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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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.

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);

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