Re: [PATCH 1/5] eoie: default to not writing EOIE section

> Since 3b1d9e04 (eoie: add End of Index Entry (EOIE) extension,
> 2018-10-10) Git defaults to writing the new EOIE section when writing
> out an index file.  Usually that is a good thing because it improves
> threaded performance, but when a Git repository is shared with older
> versions of Git, it produces a confusing warning:
> 
>    $ git status
>    ignoring EOIE extension
>    HEAD detached at 371ed0defa
>    nothing to commit, working tree clean
> 
> Let's introduce the new index extension more gently.  First we'll roll
> out the new version of Git that understands it, and then once
> sufficiently many users are using such a version, we can flip the
> default to writing it by default.
> 
> Introduce a '[index] recordEndOfIndexEntries' configuration variable
> to allow interested users to benefit from this index extension early.
> 
> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
> ---
> Rebased.  No other change from v1.
> 
> As Jonathan pointed out, it would be nice to have tests here.  Ben,
> any advice for how I could write some in a followup change?  E.g. does
> Derrick Stolee's tracing based testing trick apply here?
> 
>   Documentation/config/index.txt |  7 +++++++
>   read-cache.c                   | 11 ++++++++++-
>   t/t1700-split-index.sh         | 11 +++++++----
>   3 files changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/config/index.txt b/Documentation/config/index.txt
> index 4b94b6bedc..8e138aba7a 100644
> --- a/Documentation/config/index.txt
> +++ b/Documentation/config/index.txt
> @@ -1,3 +1,10 @@
> +index.recordEndOfIndexEntries::
> +	Specifies whether the index file should include an "End Of Index
> +	Entry" section. This reduces index load time on multiprocessor
> +	machines but produces a message "ignoring EOIE extension" when
> +	reading the index using Git versions before 2.20. Defaults to
> +	'false'.
> +
>   index.threads::
>   	Specifies the number of threads to spawn when loading the index.
>   	This is meant to reduce index load time on multiprocessor machines.
> diff --git a/read-cache.c b/read-cache.c
> index 4ca81286c0..1e9c772603 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -2689,6 +2689,15 @@ void update_index_if_able(struct index_state *istate, struct lock_file *lockfile
>   		rollback_lock_file(lockfile);
>   }
>   
> +static int record_eoie(void)
> +{
> +	int val;

> +
> +	if (!git_config_get_bool("index.recordendofindexentries", &val))
> +		return val;
> +	return 0;
> +}
> +
>   /*
>    * On success, `tempfile` is closed. If it is the temporary file
>    * of a `struct lock_file`, we will therefore effectively perform
> @@ -2936,7 +2945,7 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
>   	 * read.  Write it out regardless of the strip_extensions parameter as we need it
>   	 * when loading the shared index.
>   	 */
> -	if (offset) {
> +	if (offset && record_eoie()) {
>   		struct strbuf sb = STRBUF_INIT;
>   
>   		write_eoie_extension(&sb, &eoie_c, offset);
> diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
> index 2ac47aa0e4..0cbac64e28 100755
> --- a/t/t1700-split-index.sh
> +++ b/t/t1700-split-index.sh
> @@ -25,14 +25,17 @@ test_expect_success 'enable split index' '
>   	git update-index --split-index &&
>   	test-tool dump-split-index .git/index >actual &&
>   	indexversion=$(test-tool index-version <.git/index) &&
> +
> +	# NEEDSWORK: Stop hard-coding checksums.
>   	if test "$indexversion" = "4"
>   	then
> -		own=3527df833c6c100d3d1d921a9a782d62a8be4b58
> -		base=746f7ab2ed44fb839efdfbffcf399d0b113fb4cb
> +		own=432ef4b63f32193984f339431fd50ca796493569
> +		base=508851a7f0dfa8691e9f69c7f055865389012491
>   	else
> -		own=5e9b60117ece18da410ddecc8b8d43766a0e4204
> -		base=4370042739b31cd17a5c5cd6043a77c9a00df113
> +		own=8299b0bcd1ac364e5f1d7768efb62fa2da79a339
> +		base=39d890139ee5356c7ef572216cebcd27aa41f9df
>   	fi &&
> +
>   	cat >expect <<-EOF &&
>   	own $own
>   	base $base
>