Subject: Re: [PATCH v2 14/16] fsmonitor: support case-insensitive events

From: Junio C Hamano <[email-masked]>
Date: Fri, 23 Feb 2024 10:14:48 -0800

"Jeff Hostetler via GitGitGadget" <[email-masked]> writes:

> +/*
> + * Use the name-hash to do a case-insensitive cache-entry lookup with
> + * the pathname and invalidate the cache-entry.
> + *
> + * Returns the number of cache-entries that we invalidated.
> + */
> +static size_t handle_using_name_hash_icase(
> +	struct index_state *istate, const char *name)
> +{
> +	struct cache_entry *ce = NULL;
> +
> +	ce = index_file_exists(istate, name, strlen(name), 1);
> +	if (!ce)
> +		return 0;
> +
> +	/*
> +	 * A case-insensitive search in the name-hash using the
> +	 * observed pathname found a cache-entry, so the observed path
> +	 * is case-incorrect.  Invalidate the cache-entry and use the
> +	 * correct spelling from the cache-entry to invalidate the
> +	 * untracked-cache.  Since we now have sparse-directories in
> +	 * the index, the observed pathname may represent a regular
> +	 * file or a sparse-index directory.
> +	 *
> +	 * Note that we should not have seen FSEvents for a
> +	 * sparse-index directory, but we handle it just in case.
> +	 *
> +	 * Either way, we know that there are not any cache-entries for
> +	 * children inside the cone of the directory, so we don't need to
> +	 * do the usual scan.
> +	 */
> +	trace_printf_key(&trace_fsmonitor,
> +			 "fsmonitor_refresh_callback MAP: '%s' '%s'",
> +			 name, ce->name);
> +
> +	untracked_cache_invalidate_trimmed_path(istate, ce->name, 0);
> +	ce->ce_flags &= ~CE_FSMONITOR_VALID;
> +	return 1;
> +}

You first ask the name-hash to turn the incoming "name" into the
case variant that we know about, i.e. ce->name, and use that to
access the untracked cache.  Clever and makes sense.  But if we have
ce->name, doesn't it mean the name is tracked?  Do we find anything
useful to do in the untracked cache invalidation codepath in that
case?

An FSmonitor event with case-incorrect pathname for a directory may
not be this trivial, I presume, and I expect that is what the
remainder of this patch is about.

> +
> +/*
> + * Use the dir-name-hash to find the correct-case spelling of the
> + * directory.  Use the canonical spelling to invalidate all of the
> + * cache-entries within the matching cone.
> + *
> + * Returns the number of cache-entries that we invalidated.
> + */
> +static size_t handle_using_dir_name_hash_icase(
> +	struct index_state *istate, const char *name)

It is a bit unfortunate that here on the name-hash side we contrast
the two helper function variants as "dir-name" vs "name", while the
original handle_path side use "without_slash" vs "with_slash".

If I understand correctly, it is not like there are two distinct
hashes, "name-hash" vs "dir-name-hash".  Both of these helpers use
the same "name-hash" mechanism, and this function differs from the
previous one in that it is about a directory, which is why it has
"dir" in its name.  I wonder if we renamed the other one with
"nondir" in its name, and the other without_slash and with_slash
pair to match, e.g., handle_nondir_path() vs handle_dir_path(), or
something like that, the resulting names for these four functions
become easier to contrast and understand?

> +{
> +	struct strbuf canonical_path = STRBUF_INIT;
> +	int pos;
> +	size_t len = strlen(name);
> +	size_t nr_in_cone;
> +
> +	if (name[len - 1] == '/')
> +		len--;
> +
> +	if (!index_dir_find(istate, name, len, &canonical_path))
> +		return 0; /* name is untracked */
> +
> +	if (!memcmp(name, canonical_path.buf, canonical_path.len)) {
> +		strbuf_release(&canonical_path);
> +		/*
> +		 * NEEDSWORK: Our caller already tried an exact match
> +		 * and failed to find one.  They called us to do an
> +		 * ICASE match, so we should never get an exact match,
> +		 * so we could promote this to a BUG() here if we
> +		 * wanted to.  It doesn't hurt anything to just return
> +		 * 0 and go on becaus we should never get here.  Or we
> +		 * could just get rid of the memcmp() and this "if"
> +		 * clause completely.
> +		 */
> +		return 0; /* should not happen */
> +	}

"becaus" -> "because".

If we should never get here, having BUG("we should never get here")
would not hurt anything, either.  On the other hand, silently
returning 0 will hide the bug under the carpet, and I am not sure it
is fair to call it "doesn't hurt anything".

> +
> +	trace_printf_key(&trace_fsmonitor,
> +			 "fsmonitor_refresh_callback MAP: '%s' '%s'",
> +			 name, canonical_path.buf);
> +
> +	/*
> +	 * The dir-name-hash only tells us the corrected spelling of
> +	 * the prefix.  We have to use this canonical path to do a
> +	 * lookup in the cache-entry array so that we repeat the
> +	 * original search using the case-corrected spelling.
> +	 */
> +	strbuf_addch(&canonical_path, '/');
> +	pos = index_name_pos(istate, canonical_path.buf,
> +			     canonical_path.len);
> +	nr_in_cone = handle_path_with_trailing_slash(
> +		istate, canonical_path.buf, pos);
> +	strbuf_release(&canonical_path);
> +	return nr_in_cone;
> +}

Nice.  Do we need to give this corrected name to help untracked
cache invalidation from the caller that called us?

> @@ -319,6 +416,19 @@ static void fsmonitor_refresh_callback(struct index_state *istate, char *name)
>  	else
>  		nr_in_cone = handle_path_without_trailing_slash(istate, name, pos);
>  
> +	/*
> +	 * If we did not find an exact match for this pathname or any
> +	 * cache-entries with this directory prefix and we're on a
> +	 * case-insensitive file system, try again using the name-hash
> +	 * and dir-name-hash.
> +	 */
> +	if (!nr_in_cone && ignore_case) {
> +		nr_in_cone = handle_using_name_hash_icase(istate, name);
> +		if (!nr_in_cone)
> +			nr_in_cone = handle_using_dir_name_hash_icase(
> +				istate, name);
> +	}

It might be interesting to learn how often we go through these
"fallback" code paths by tracing.  Maybe it will become too noisy?
I dunno.

>  	if (nr_in_cone)
>  		trace_printf_key(&trace_fsmonitor,
>  				 "fsmonitor_refresh_callback CNT: %d",