Re: [Qemu-devel] [PATCH 7/7] error: On abort, report where the error was created

[Laszlo Ersek <lersek@redhat.com>]

On 06/22/15 21:26, Markus Armbruster wrote:
> This is particularly useful when we abort in error_propagate(),
> because there the stack backtrace doesn't lead to where the error was
> created.  Looks like this:
> 
>     Unexpected error at /work/armbru/qemu/blockdev.c:322:
>     qemu-system-x86_64: -drive if=none,werror=foo: 'foo' invalid write error action
>     Aborted (core dumped)
>     [Exit 134 (SIGABRT)]
> 
> Note: to get this example output, I monkey-patched drive_new() to pass
> &error_abort to blockdev_init().
> 
> To keep the error handling boiler plate from growing even more, all
> error_setFOO() become macros expanding into error_setFOO_internal()
> with additional __FILE__, __LINE__ arguments.  Not exactly pretty, but
> it works.

Please consider squeezing in __func__ too. The information given by
__FILE__:__LINE__ goes stale quite a bit faster than when __func__ is
included (in a triplet then).

In a poorly written bug report (eg. no exact version / git commit
identified), the function name could be the most helpful bit.

Just an idea, of course. :)

Thanks
Laszlo

> 
> The macro trickery breaks down when you take the address of an
> error_setFOO().  Fortunately, we do that in just one place: qemu-ga's
> Windows VSS provider and requester DLL wants to call
> error_setg_win32() through a function pointer "to avoid linking glib
> to the DLL".  Use error_setg_win32_internal() there.  The use of the
> function pointer is already wrapped in a macro, so the churn isn't
> bad.
> 
> Code size increases by some 14KiB for me (0.3%).  Tolerable.  Could be
> less if we passed relative rather than absolute source file names to
> the compiler.
> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  include/qapi/error.h        | 36 +++++++++++++++++++++++++---------
>  qga/vss-win32.c             |  2 +-
>  qga/vss-win32/requester.cpp |  5 +++--
>  qga/vss-win32/requester.h   |  5 +++--
>  util/error.c                | 47 ++++++++++++++++++++++++++++++---------------
>  5 files changed, 65 insertions(+), 30 deletions(-)
> 
> diff --git a/include/qapi/error.h b/include/qapi/error.h
> index 9466b09..501e110 100644
> --- a/include/qapi/error.h
> +++ b/include/qapi/error.h
> @@ -104,16 +104,22 @@ ErrorClass error_get_class(const Error *err);
>   * The new error's class is ERROR_CLASS_GENERIC_ERROR, and its
>   * human-readable error message is made from printf-style @fmt, ...
>   */
> -void error_setg(Error **errp, const char *fmt, ...)
> -    GCC_FMT_ATTR(2, 3);
> +#define error_setg(errp, fmt, ...) \
> +    error_setg_internal((errp), __FILE__, __LINE__, (fmt), ## __VA_ARGS__)
> +void error_setg_internal(Error **errp, const char *src, int line,
> +                         const char *fmt, ...) GCC_FMT_ATTR(4, 5);
>  
>  /*
>   * Just like error_setg(), with @os_error info added to the message.
>   * If @os_error is non-zero, ": " + strerror(os_error) is appended to
>   * the human-readable error message.
>   */
> -void error_setg_errno(Error **errp, int os_error, const char *fmt, ...)
> -    GCC_FMT_ATTR(3, 4);
> +#define error_setg_errno(errp, os_error, fmt, ...)                      \
> +    error_setg_errno_internal((errp), __FILE__, __LINE__, (os_error),   \
> +                              (fmt), ## __VA_ARGS__)
> +void error_setg_errno_internal(Error **errp, const char *fname, int line,
> +                               int os_error, const char *fmt, ...)
> +    GCC_FMT_ATTR(5, 6);
>  
>  #ifdef _WIN32
>  /*
> @@ -121,8 +127,12 @@ void error_setg_errno(Error **errp, int os_error, const char *fmt, ...)
>   * If @win32_error is non-zero, ": " + g_win32_error_message(win32_err)
>   * is appended to the human-readable error message.
>   */
> -void error_setg_win32(Error **errp, int win32_err, const char *fmt, ...)
> -    GCC_FMT_ATTR(3, 4);
> +#define error_setg_win32(errp, win32_err, fmt, ...)                     \
