Re: [PATCH v4 2/3] module: add Error arguments to module_load_one and module_load_qom_one

[Markus Armbruster]

Claudio Fontana <cfontana@suse.de> writes:

> improve error handling during module load, by changing:
>
> bool module_load_one(const char *prefix, const char *lib_name);
> void module_load_qom_one(const char *type);
>
> to:
>
> bool module_load_one(const char *prefix, const char *name, Error **errp);
> bool module_load_qom_one(const char *type, Error **errp);
>
> module_load_qom_one has been introduced in:
>
> commit 28457744c345 ("module: qom module support"), which built on top of
> module_load_one, but discarded the bool return value. Restore it.
>
> Adapt all callers to emit errors, or ignore them, or fail hard,
> as appropriate in each context.

How exactly does behavior change?  The commit message is mum on the
behavior before the patch, and vague on the behavior afterwards.

> Signed-off-by: Claudio Fontana <cfontana@suse.de>
> ---
>  audio/audio.c         |   9 ++-
>  block.c               |  15 ++++-
>  block/dmg.c           |  18 +++++-
>  hw/core/qdev.c        |  10 ++-
>  include/qemu/module.h |  38 ++++++++++--
>  qom/object.c          |  18 +++++-
>  softmmu/qtest.c       |   6 +-
>  ui/console.c          |  18 +++++-
>  util/module.c         | 140 ++++++++++++++++++++++++------------------
>  9 files changed, 194 insertions(+), 78 deletions(-)
>
> diff --git a/audio/audio.c b/audio/audio.c
> index 76b8735b44..cff7464c07 100644
> --- a/audio/audio.c
> +++ b/audio/audio.c
> @@ -72,6 +72,7 @@ void audio_driver_register(audio_driver *drv)
>  audio_driver *audio_driver_lookup(const char *name)
>  {
>      struct audio_driver *d;
> +    Error *local_err = NULL;
>  
>      QLIST_FOREACH(d, &audio_drivers, next) {
>          if (strcmp(name, d->name) == 0) {
> @@ -79,7 +80,13 @@ audio_driver *audio_driver_lookup(const char *name)
>          }
>      }
>  
> -    audio_module_load_one(name);
> +    if (!audio_module_load_one(name, &local_err)) {
> +        if (local_err) {
> +            error_report_err(local_err);
> +        }
> +        return NULL;
> +    }
> +
>      QLIST_FOREACH(d, &audio_drivers, next) {
>          if (strcmp(name, d->name) == 0) {
>              return d;
> diff --git a/block.c b/block.c
> index bc85f46eed..8b610c6d95 100644
> --- a/block.c
> +++ b/block.c
> @@ -464,7 +464,14 @@ BlockDriver *bdrv_find_format(const char *format_name)
>      /* The driver isn't registered, maybe we need to load a module */
>      for (i = 0; i < (int)ARRAY_SIZE(block_driver_modules); ++i) {
>          if (!strcmp(block_driver_modules[i].format_name, format_name)) {
> -            block_module_load_one(block_driver_modules[i].library_name);
> +            Error *local_err = NULL;
> +            if (!block_module_load_one(block_driver_modules[i].library_name,
> +                                       &local_err)) {
> +                if (local_err) {
> +                    error_report_err(local_err);
> +                }
> +                return NULL;
> +            }
>              break;
>          }
>      }

Before the patch, bdrv_find_format() fails silently[*].

Afterwards, it reports an error on some failures, but not on others.
Sure this is what we want?

> @@ -976,7 +983,11 @@ BlockDriver *bdrv_find_protocol(const char *filename,
>      for (i = 0; i < (int)ARRAY_SIZE(block_driver_modules); ++i) {
>          if (block_driver_modules[i].protocol_name &&
>              !strcmp(block_driver_modules[i].protocol_name, protocol)) {
> -            block_module_load_one(block_driver_modules[i].library_name);
> +            Error *local_err = NULL;
> +            if (!block_module_load_one(block_driver_modules[i].library_name,
> +                                       &local_err) && local_err) {

Break the line before && local_err, please, like you do elsewhere.

> +                error_report_err(local_err);
> +            }
>              break;
>          }
>      }

Uh-oh: error_report() or equivalent in a function with an Error **
parameter.  This is almost always wrong.  Shouldn't you pass the error
to the caller?

Please check all uses of your FOO_load_one() for this issue.

