Re: [PATCH v2 7/7] trace2: make SIDs more unique

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Fri, Mar 29 2019, Jeff Hostetler via GitGitGadget wrote:

> From: Jeff Hostetler <jeffhost@microsoft.com>
>
> Update SID component construction to use the current UTC datetime
> and a portion of the SHA1 of the hostname.
>
> Use an simplified date/time format to make it easier to use the
> SID component as a logfile filename.
> [...]
> +static void tr2_sid_append_my_sid_component(void)
> +{
> +	const struct git_hash_algo *algo = &hash_algos[GIT_HASH_SHA1];
> +	struct tr2_tbuf tb_now;
> +	git_hash_ctx ctx;
> +	unsigned char hash[GIT_MAX_RAWSZ + 1];
> +	char hex[GIT_MAX_HEXSZ + 1];
> +	char hostname[HOST_NAME_MAX + 1];
> +
> +	tr2_tbuf_utc_datetime_for_filename(&tb_now);
> +	strbuf_addstr(&tr2sid_buf, tb_now.buf);
> +	strbuf_addch(&tr2sid_buf, '-');
> +
> +	if (xgethostname(hostname, sizeof(hostname)))
> +		xsnprintf(hostname, sizeof(hostname), "localhost");
> +
> +	algo->init_fn(&ctx);
> +	algo->update_fn(&ctx, hostname, strlen(hostname));
> +	algo->final_fn(hash, &ctx);
> +	hash_to_hex_algop_r(hex, hash, algo);
> +	strbuf_add(&tr2sid_buf, hex, 8);
> +
> +	strbuf_addch(&tr2sid_buf, '-');
> +	strbuf_addf(&tr2sid_buf, "%06"PRIuMAX, (uintmax_t)getpid());
> +}
> +

Thanks for turning my shitty half-formed idea into a patch :)

I wrote this on top to bikeshed this a bit further, wonder what you
think:
https://github.com/gitgitgadget/git/compare/pr-169/jeffhostetler/core-tr2-startup-and-sysenv-v2...avar:pr-169/jeffhostetler/core-tr2-startup-and-sysenv-v2

So e.g.:

    Before: 20190329-220413-446441-c2f5b994-018702
    After:  20190329T220431.244562Z-Hc2f5b994-P19812F

I.e:

 * Using <date>T<time> as is ISO 8601 convention/easier to read

 * <dateime>.<microseconds>Z, so seperating with "." to indicate it's
   the same value + add "Z" for "it's UTC". I'm least sure about the
   ".". Is that going to cause issues on Windows these days (the rest
   being the "extension"...).

 * I changed the hostname discovery so if gethostbyname() fails we'll
   print "-H00000000-" for that part, instead of "-H<first 8 chars of
   the sha1 for 'localhost'>-". Also prefix with "H" for "Host".

 * Wrap pids to 0xffff, prefix with "P" (Pid)" and trail with either "F"
   = Full or "W" = Wrapped (not the real PID).

 * I didn't add "T<datetime>" like "H" and "P" for the rest, since it's
   obvious what sort of value it is.

Maybe this is going a bit overboard, but I think it's easier to read at
a glance for humans, and since it's meant to be opaque to machines
anyway and the length is simliar enough not to matter I figure it's
worth it.