> +    error_setg_win32_internal((errp), __FILE__, __LINE__, (win32_err),  \
> +                              (fmt), ## __VA_ARGS__)
> +void error_setg_win32_internal(Error **errp, const char *src, int line,
> +                               int win32_err, const char *fmt, ...)
> +    GCC_FMT_ATTR(5, 6);
>  #endif
>  
>  /*
> @@ -143,7 +153,11 @@ void error_propagate(Error **dst_errp, Error *local_err);
>  /*
>   * Convenience function to report open() failure.
>   */
> -void error_setg_file_open(Error **errp, int os_errno, const char *filename);
> +#define error_setg_file_open(errp, os_errno, filename)          \
> +    error_setg_file_open_internal((errp), __FILE__, __LINE__,   \
> +                                  (os_errno), (filename))
> +void error_setg_file_open_internal(Error **errp, const char *src, int line,
> +                                   int os_errno, const char *filename);
>  
>  /*
>   * Return an exact copy of @err.
> @@ -165,8 +179,12 @@ void error_report_err(Error *);
>   * Note: use of error classes other than ERROR_CLASS_GENERIC_ERROR is
>   * strongly discouraged.
>   */
> -void error_set(Error **errp, ErrorClass err_class, const char *fmt, ...)
> -    GCC_FMT_ATTR(3, 4);
> +#define error_set(errp, err_class, fmt, ...)                    \
> +    error_set_internal((errp), __FILE__, __LINE__, (err_class), \
> +                       (fmt), ## __VA_ARGS__)
> +void error_set_internal(Error **errp, const char *src, int line,
> +                        ErrorClass err_class, const char *fmt, ...)
> +    GCC_FMT_ATTR(5, 6);
>  
>  /*
>   * Pass to error_setg() & friends to abort() on error.
> diff --git a/qga/vss-win32.c b/qga/vss-win32.c
> index d75d7bb..2142b49 100644
> --- a/qga/vss-win32.c
> +++ b/qga/vss-win32.c
> @@ -150,7 +150,7 @@ void qga_vss_fsfreeze(int *nr_volume, Error **errp, bool freeze)
>      const char *func_name = freeze ? "requester_freeze" : "requester_thaw";
>      QGAVSSRequesterFunc func;
>      ErrorSet errset = {
> -        .error_setg_win32 = error_setg_win32,
> +        .error_setg_win32 = error_setg_win32_internal,
>          .errp = errp,
>      };
>  
> diff --git a/qga/vss-win32/requester.cpp b/qga/vss-win32/requester.cpp
> index aae0d5f..0373491 100644
> --- a/qga/vss-win32/requester.cpp
> +++ b/qga/vss-win32/requester.cpp
> @@ -23,8 +23,9 @@
>  /* Call QueryStatus every 10 ms while waiting for frozen event */
>  #define VSS_TIMEOUT_EVENT_MSEC 10
>  
> -#define err_set(e, err, fmt, ...) \
> -    ((e)->error_setg_win32((e)->errp, err, fmt, ## __VA_ARGS__))
> +#define err_set(e, err, fmt, ...)                               \
> +    ((e)->error_setg_win32((e)->errp, __FILE__, __LINE__,       \
> +                                    err, fmt, ## __VA_ARGS__))
>  /* Bad idea, works only when (e)->errp != NULL: */
>  #define err_is_set(e) ((e)->errp && *(e)->errp)
>  /* To lift this restriction, error_propagate(), like we do in QEMU code */
> diff --git a/qga/vss-win32/requester.h b/qga/vss-win32/requester.h
> index 34be5c1..7dd8102 100644
> --- a/qga/vss-win32/requester.h
> +++ b/qga/vss-win32/requester.h
> @@ -23,8 +23,9 @@ extern "C" {
>  struct Error;
>  
>  /* Callback to set Error; used to avoid linking glib to the DLL */
> -typedef void (*ErrorSetFunc)(struct Error **errp, int win32_err,
> -                             const char *fmt, ...) GCC_FMT_ATTR(3, 4);
> +typedef void (*ErrorSetFunc)(struct Error **errp, const char *src, int line,
> +                             int win32_err, const char *fmt, ...)
> +    GCC_FMT_ATTR(5, 6);
>  typedef struct ErrorSet {
>      ErrorSetFunc error_setg_win32;
>      struct Error **errp;        /* restriction: must not be null */
> diff --git a/util/error.c b/util/error.c
> index fb575ac..a655cfe 100644
> --- a/util/error.c
> +++ b/util/error.c
> @@ -18,12 +18,21 @@ struct Error
>  {
>      char *msg;
>      ErrorClass err_class;
> +    const char *src;
> +    int line;
>  };
>  
>  Error *error_abort;
>  
