As the comment in qapi/error, dereferencing @errp requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
* - It must not be dereferenced, because it may be null.
...
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.
*
* Using it when it's not needed is safe, but please avoid cluttering
* the source with useless code.

Though currently ga_run_command() only gets &local_err instead of NULL
@errp, it's still better to follow the requirement to add the
ERRP_GUARD().

But as error.h suggested, the best practice for callee is to return
something to indicate success / failure.

So make ga_wait_child() return boolean and check the returned boolean in
ga_run_command() instead of dereferencing @errp, which eliminates the
need of ERRP_GUARD().

Cc: Michael Roth <michael.roth@amd.com>
Cc: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
---
 qga/commands-posix.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 7f05996495a2..64bb0be94479 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -59,7 +59,7 @@
 #endif
 #endif
 
-static void ga_wait_child(pid_t pid, int *status, Error **errp)
+static bool ga_wait_child(pid_t pid, int *status, Error **errp)
 {
     pid_t rpid;
 
@@ -70,10 +70,11 @@ static void ga_wait_child(pid_t pid, int *status, Error **errp)
     if (rpid == -1) {
         error_setg_errno(errp, errno, "failed to wait for child (pid: %d)",
                          pid);
-        return;
+        return false;
     }
 
     g_assert(rpid == pid);
+    return true;
 }
 
 static ssize_t ga_pipe_read_str(int fd[2], char **str)
@@ -178,8 +179,7 @@ static int ga_run_command(const char *argv[], const char *in_str,
         goto out;
     }
 
-    ga_wait_child(pid, &status, errp);
-    if (*errp) {
+    if (!ga_wait_child(pid, &status, errp)) {
         goto out;
     }
 
-- 
2.34.1

On 15/7/24 11:59, Zhao Liu wrote:
> As the comment in qapi/error, dereferencing @errp requires
> ERRP_GUARD():
> 
> * = Why, when and how to use ERRP_GUARD() =
> *
> * Without ERRP_GUARD(), use of the @errp parameter is restricted:
> * - It must not be dereferenced, because it may be null.
> ...
> * ERRP_GUARD() lifts these restrictions.
> *
> * To use ERRP_GUARD(), add it right at the beginning of the function.
> * @errp can then be used without worrying about the argument being
> * NULL or &error_fatal.
> *
> * Using it when it's not needed is safe, but please avoid cluttering
> * the source with useless code.
> 
> Though currently ga_run_command() only gets &local_err instead of NULL
> @errp, it's still better to follow the requirement to add the
> ERRP_GUARD().
> 
> But as error.h suggested, the best practice for callee is to return
> something to indicate success / failure.
> 
> So make ga_wait_child() return boolean and check the returned boolean in
> ga_run_command() instead of dereferencing @errp, which eliminates the
> need of ERRP_GUARD().

I'd avoid mentioning ERRP_GUARD and just describe:

   Make ga_wait_child() return boolean and check the returned boolean
   in ga_run_command() instead of dereferencing @errp.

For the code change:

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

> Cc: Michael Roth <michael.roth@amd.com>
> Cc: Konstantin Kostiuk <kkostiuk@redhat.com>
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> ---
>   qga/commands-posix.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/qga/commands-posix.c b/qga/commands-posix.c
> index 7f05996495a2..64bb0be94479 100644
> --- a/qga/commands-posix.c
> +++ b/qga/commands-posix.c
> @@ -59,7 +59,7 @@
>   #endif
>   #endif
>   
> -static void ga_wait_child(pid_t pid, int *status, Error **errp)
> +static bool ga_wait_child(pid_t pid, int *status, Error **errp)
>   {
>       pid_t rpid;
>   
> @@ -70,10 +70,11 @@ static void ga_wait_child(pid_t pid, int *status, Error **errp)
>       if (rpid == -1) {
>           error_setg_errno(errp, errno, "failed to wait for child (pid: %d)",
>                            pid);
> -        return;
> +        return false;
>       }
>   
>       g_assert(rpid == pid);
> +    return true;
>   }
>   
>   static ssize_t ga_pipe_read_str(int fd[2], char **str)
> @@ -178,8 +179,7 @@ static int ga_run_command(const char *argv[], const char *in_str,
>           goto out;
>       }
>   
> -    ga_wait_child(pid, &status, errp);
> -    if (*errp) {
> +    if (!ga_wait_child(pid, &status, errp)) {
>           goto out;
>       }
>   

On Mon, Jul 15, 2024 at 11:59:29AM +0200, Philippe Mathieu-Daudé wrote:
> Date: Mon, 15 Jul 2024 11:59:29 +0200
> From: Philippe Mathieu-Daudé <philmd@linaro.org>
> Subject: Re: [PATCH] qga/commands-posix: Make ga_wait_child() return boolean
> 
> On 15/7/24 11:59, Zhao Liu wrote:
> > As the comment in qapi/error, dereferencing @errp requires
> > ERRP_GUARD():
> > 
> > * = Why, when and how to use ERRP_GUARD() =
> > *
> > * Without ERRP_GUARD(), use of the @errp parameter is restricted:
> > * - It must not be dereferenced, because it may be null.
> > ...
> > * ERRP_GUARD() lifts these restrictions.
> > *
> > * To use ERRP_GUARD(), add it right at the beginning of the function.
> > * @errp can then be used without worrying about the argument being
> > * NULL or &error_fatal.
> > *
> > * Using it when it's not needed is safe, but please avoid cluttering
> > * the source with useless code.
> > 
> > Though currently ga_run_command() only gets &local_err instead of NULL
> > @errp, it's still better to follow the requirement to add the
> > ERRP_GUARD().
> > 
> > But as error.h suggested, the best practice for callee is to return
> > something to indicate success / failure.
> > 
> > So make ga_wait_child() return boolean and check the returned boolean in
> > ga_run_command() instead of dereferencing @errp, which eliminates the
> > need of ERRP_GUARD().
> 
> I'd avoid mentioning ERRP_GUARD and just describe:
> 
>   Make ga_wait_child() return boolean and check the returned boolean
>   in ga_run_command() instead of dereferencing @errp.
> 
> For the code change:
> 
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

Many thanks for your words and review!

Will use your words in the next version.