Re: [PATCH v2 17/21] libmultipath: add udev and logsink symbols

[Benjamin Marzinski <bmarzins@redhat.com>]

On Thu, Sep 24, 2020 at 03:37:12PM +0200, mwilck@suse.com wrote:
> From: Martin Wilck <mwilck@suse.com>
> 
> With these symbols added, applications using libmultipath don't
> need to define global variables "udev" and "logsink" any more.
> This comes at the cost of having to call an init function.
> Currently, libmultipath_init() does nothing but initialize
> "udev".
> 
> The linker's symbol lookup order still allows applications to use
> their own "logsink" and "udev" variables, which will take precendence
> over libmultipath's internal ones. In this case, calling
> libmultipath_init() can be skipped, but like before,
> udev should be initialized (using udev_new()) before making any
> libmultipath calls.
> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  libmultipath/config.c             | 46 +++++++++++++++++++++++++++++++
>  libmultipath/config.h             | 46 ++++++++++++++++++++++++++++++-
>  libmultipath/debug.c              |  2 ++
>  libmultipath/libmultipath.version |  8 ++++++
>  4 files changed, 101 insertions(+), 1 deletion(-)
> 
> diff --git a/libmultipath/config.c b/libmultipath/config.c
> index 01b77df..fbb66b3 100644
> --- a/libmultipath/config.c
> +++ b/libmultipath/config.c
> @@ -27,6 +27,52 @@
>  #include "mpath_cmd.h"
>  #include "propsel.h"
>  
> +/*
> + * We don't support re-initialization after
> + * libmultipath_exit().
> + */
> +static bool libmultipath_exit_called;
> +static pthread_once_t _init_once = PTHREAD_ONCE_INIT;
> +static pthread_once_t _exit_once = PTHREAD_ONCE_INIT;
> +struct udev *udev;
> +
> +static void _udev_init(void)
> +{
> +	if (udev)
> +		udev_ref(udev);
> +	else
> +		udev = udev_new();
> +	if (!udev)
> +		condlog(0, "%s: failed to initialize udev", __func__);
> +}
> +
> +static void _libmultipath_init(void)
> +{
> +	_udev_init();
> +}

I don't understand why we need both _udev_init() and
_libmultipath_init().

-Ben

> +
> +static bool _is_libmultipath_initialized(void)
> +{
> +	return !libmultipath_exit_called && !!udev;
> +}
> +
> +int libmultipath_init(void)
> +{
> +	pthread_once(&_init_once, _libmultipath_init);
> +	return !_is_libmultipath_initialized();
> +}
> +
> +static void _libmultipath_exit(void)
> +{
> +	libmultipath_exit_called = true;
> +	udev_unref(udev);
> +}
> +
> +void libmultipath_exit(void)
> +{
> +	pthread_once(&_exit_once, _libmultipath_exit);
> +}
> +
>  static struct config __internal_config;
>  struct config *libmp_get_multipath_config(void)
>  {
> diff --git a/libmultipath/config.h b/libmultipath/config.h
> index 5997b71..dac4e8f 100644
> --- a/libmultipath/config.h
> +++ b/libmultipath/config.h
> @@ -232,7 +232,51 @@ struct config {
>  	char *enable_foreign;
>  };
>  
> -extern struct udev * udev;
> +/**
> + * extern variable: udev
> + *
> + * A &struct udev instance used by libmultipath. libmultipath expects
> + * a valid, initialized &struct udev in this variable.
> + * An application can define this variable itself, in which case
> + * the applications's instance will take precedence.
> + * The application can initialize and destroy this variable by
> + * calling libmultipath_init() and libmultipath_exit(), respectively,
> + * whether or not it defines the variable itself.
> + * An application can initialize udev with udev_new() before calling
> + * libmultipath_init(), e.g. if it has to make libudev calls before
> + * libmultipath calls. If an application wants to keep using the
> + * udev variable after calling libmultipath_exit(), it should have taken
> + * an additional reference on it beforehand. This is the case e.g.
> + * after initiazing udev with udev_new().
> + */
> +extern struct udev *udev;
> +
> +/**
> + * libmultipath_init() - library initialization
> + *
> + * This function initializes libmultipath data structures.
> + * It is light-weight; some other initializations, like device-mapper
> + * initialization, are done lazily when the respective functionality
> + * is required.
> + *
> + * Clean up by libmultipath_exit() when the program terminates.
> + * It is an error to call libmultipath_init() after libmultipath_exit().
> + * Return: 0 on success, 1 on failure.
> + */
> +int libmultipath_init(void);
> +
> +/**
> + * libmultipath_exit() - library un-initialization
> + *
> + * This function un-initializes libmultipath data structures.
> + * It is recommended to call this function at program exit.
> + *
> + * Calls to libmultipath_init() after libmultipath_exit() will fail
> + * (in other words, libmultipath can't be re-initialized).
> + * Any other libmultipath calls after libmultipath_exit() may cause
> + * undefined behavior.
> + */
> +void libmultipath_exit(void);
>  
>  int find_hwe (const struct _vector *hwtable,
>  	      const char * vendor, const char * product, const char *revision,
> diff --git a/libmultipath/debug.c b/libmultipath/debug.c
> index 4128cb9..b3a1de9 100644
> --- a/libmultipath/debug.c
> +++ b/libmultipath/debug.c
> @@ -15,6 +15,8 @@
>  #include "defaults.h"
>  #include "debug.h"
>  
> +int logsink;
> +
>  void dlog (int sink, int prio, const char * fmt, ...)
>  {
>  	va_list ap;
> diff --git a/libmultipath/libmultipath.version b/libmultipath/libmultipath.version
> index 81bcc9d..2e531ef 100644
> --- a/libmultipath/libmultipath.version
> +++ b/libmultipath/libmultipath.version
> @@ -228,3 +228,11 @@ global:
>  	init_config;
>  	uninit_config;
>  } LIBMULTIPATH_0.8.4.2;
> +
> +LIBMULTIPATH_0.8.4.4 {
> +global:
> +	udev;
> +	logsink;
> +	libmultipath_init;
> +	libmultipath_exit;
> +} LIBMULTIPATH_0.8.4.3;
> -- 
> 2.28.0