> -static void error_setv(Error **errp, ErrorClass err_class,
> -                       const char *fmt, va_list ap)
> +static void error_do_abort(Error *err)
> +{
> +    fprintf(stderr, "Unexpected error at %s:%d:\n", err->src, err->line);
> +    error_report_err(err);
> +    abort();
> +}
> +
> +static void error_setv(Error **errp, const char *src, int line,
> +                       ErrorClass err_class, const char *fmt, va_list ap)
>  {
>      Error *err;
>      int saved_errno = errno;
> @@ -36,10 +45,11 @@ static void error_setv(Error **errp, ErrorClass err_class,
>      err = g_malloc0(sizeof(*err));
>      err->msg = g_strdup_vprintf(fmt, ap);
>      err->err_class = err_class;
> +    err->src = src;
> +    err->line = line;
>  
>      if (errp == &error_abort) {
> -        error_report_err(err);
> -        abort();
> +        error_do_abort(err);
>      }
>  
>      *errp = err;
> @@ -47,25 +57,28 @@ static void error_setv(Error **errp, ErrorClass err_class,
>      errno = saved_errno;
>  }
>  
> -void error_set(Error **errp, ErrorClass err_class, const char *fmt, ...)
> +void error_set_internal(Error **errp, const char *src, int line,
> +                        ErrorClass err_class, const char *fmt, ...)
>  {
>      va_list ap;
>  
>      va_start(ap, fmt);
> -    error_setv(errp, err_class, fmt, ap);
> +    error_setv(errp, src, line, err_class, fmt, ap);
>      va_end(ap);
>  }
>  
> -void error_setg(Error **errp, const char *fmt, ...)
> +void error_setg_internal(Error **errp, const char *src, int line,
> +                         const char *fmt, ...)
>  {
>      va_list ap;
>  
>      va_start(ap, fmt);
> -    error_setv(errp, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
> +    error_setv(errp, src, line, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
>      va_end(ap);
>  }
>  
> -void error_setg_errno(Error **errp, int os_errno, const char *fmt, ...)
> +void error_setg_errno_internal(Error **errp, const char *src, int line,
> +                              int os_errno, const char *fmt, ...)
>  {
>      va_list ap;
>      char *msg;
> @@ -76,7 +89,7 @@ void error_setg_errno(Error **errp, int os_errno, const char *fmt, ...)
>      }
>  
>      va_start(ap, fmt);
> -    error_setv(errp, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
> +    error_setv(errp, src, line, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
>      va_end(ap);
>  
>      if (os_errno != 0) {
> @@ -88,14 +101,17 @@ void error_setg_errno(Error **errp, int os_errno, const char *fmt, ...)
>      errno = saved_errno;
>  }
>  
> -void error_setg_file_open(Error **errp, int os_errno, const char *filename)
> +void error_setg_file_open_internal(Error **errp, const char *src, int line,
> +                                   int os_errno, const char *filename)
>  {
> -    error_setg_errno(errp, os_errno, "Could not open '%s'", filename);
> +    error_setg_errno_internal(errp, src, line, os_errno,
> +                              "Could not open '%s'", filename);
>  }
>  
>  #ifdef _WIN32
>  
> -void error_setg_win32(Error **errp, int win32_err, const char *fmt, ...)
> +void error_setg_win32_internal(Error **errp, const char *src, int line,
> +                              int win32_err, const char *fmt, ...)
>  {
>      va_list ap;
>      char *msg1, *msg2;
> @@ -105,7 +121,7 @@ void error_setg_win32(Error **errp, int win32_err, const char *fmt, ...)
>      }
>  
>      va_start(ap, fmt);
> -    error_setv(errp, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
> +    error_setv(errp, src, line, ERROR_CLASS_GENERIC_ERROR, fmt, ap);
>      va_end(ap);
>  
>      if (win32_err != 0) {
> @@ -157,8 +173,7 @@ void error_free(Error *err)
>  void error_propagate(Error **dst_errp, Error *local_err)
>  {
>      if (local_err && dst_errp == &error_abort) {
> -        error_report_err(local_err);
> -        abort();
> +        error_do_abort(local_err);
>      } else if (dst_errp && !*dst_errp) {
>          *dst_errp = local_err;
>      } else if (local_err) {
>