Re: [PATCH 03/21] libmultipath/checkers: replace message by msgid

["Benjamin Marzinski" <bmarzins@redhat.com>]

On Fri, Oct 12, 2018 at 12:26:49AM +0200, Martin Wilck wrote:
> Replace the character array "message" in struct checker with
> a "message ID" field.
> 
> The generic checker code defines a couple of standard message IDs
> and corresponding messages. Checker-specific message IDs start
> at CHECKER_FIRST_MSG. Checkers that implement specific message
> IDs must provide a table for converting the IDs into actual log
> messages.
> 
> This simplifies the checker data structure and the handling of
> checker messages in the critical checker code path. It comes at
> the cost that only constant message strings are supported. It
> turns out that only a single checker log message (in the emc_clariion
> checker) was dynamically generated, and the missing information can
> be provided with a standard condlog message.
> 
> Follow-up patches implement this for the existing checkers.

I have some cosmetic nits that I'll add inline, but none of them are
real problems so, 

Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>

> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  libmultipath/checkers.c | 70 +++++++++++++++++++++++++++++++++++------
>  libmultipath/checkers.h | 39 +++++++++++++++++++----
>  2 files changed, 94 insertions(+), 15 deletions(-)
> 
> diff --git a/libmultipath/checkers.c b/libmultipath/checkers.c
> index 0bacc864..359791ff 100644
> --- a/libmultipath/checkers.c
> +++ b/libmultipath/checkers.c
> @@ -141,6 +141,22 @@ struct checker * add_checker (char *multipath_dir, char * name)
>  	if (!c->free)
>  		goto out;
>  
> +	c->msgtable_size = 0;
> +	c->msgtable = dlsym(c->handle, "libcheck_msgtable");
> +
> +	if (c->msgtable != NULL) {
> +		const char **p;
> +
> +		for (p = c->msgtable;
> +		     *p && (p - c->msgtable) < CHECKER_MSGTABLE_SIZE; p++)
> +			/* nothing */;
> +
> +		c->msgtable_size = p - c->msgtable;
> +	} else
> +		c->msgtable_size = 0;
> +	condlog(3, "checker %s: message table size = %d",
> +		c->name, c->msgtable_size);
> +
>  done:
>  	c->fd = -1;
>  	c->sync = 1;
> @@ -222,16 +238,16 @@ int checker_check (struct checker * c, int path_state)
>  	if (!c)
>  		return PATH_WILD;
>  
> -	c->message[0] = '\0';
> +	c->msgid = CHECKER_MSGID_NONE;
>  	if (c->disable) {
> -		MSG(c, "checker disabled");
> +		c->msgid = CHECKER_MSGID_DISABLED;
>  		return PATH_UNCHECKED;
>  	}
>  	if (!strncmp(c->name, NONE, 4))
>  		return path_state;
>  
>  	if (c->fd < 0) {
> -		MSG(c, "no usable fd");
> +		c->msgid = CHECKER_MSGID_NO_FD;
>  		return PATH_WILD;
>  	}
>  	r = c->check(c);
> @@ -248,25 +264,59 @@ int checker_selected (struct checker * c)
>  	return (c->check) ? 1 : 0;
>  }
>  
> -char * checker_name (struct checker * c)
> +const char *checker_name(const struct checker *c)
>  {
>  	if (!c)
>  		return NULL;
>  	return c->name;
>  }
>  
> -char * checker_message (struct checker * c)
> +static const char *generic_msg[CHECKER_LAST_GENERIC_MSGID] = {
> +	[CHECKER_MSGID_NONE] = "",
> +	[CHECKER_MSGID_DISABLED] = " is disabled",
> +	[CHECKER_MSGID_NO_FD] = " has no usable fd",
> +	[CHECKER_MSGID_INVALID] = " provided invalid message id",
> +	[CHECKER_MSGID_UP] = " reports path is up",
> +	[CHECKER_MSGID_DOWN] = " reports path is down",
> +	[CHECKER_MSGID_GHOST] = " reports path is ghost",
> +};
> +
> +static const char *_checker_message(const struct checker *c)
>  {
> -	if (!c)
> +	int id;
> +
> +	if (!c || c->msgid < 0 ||
> +	    (c->msgid >= CHECKER_LAST_GENERIC_MSGID &&
> +	     c->msgid < CHECKER_FIRST_MSGID))
>  		return NULL;
> -	return c->message;
> +
> +	if (c->msgid < CHECKER_LAST_GENERIC_MSGID)
> +		return generic_msg[c->msgid];
> +
> +	id = c->msgid - CHECKER_FIRST_MSGID;
> +	if (id < c->msgtable_size)
> +		return c->msgtable[id];
> +	return NULL;
> +}
> +
> +const char *checker_message(const struct checker *c)
> +{
> +	static char buf[CHECKER_MSG_LEN];
> +	const char *msg = _checker_message(c);
> +
> +	if (msg == NULL || *msg == '\0')
> +		*buf = '\0';
> +	else
> +		snprintf(buf, sizeof(buf), "%s checker%s",
> +			 c->name, msg);
> +	return buf;
>  }

Since we are returning a pointer to a static buffer, it is theoretically
possible for multiple threads to be updating the buffer at the same
time. In reality, we are always calling checker_message with the vecs
lock held, seeing as we need to guarantee that the path we are checking
doesn't get deleted from pathvec while we are using it. However, it is
easy to imagine future code calling checker_message for a path that
hasn't been added to pathvec, and thus couldn't be deleted by another
thread. In this case we wouldn't need the vecs lock, except to guarantee
that we are the only thread using the static buffer. I don't really have
a problem with keeping the static buffer, but we should add a comment
that checker_message needs to be called with the vecs lock held.
  
