Re: [PATCH 2/2] read-cache: use time_t instead of unsigned long

From: Junio C Hamano <gitster@pobox.com>
Date: Nov 13, 2018, 09:43 UTC
Message-ID: <xmqq36s5wbvn.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20181113074017.17292-1-carenas@gmail.com>

Carlo Marcelo Arenas Belón  <carenas@gmail.com> writes:

Show 22 quoted lines

> There are still some more possible improvements around this code but
> they are orthogonal to this change :
>
> * migrate to approxidate_careful or parse_expiry_date
> * maybe make sure only approxidate are used for expiration
>
> Changes in v2:
> * improved commit message as suggested by Eric
> * failsafe against time_t truncation as suggested by Junio
>
> -- >8 --
> Subject: [PATCH v2 2/2] read-cache: use time specific types instead of
>  unsigned long
>
> b968372279 ("read-cache: unlink old sharedindex files", 2017-03-06)
> introduced get_shared_index_expire_date using unsigned long to track
> the modification times of a shared index.
>
> dddbad728c ("timestamp_t: a new data type for timestamps", 2017-04-26)
> shows why that might be problematic so move to timestamp_t/time_t.
>
> if time_t can't represent a valid time keep the indexes for failsafe

I am not sure if this "failsafe" is a good idea in the first place. FWIW, I had a total opposite in mind when I suggested it.

The user gave you a timestamp_t value, telling you that anything before that time is too old to matter and can be discarded.

And date_overflows() helper tells you that that the given expiry date is more in the future than _any_ possible timestamp expressed in time_t.

So the result of running stat() on the shared index file _will_ have a timestamp that is older than the specified expiry.

Which means that the user essentially is telling you that any shared index that is not referenced can immediately be expired, similar to saying --expire=now, no?

I kind of sort of would understand if we (1) read the configured expiry, (2) checked it with date_overflows(), and (3) told the user that it is an impossible condition, i.e. an error, to use such a large timestamp far in the future. Then the user has a chance to update the configuration to a more reasonable time---even the most aggressive one does not need to specify anything more in the future than "now", and that will not "date_overflows()" for 20 years or so.

Show 38 quoted lines

> Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
> ---
>  read-cache.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/read-cache.c b/read-cache.c
> index 7b1354d759..7d322f11c8 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -2625,9 +2625,9 @@ static int write_split_index(struct index_state *istate,
>  
>  static const char *shared_index_expire = "2.weeks.ago";
>  
> -static unsigned long get_shared_index_expire_date(void)
> +static timestamp_t get_shared_index_expire_date(void)
>  {
> -	static unsigned long shared_index_expire_date;
> +	static timestamp_t shared_index_expire_date;
>  	static int shared_index_expire_date_prepared;
>  
>  	if (!shared_index_expire_date_prepared) {
> @@ -2643,12 +2643,12 @@ static unsigned long get_shared_index_expire_date(void)
>  static int should_delete_shared_index(const char *shared_index_path)
>  {
>  	struct stat st;
> -	unsigned long expiration;
> +	time_t expiration;
> +	timestamp_t t = get_shared_index_expire_date();
>  
> -	/* Check timestamp */
> -	expiration = get_shared_index_expire_date();
> -	if (!expiration)
> +	if (!t || date_overflows(t))
>  		return 0;
> +	expiration = t;
>  	if (stat(shared_index_path, &st))
>  		return error_errno(_("could not stat '%s'"), shared_index_path);
>  	if (st.st_mtime > expiration)