> diff --git a/block/dmg.c b/block/dmg.c
> index 98db18d82a..11d184d39c 100644
> --- a/block/dmg.c
> +++ b/block/dmg.c
> @@ -434,6 +434,7 @@ static int dmg_open(BlockDriverState *bs, QDict *options, 
> int flags,
>      uint64_t plist_xml_offset, plist_xml_length;
>      int64_t offset;
>      int ret;
> +    Error *local_err = NULL;
>  
>      ret = bdrv_apply_auto_read_only(bs, NULL, errp);
>      if (ret < 0) {
> @@ -446,8 +447,21 @@ static int dmg_open(BlockDriverState *bs, QDict 
> *options, int flags,
>          return -EINVAL;
>      }
>  
> -    block_module_load_one("dmg-bz2");
> -    block_module_load_one("dmg-lzfse");
> +    if (!block_module_load_one("dmg-bz2", &local_err)) {
> +        if (local_err) {
> +            error_report_err(local_err);
> +            return -EINVAL;
> +        }
> +        warn_report("dmg-bz2 module not present, bz2 decomp unavailable");
> +    }
> +    local_err = NULL;
> +    if (!block_module_load_one("dmg-lzfse", &local_err)) {
> +        if (local_err) {
> +            error_report_err(local_err);
> +            return -EINVAL;
> +        }
> +        warn_report("dmg-lzfse module not present, lzfse decomp 
> unavailable");
> +    }
>  
>      s->n_chunks = 0;
>      s->offsets = s->lengths = s->sectors = s->sectorcounts = NULL;
> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> index 0806d8fcaa..5902c59c94 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -148,7 +148,15 @@ bool qdev_set_parent_bus(DeviceState *dev, BusState 
> *bus, Error **errp)
>  DeviceState *qdev_new(const char *name)
>  {
>      if (!object_class_by_name(name)) {
> -        module_load_qom_one(name);
> +        Error *local_err = NULL;
> +        if (!module_load_qom_one(name, &local_err)) {
> +            if (local_err) {
> +                error_report_err(local_err);
> +            } else {
> +                error_report("could not find a module for type '%s'", name);
> +            }
> +            abort();

Why is this a programming error?

> +        }
>      }
>      return DEVICE(object_new(name));
>  }
> diff --git a/include/qemu/module.h b/include/qemu/module.h
> index 8c012bbe03..78d4c4de96 100644
> --- a/include/qemu/module.h
> +++ b/include/qemu/module.h
> @@ -61,16 +61,44 @@ typedef enum {
>  #define fuzz_target_init(function) module_init(function, \
>                                                 MODULE_INIT_FUZZ_TARGET)
>  #define migration_init(function) module_init(function, MODULE_INIT_MIGRATION)
> -#define block_module_load_one(lib) module_load_one("block-", lib)
> -#define ui_module_load_one(lib) module_load_one("ui-", lib)
> -#define audio_module_load_one(lib) module_load_one("audio-", lib)
> +#define block_module_load_one(lib, errp) module_load_one("block-", lib, errp)
> +#define ui_module_load_one(lib, errp) module_load_one("ui-", lib, errp)
> +#define audio_module_load_one(lib, errp) module_load_one("audio-", lib, errp)
>  
>  void register_module_init(void (*fn)(void), module_init_type type);
>  void register_dso_module_init(void (*fn)(void), module_init_type type);
>  
>  void module_call_init(module_init_type type);
> -bool module_load_one(const char *prefix, const char *lib_name);
> -void module_load_qom_one(const char *type);
> +
> +/*
> + * module_load_one: attempt to load a module from a set of directories
> + *
> + * directories searched are:
> + * - getenv("QEMU_MODULE_DIR")
> + * - get_relocated_path(CONFIG_QEMU_MODDIR);
> + * - /var/run/qemu/${version_dir}
> + *
> + * prefix:         a subsystem prefix, or the empty string ("audio-", ..., 
> "")
> + * name:           name of the module
> + * errp:           error to set in case the module is found, but load failed.
> + *
> + * Return value:   true on success (found and loaded);
> + *                 if module if found, but load failed, errp will be set.
> + *                 if module is not found, errp will not be set.

I understand you need to distingush two failure modes "found, but load
failed" and "not found".

Functions that set an error on some failures only tend to be awkward: in
addition to checking the return value for failure, you have to check
@errp for special failures.  This is particularly cumbersome when it
requires a @local_err and an error_propagate() just for that.  I
generally prefer to return an error code and always set an error.

That said, the patch doesn't look bad.  Perhaps it will be once the
issues I pointed out above have been addressed.  Let's not worry about
it right now.

> + */
> +bool module_load_one(const char *prefix, const char *name, Error **errp);
> +
> +/*
> + * module_load_qom_one: attempt to load a module to provide a QOM type
> + *
> + * type:           the type to be provided
> + * errp:           error to set.
> + *
> + * Return value:   true on success (found and loaded), false on failure.
> + *                 If a module is simply not found for the type,
> + *                 errp will not be set.
> + */
> +bool module_load_qom_one(const char *type, Error **errp);
>  void module_load_qom_all(void);
>  void module_allow_arch(const char *arch);
>  

[...]


[*] Except it prints "Module is not supported by system\n" to stderr
when !g_module_supported(), which doesn't look right to me.