>  void checker_clear_message (struct checker *c)
>  {
>  	if (!c)
>  		return;
> -	c->message[0] = '\0';
> +	c->msgid = CHECKER_MSGID_NONE;
>  }
>  
>  void checker_get (char *multipath_dir, struct checker * dst, char * name)
> @@ -288,10 +338,12 @@ void checker_get (char *multipath_dir, struct checker * dst, char * name)
>  	dst->fd = src->fd;
>  	dst->sync = src->sync;
>  	strncpy(dst->name, src->name, CHECKER_NAME_LEN);
> -	strncpy(dst->message, src->message, CHECKER_MSG_LEN);
> +	dst->msgid = src->msgid;

It looks odd setting dst->msgid to src->msgid, when we've never set
src->msgid.  I realize that CHECKER_MSGID_NONE = 0, and we zero out the
checker when we alloc is, so src->msgid is set to CHECKER_MSGID_NONE.
But since we are always referring to msgid as a named constant, it would
be clearer if we explicitly set it to CHECKER_MSGID_NONE in add_checker
(even though it is unnecessary). This is purely cosmetic.

>  	dst->check = src->check;
>  	dst->init = src->init;
>  	dst->free = src->free;
> +	dst->msgtable = src->msgtable;
> +	dst->msgtable_size = src->msgtable_size;
>  	dst->handle = NULL;
>  	src->refcount++;
>  }
> diff --git a/libmultipath/checkers.h b/libmultipath/checkers.h
> index 7b18a1ac..b48f4dc2 100644
> --- a/libmultipath/checkers.h
> +++ b/libmultipath/checkers.h
> @@ -97,6 +97,22 @@ enum path_check_state {
>  #define CHECKER_DEV_LEN 256
>  #define LIB_CHECKER_NAMELEN 256
>  
> +/*
> + * Generic message IDs for use in checkers.
> + */
> +enum {
> +	CHECKER_MSGID_NONE = 0,
> +	CHECKER_MSGID_DISABLED,
> +	CHECKER_MSGID_NO_FD,
> +	CHECKER_MSGID_INVALID,
> +	CHECKER_MSGID_UP,
> +	CHECKER_MSGID_DOWN,
> +	CHECKER_MSGID_GHOST,
> +	CHECKER_LAST_GENERIC_MSGID,

I prefer CHECKER_GENERIC_MSGTABLE_SIZE to CHECKER_LAST_GENERIC_MSGID.
"char *generic_msg[CHECKER_LAST_GENERIC_MSGID]" just looks like it's an
off-by-one error when you see it. Again, this is clearly cosmetic. It
just made me worried when I first saw it.

> +	CHECKER_FIRST_MSGID = 100,	/* lowest msgid for checkers */
> +	CHECKER_MSGTABLE_SIZE = 100,	/* max msg table size for checkers */
> +};
> +
>  struct checker {
>  	struct list_head node;
>  	void *handle;
> @@ -106,7 +122,7 @@ struct checker {
>  	unsigned int timeout;
>  	int disable;
>  	char name[CHECKER_NAME_LEN];
> -	char message[CHECKER_MSG_LEN];       /* comm with callers */
> +	short msgid;		             /* checker-internal extra status */
>  	void * context;                      /* store for persistent data */
>  	void ** mpcontext;                   /* store for persistent data shared
>  						multipath-wide. Use MALLOC if
> @@ -114,10 +130,10 @@ struct checker {
>  	int (*check)(struct checker *);
>  	int (*init)(struct checker *);       /* to allocate the context */
>  	void (*free)(struct checker *);      /* to free the context */
> +	const char**msgtable;
> +	short msgtable_size;
>  };
>  
> -#define MSG(c, fmt, args...) snprintf((c)->message, CHECKER_MSG_LEN, fmt, ##args);
> -
>  char * checker_state_name (int);
>  int init_checkers (char *);
>  void cleanup_checkers (void);
> @@ -134,14 +150,25 @@ void checker_enable (struct checker *);
>  void checker_disable (struct checker *);
>  int checker_check (struct checker *, int);
>  int checker_selected (struct checker *);
> -char * checker_name (struct checker *);
> -char * checker_message (struct checker *);
> +const char *checker_name (const struct checker *);
> +const char *checker_message (const struct checker *);
>  void checker_clear_message (struct checker *c);
>  void checker_get (char *, struct checker *, char *);
>  
> -/* Functions exported by path checker dynamic libraries (.so) */
> +/* Prototypes for symbols exported by path checker dynamic libraries (.so) */
>  int libcheck_check(struct checker *);
>  int libcheck_init(struct checker *);
>  void libcheck_free(struct checker *);
> +/*
> + * msgid => message map.
> + *
> + * It only needs to be provided if the checker defines specific
> + * message IDs.
> + * Message IDs available to checkers start at CHECKER_FIRST_MSG.
> + * The msgtable array is 0-based, i.e. msgtable[0] is the message
> + * for msgid == __CHECKER_FIRST_MSG.
> + * The table ends with a NULL element.
> + */
> +extern const char *libcheck_msgtable[];
>  
>  #endif /* _CHECKERS_H */
> -- 
> 2.19.0