` itself as the second argument to
`pthread_setspecific`. This guaranteed non-NULL, valid pointer matches the
intent of the current usage while not triggering the build error.

[1] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=a1561c3bbe8
[2] https://lore.kernel.org/all/[email-masked]/

Co-authored-by: Johannes Schindelin <[email-masked]>
Signed-off-by: Victoria Dye <[email-masked]>
---
    async_die_is_recursing: fix use of pthread_getspecific for Fedora
    
    Following up on Johannes' report earlier [1], this patch fixes a
    compiler error in the Fedora CI build (the same issue was identified in
    a local developer build about a week ago [2]). This fix changes the
    second argument in the call to pthread_setspecific from '(void *)1' to a
    valid pointer, thus preventing the error in the use of
    __attr_access_none.

    Changes since V1
    ================
    
     * Change &ret to &async_die_counter ("dummy" argument to
       pthread_setspecific) so that it's using a global (rather than local)
       variable
     * Fix typo in commit message (pthread_getspecific ->
       pthread_setspecific)
    
    [1]
    https://lore.kernel.org/git/[email-masked]/
    [2]
    https://lore.kernel.org/git/[email-masked]/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1072%2Fvdye%2Ffix-fedora-build-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1072/vdye/fix-fedora-build-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1072

Range-diff vs v1:

 1:  efe0b398f54 ! 1:  7f10e09269a async_die_is_recursing: fix use of pthread_getspecific for Fedora
     @@ Metadata
      Author: Victoria Dye <[email-masked]>
      
       ## Commit message ##
     -    async_die_is_recursing: fix use of pthread_getspecific for Fedora
     +    async_die_is_recursing: work around GCC v11.x issue on Fedora
      
     -    Correct an issue encountered in the `dockerized(pedantic, fedora)` CI build,
     -    first appearing after the release of v36 as the latest stable version of the
     -    Fedora docker image. This image includes a version of `glibc` with the
     -    attribute `__attr_access_none` added to `pthread_getspecific` [1], the
     +    This fix corrects an issue found in the `dockerized(pedantic, fedora)` CI
     +    build, first appearing after the introduction of a new version of the Fedora
     +    docker image version. This image includes a version of `glibc` with the
     +    attribute `__attr_access_none` added to `pthread_setspecific` [1], the
          implementation of which only exists for GCC 11.X - the version included in
     -    Fedora. The attribute requires that the pointer provided in the second
     -    argument of `pthread_getspecific` must, if not NULL, be a pointer to a valid
     -    object. In the usage in `async_die_is_recursing`, `(void *)1` is not valid,
     -    resulting in the error.
     +    the Fedora image. The attribute requires that the pointer provided in the
     +    second argument of `pthread_getspecific` must, if not NULL, be a pointer to
     +    a valid object. In the usage in `async_die_is_recursing`, `(void *)1` is not
     +    valid, causing the error.
      
     -    The fix imitates a workaround added in SELinux [2] by using the pointer to
     -    `ret` as the second argument to `pthread_getspecific`. This guaranteed
     -    non-NULL, valid pointer adheres to the current intended usage of
     -    `pthread_getspecific` while not triggering the build error.
     +    This fix imitates a workaround added in SELinux [2] by using the pointer to
     +    the static `async_die_counter` itself as the second argument to
     +    `pthread_setspecific`. This guaranteed non-NULL, valid pointer matches the
     +    intent of the current usage while not triggering the build error.
      
          [1] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=a1561c3bbe8
          [2] https://lore.kernel.org/all/[email-masked]/
     @@ run-command.c: static NORETURN void die_async(const char *err, va_list params)
       {
       	void *ret = pthread_getspecific(async_die_counter);
      -	pthread_setspecific(async_die_counter, (void *)1);
     -+	pthread_setspecific(async_die_counter, &ret); /* set to any non-NULL valid pointer */
     ++	pthread_setspecific(async_die_counter, &async_die_counter); /* set to any non-NULL valid pointer */
       	return ret != NULL;
       }

 run-command.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/run-command.c b/run-command.c
index 7ef5cc712a9..f40df01c772 100644
--- a/run-command.c
+++ b/run-command.c
@@ -1099,7 +1099,7 @@ static NORETURN void die_async(const char *err, va_list params)
 static int async_die_is_recursing(void)
 {
 	void *ret = pthread_getspecific(async_die_counter);
-	pthread_setspecific(async_die_counter, (void *)1);
+	pthread_setspecific(async_die_counter, &async_die_counter); /* set to any non-NULL valid pointer */
 	return ret != NULL;
 }

base-commit: 876b1423317071f43c99666f3fc3db3642dfbe14
-- 
gitgitgadget