Subject: Re: [PATCH 08/12] fsmonitor: support case-insensitive directory events

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 15 Feb 2024 10:32:43 +0100

On Tue, Feb 13, 2024 at 08:52:17PM +0000, Jeff Hostetler via GitGitGadget wrote:
> From: Jeff Hostetler <[email-masked]>
> 
> Teach fsmonitor_refresh_callback() to handle case-insensitive
> lookups if case-sensitive lookups fail on case-insensitive systems.
> This can cause 'git status' to report stale status for files if there
> are case issues/errors in the worktree.
> 
> The FSMonitor daemon sends FSEvents using the observed spelling
> of each pathname.  On case-insensitive file systems this may be
> different than the expected case spelling.
> 
> The existing code uses index_name_pos() to find the cache-entry for
> the pathname in the FSEvent and clear the CE_FSMONITOR_VALID bit so
> that the worktree scan/index refresh will revisit and revalidate the
> path.
> 
> On a case-insensitive file system, the exact match lookup may fail
> to find the associated cache-entry. This causes status to think that
> the cached CE flags are correct and skip over the file.
> 
> Update the handling of directory-style FSEvents (ones containing a
> path with a trailing slash) to optionally use the name-hash if the
> case-correct search does not find a match.
> 
> (The FSMonitor daemon can send directory FSEvents if the OS provides
> that information.)
> 
> Signed-off-by: Jeff Hostetler <[email-masked]>
> ---
>  fsmonitor.c | 122 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 120 insertions(+), 2 deletions(-)
> 
> diff --git a/fsmonitor.c b/fsmonitor.c
> index 14585b6c516..73e6ac82af7 100644
> --- a/fsmonitor.c
> +++ b/fsmonitor.c
> @@ -5,6 +5,7 @@
>  #include "ewah/ewok.h"
>  #include "fsmonitor.h"
>  #include "fsmonitor-ipc.h"
> +#include "name-hash.h"
>  #include "run-command.h"
>  #include "strbuf.h"
>  #include "trace2.h"
> @@ -183,6 +184,9 @@ static int query_fsmonitor_hook(struct repository *r,
>  	return result;
>  }
>  
> +static int fsmonitor_refresh_callback_slash(
> +	struct index_state *istate, const char *name, int len, int pos);
> +
>  /*
>   * Invalidate the untracked cache for the given pathname.  Copy the
>   * buffer to a proper null-terminated string (since the untracked
> @@ -205,6 +209,84 @@ static void my_invalidate_untracked_cache(
>  	strbuf_release(&work_path);
>  }
>  
> +/*
> + * Use the name-hash to lookup the pathname.
> + *
> + * Returns the number of cache-entries that we invalidated.
> + */

The function not only looks up the path name, but also invalidates the
corresponding cache entry. You imply this with the second sentence, but
this could be a bit more explicit.

> +static int my_callback_name_hash(
> +	struct index_state *istate, const char *name, int len)

I find the naming conventions here to be weird with the `my_` prefix.

> +{
> +	struct cache_entry *ce = NULL;
> +
> +	ce = index_file_exists(istate, name, len, 1);
> +	if (!ce)
> +		return 0;

Okay, `index_file_exists()` is called with `icase == 1` here. But is
that the correct thing to do on case-sensitive platforms? I would have
expected us to honor `core.ignoreCase` here.

Turns out, we only end up calling this function when `ignore_case` is
set, so we already do. I think this can be clarified both by giving the
function a better name and by documenting this in the comment. Also,
neither of this or the next function really are callbacks -- they only
happen to be called by a callback function.

I'd think something like `lookup_and_invalidate_path_icase()` and
`lookup_and_invalidate_dir_icase()` could help to clarify intent.

