Re: [PATCH 0/1] fsmonitor: skip sanity check if the index is split

> Thanks.  Here is what I came up with to tie the loose ends of this
> thread.
> 
> -- >8 --
> From: Junio C Hamano <gitster@pobox.com>
> Subject: [PATCH] fsmonitor: do not compare bitmap size with size of split index
> 
> 3444ec2e ("fsmonitor: don't fill bitmap with entries to be removed",
> 2019-10-11) added a handful of sanity checks that make sure that a
> bit position in fsmonitor bitmap does not go beyond the end of the
> index.  As each bit in the bitmap corresponds to a path in the
> index, this is the right check most of the time.
> 
> Except for the case when we are in the split-index mode and looking
> at a delta index that is to be overlayed on the base index but
> before the base index has actually been merged in, namely in read_
> and write_fsmonitor_extension().  In these codepaths, the entries in
> the split/delta index is typically a small subset of the entire set
> of paths (otherwise why would we be using split-index?), so the
> bitmap used by the fsmonitor is almost always larger than the number
> of entries in the partial index, and the incorrect comparison would
> trigger the BUG().
> 
> Reported-by: Utsav Shah <ukshah2@illinois.edu>
> Helped-by: Kevin Willford <Kevin.Willford@microsoft.com>
> Helped-by: William Baker <William.Baker@microsoft.com>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>  fsmonitor.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fsmonitor.c b/fsmonitor.c
> index 1f4aa1b150..0477500b39 100644
> --- a/fsmonitor.c
> +++ b/fsmonitor.c
> @@ -55,7 +55,8 @@ int read_fsmonitor_extension(struct index_state *istate, const void *data,
>  	}
>  	istate->fsmonitor_dirty = fsmonitor_dirty;
>  
> -	if (istate->fsmonitor_dirty->bit_size > istate->cache_nr)
> +	if (!istate->split_index &&
> +	    istate->fsmonitor_dirty->bit_size > istate->cache_nr)
>  		BUG("fsmonitor_dirty has more entries than the index (%"PRIuMAX" > %u)",
>  		    (uintmax_t)istate->fsmonitor_dirty->bit_size, istate->cache_nr);
>  
> @@ -83,7 +84,8 @@ void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
>  	uint32_t ewah_size = 0;
>  	int fixup = 0;
>  
> -	if (istate->fsmonitor_dirty->bit_size > istate->cache_nr)
> +	if (!istate->split_index &&
> +	    istate->fsmonitor_dirty->bit_size > istate->cache_nr)
>  		BUG("fsmonitor_dirty has more entries than the index (%"PRIuMAX" > %u)",
>  		    (uintmax_t)istate->fsmonitor_dirty->bit_size, istate->cache_nr);
>  
>