Subject: Re: [PATCH/WIP v2 02/14] read-cache: new extension to mark what file is watched

From: Thomas Gummerer <[email-masked]>
Date: Fri, 17 Jan 2014 12:19:05 +0100

Nguyễn Thái Ngọc Duy <[email-masked]> writes:

> If an entry is "watched", git lets an external program decide if the
> entry is modified or not. It's more like --assume-unchanged, but
> designed to be controlled by machine.
>
> We are running out of on-disk ce_flags, so instead of extending
> on-disk entry format again, "watched" flags are in-core only and
> stored as extension instead.

As you said yourself in http://thread.gmane.org/gmane.comp.version-control.git/240339/focus=240385
this is not quite true.  As for your explanation there,

> Anyway using extended flags means 2 extra bytes per entry for
> almost every entry in this case (and for index v5 it means redoing
> crc32 for almost every entry too when the bit is updated) so it may
> still be a good idea to keep the new flag separate.

I don't think adding 2 extra bytes would be too bad, since we are
already using 62 bytes plus the bytes for the filename for each index
entry, so it would be a less than 3% increase in the index file size.
(And the extended flags may be used anyway in some cases)

As for index-v5 (if that's ever going to happen), it depends mostly on
how often the CE_WATCHED is going to be updated, to decide whether it
makes sense to store this as extension.

That said, I don't care too deeply if it's stored one way or another,
but I think it would be good to update the commit message with a better
rationale for the choice.

> Signed-off-by: Nguyễn Thái Ngọc Duy <[email-masked]>
> ---
>  cache.h      |  2 ++
>  read-cache.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 42 insertions(+), 1 deletion(-)
>
> diff --git a/cache.h b/cache.h
> index a09d622..069dce7 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -168,6 +168,8 @@ struct cache_entry {
>  /* used to temporarily mark paths matched by pathspecs */
>  #define CE_MATCHED           (1 << 26)
>
> +#define CE_WATCHED           (1 << 27)
> +
>  /*
>   * Extended on-disk flags
>   */
> diff --git a/read-cache.c b/read-cache.c
> index fe1d153..6f21e3f 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -33,6 +33,7 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int reall
>  #define CACHE_EXT(s) ( (s[0]<<24)|(s[1]<<16)|(s[2]<<8)|(s[3]) )
>  #define CACHE_EXT_TREE 0x54524545	/* "TREE" */
>  #define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
> +#define CACHE_EXT_WATCH 0x57415443	  /* "WATC" */
>
>  struct index_state the_index;
>
> @@ -1293,6 +1294,19 @@ static int verify_hdr(struct cache_header *hdr,
>  	return 0;
>  }
>
> +static void read_watch_extension(struct index_state *istate, uint8_t *data,
> +				 unsigned long sz)
> +{
> +	int i;
> +	if ((istate->cache_nr + 7) / 8 != sz) {
> +		error("invalid 'WATC' extension");
> +		return;
> +	}
> +	for (i = 0; i < istate->cache_nr; i++)
> +		if (data[i / 8] & (1 << (i % 8)))
> +			istate->cache[i]->ce_flags |= CE_WATCHED;
> +}
> +
>  static int read_index_extension(struct index_state *istate,
>  				const char *ext, void *data, unsigned long sz)
>  {
> @@ -1303,6 +1317,9 @@ static int read_index_extension(struct index_state *istate,
>  	case CACHE_EXT_RESOLVE_UNDO:
>  		istate->resolve_undo = resolve_undo_read(data, sz);
>  		break;
> +	case CACHE_EXT_WATCH:
> +		read_watch_extension(istate, data, sz);
> +		break;
>  	default:
>  		if (*ext < 'A' || 'Z' < *ext)
>  			return error("index uses %.4s extension, which we do not understand",
> @@ -1781,7 +1798,7 @@ int write_index(struct index_state *istate, int newfd)
>  {
>  	git_SHA_CTX c;
>  	struct cache_header hdr;
> -	int i, err, removed, extended, hdr_version;
> +	int i, err, removed, extended, hdr_version, has_watches = 0;
>  	struct cache_entry **cache = istate->cache;
>  	int entries = istate->cache_nr;
>  	struct stat st;
> @@ -1790,6 +1807,8 @@ int write_index(struct index_state *istate, int newfd)
>  	for (i = removed = extended = 0; i < entries; i++) {
>  		if (cache[i]->ce_flags & CE_REMOVE)
>  			removed++;
> +		else if (cache[i]->ce_flags & CE_WATCHED)
> +			has_watches++;
>
>  		/* reduce extended entries if possible */
>  		cache[i]->ce_flags &= ~CE_EXTENDED;
> @@ -1861,6 +1880,26 @@ int write_index(struct index_state *istate, int newfd)
>  		if (err)
>  			return -1;
>  	}
> +	if (has_watches) {
> +		int id, sz = (entries - removed + 7) / 8;
> +		uint8_t *data = xmalloc(sz);
> +		memset(data, 0, sz);
> +		for (i = 0, id = 0; i < entries && has_watches; i++) {
> +			struct cache_entry *ce = cache[i];
> +			if (ce->ce_flags & CE_REMOVE)
> +				continue;
> +			if (ce->ce_flags & CE_WATCHED) {
> +				data[id / 8] |= 1 << (id % 8);
> +				has_watches--;
> +			}
> +			id++;
> +		}
> +		err = write_index_ext_header(&c, newfd, CACHE_EXT_WATCH, sz) < 0
> +			|| ce_write(&c, newfd, data, sz) < 0;
> +		free(data);
> +		if (err)
> +			return -1;
> +	}
>
>  	if (ce_flush(&c, newfd) || fstat(newfd, &st))
>  		return -1;
> --
> 1.8.5.1.208.g05b12ea
>
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--
Thomas