> +	/*
> +	 * The index contains a case-insensitive match for the pathname.
> +	 * This could either be a regular file or a sparse-index directory.
> +	 *
> +	 * 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);
> +
> +	my_invalidate_untracked_cache(istate, ce->name, ce->ce_namelen);
> +
> +	ce->ce_flags &= ~CE_FSMONITOR_VALID;
> +	return 1;
> +}
> +
> +/*
> + * Use the directory 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.
> + *
> + * The pathname MUST NOT have a trailing slash.
> + *
> + * Returns the number of cache-entries that we invalidated.
> + */
> +static int my_callback_dir_name_hash(
> +	struct index_state *istate, const char *name, int len)
> +{
> +	struct strbuf canonical_path = STRBUF_INIT;
> +	int pos;
> +	int nr_in_cone;
> +
> +	if (!index_dir_exists2(istate, name, len, &canonical_path))
> +		return 0; /* name is untracked */
> +	if (!memcmp(name, canonical_path.buf, len)) {
> +		strbuf_release(&canonical_path);
> +		return 0; /* should not happen */
> +	}

So in other words, this function should only be called when we know that
casing differs, and thus the passed-in name and the canonical name
should never be the same? If this case shouldn't ever happen, shouldn't
we report this as an error or use `BUG()` instead of silently ignoring
this mismatch of expectations?

Patrick

> +	trace_printf_key(&trace_fsmonitor,
> +			 "fsmonitor_refresh_callback map '%s' '%s'",
> +			 name, canonical_path.buf);
> +
> +	/*
> +	 * The directory 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
> +	 * 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 = fsmonitor_refresh_callback_slash(
> +		istate, canonical_path.buf, canonical_path.len, pos);
> +	strbuf_release(&canonical_path);
> +	return nr_in_cone;
> +}
> +
>  static void fsmonitor_refresh_callback_unqualified(
>  	struct index_state *istate, const char *name, int len, int pos)
>  {
> @@ -269,7 +351,10 @@ static void fsmonitor_refresh_callback_unqualified(
>   *
>   * Return the number of cache-entries that we invalidated.  We will
>   * use this later to determine if we need to attempt a second
> - * case-insensitive search.
> + * case-insensitive search.  That is, if a observed-case search yields
> + * any results, we assume the prefix is case-correct.  If there are
> + * no matches, we still don't know if the observed path is simply
> + * untracked or case-incorrect.
>   */
>  static int fsmonitor_refresh_callback_slash(
>  	struct index_state *istate, const char *name, int len, int pos)
> @@ -293,17 +378,50 @@ static int fsmonitor_refresh_callback_slash(
>  	return nr_in_cone;
>  }
>  
> +/*
> + * On a case-insensitive FS, use the name-hash and directory name-hash
> + * to map the case of the observed path to the canonical case expected
> + * by the index.
> + *
> + * The given pathname includes the trailing slash.
> + *
> + * Return the number of cache-entries that we invalidated.
> + */
> +static int fsmonitor_refresh_callback_slash_icase(
> +	struct index_state *istate, const char *name, int len)
> +{
> +	int nr_in_cone;
> +
> +	/*
> +	 * Look for a case-incorrect sparse-index directory.
> +	 */
> +	nr_in_cone = my_callback_name_hash(istate, name, len);
> +	if (nr_in_cone)
> +		return nr_in_cone;
> +
> +	/*
> +	 * (len-1) because we do not include the trailing slash in the
> +	 * pathname.
> +	 */
> +	nr_in_cone = my_callback_dir_name_hash(istate, name, len-1);
> +	return nr_in_cone;
> +}
> +
>  static void fsmonitor_refresh_callback(struct index_state *istate, char *name)
>  {
>  	int len = strlen(name);
>  	int pos = index_name_pos(istate, name, len);
> +	int nr_in_cone;
> +
>  
>  	trace_printf_key(&trace_fsmonitor,
>  			 "fsmonitor_refresh_callback '%s' (pos %d)",
>  			 name, pos);
>  
>  	if (name[len - 1] == '/') {
> -		fsmonitor_refresh_callback_slash(istate, name, len, pos);
> +		nr_in_cone = fsmonitor_refresh_callback_slash(istate, name, len, pos);
> +		if (ignore_case && !nr_in_cone)
> +			fsmonitor_refresh_callback_slash_icase(istate, name, len);
>  	} else {
>  		fsmonitor_refresh_callback_unqualified(istate, name, len, pos);
>  	}
> -- 
> gitgitgadget
> 
>