I stole the printk_once() macro.

I always wanted to be able to print some error directly if there is a
buffer to dump, however we can't use error_report() where the code path
can be triggered by DDOS attack.  To avoid that, we can introduce a
print-once-like function for it.  Meanwhile, we also introduce the
corresponding helper for warn_report().

CC: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/qemu/error-report.h | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/include/qemu/error-report.h b/include/qemu/error-report.h
index e1c8ae1a52..3e6e84801f 100644
--- a/include/qemu/error-report.h
+++ b/include/qemu/error-report.h
@@ -44,6 +44,32 @@ void error_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
 void warn_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
 void info_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
 
+/* Similar to error_report(), but it only prints the message once. */
+#define error_report_once(fmt, ...)             \
+    ({                                          \
+        static bool __print_once;               \
+        bool __ret_print_once = !__print_once;  \
+                                                \
+        if (!__print_once) {                    \
+            __print_once = true;                \
+            error_report(fmt, ##__VA_ARGS__);   \
+        }                                       \
+        unlikely(__ret_print_once);             \
+    })
+
+/* Similar to warn_report(), but it only prints the message once. */
+#define warn_report_once(fmt, ...)              \
+    ({                                          \
+        static bool __print_once;               \
+        bool __ret_print_once = !__print_once;  \
+                                                \
+        if (!__print_once) {                    \
+            __print_once = true;                \
+            warn_report(fmt, ##__VA_ARGS__);   \
+        }                                       \
+        unlikely(__ret_print_once);             \
+    })
+
 const char *error_get_progname(void);
 extern bool enable_timestamp_msg;
 
-- 
2.17.0

On 05/22/2018 10:18 PM, Peter Xu wrote:
> I stole the printk_once() macro.
> 
> I always wanted to be able to print some error directly if there is a
> buffer to dump, however we can't use error_report() where the code path
> can be triggered by DDOS attack.  To avoid that, we can introduce a
> print-once-like function for it.  Meanwhile, we also introduce the
> corresponding helper for warn_report().
> 
> CC: Markus Armbruster <armbru@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   include/qemu/error-report.h | 26 ++++++++++++++++++++++++++
>   1 file changed, 26 insertions(+)
> 
> diff --git a/include/qemu/error-report.h b/include/qemu/error-report.h
> index e1c8ae1a52..3e6e84801f 100644
> --- a/include/qemu/error-report.h
> +++ b/include/qemu/error-report.h
> @@ -44,6 +44,32 @@ void error_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
>   void warn_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
>   void info_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
>   
> +/* Similar to error_report(), but it only prints the message once. */
> +#define error_report_once(fmt, ...)             \
> +    ({                                          \
> +        static bool __print_once;               \
> +        bool __ret_print_once = !__print_once;  \

In v1 you were asked to avoid leading double underscore 
(https://lists.gnu.org/archive/html/qemu-devel/2018-05/msg03442.html; 
use things like 'print_once_' instead) and to document the return value 
of these macros 
(https://lists.gnu.org/archive/html/qemu-devel/2018-05/msg03503.html; 
having the return value makes it possible to write conditional code that 
caches further information about a first, but not repeat, failures). 
Why hasn't that happened?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org