[PATCH 3/3] Write index when populating empty untracked cache

["Tao Klerks via GitGitGadget" <gitgitgadget@gmail.com>]

From: Tao Klerks <tao@klerks.biz>

It is expected that an empty/unpopulated untracked
cache structure can be written to the index - by update-
index, or by a "git status" call that sees the untracked cache
should be enabled, but is running with options that make
it non-applicable in that run.

Currently, if that happens, then subsequent "git status"
calls end up populating the untracked cache, but not
writing the index (not saving their work) - so the
performance outcome is almost identical to the cache
being altogether disabled.

This continues until the index gets written with the cache
populated, for some *other* reason.

In this change, we detect the "existing cache is empty
and it looks like we are using it" condition, and queues
an index write when this happens.

This change depends on previous fixes to t7519 for the
"ignore .git changes when invalidating UNTR" test case to
pass - before this fix, the test never actually did anything
as it was not set up correctly.

Signed-off-by: Tao Klerks <tao@klerks.biz>
---
 dir.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/dir.c b/dir.c
index ebe5ec046e0..a326e40e1c1 100644
--- a/dir.c
+++ b/dir.c
@@ -2703,7 +2703,8 @@ void remove_untracked_cache(struct index_state *istate)
 
 static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *dir,
 						      int base_len,
-						      const struct pathspec *pathspec)
+						      const struct pathspec *pathspec,
+						      struct index_state *istate)
 {
 	struct untracked_cache_dir *root;
 	static int untracked_cache_disabled = -1;
@@ -2767,8 +2768,15 @@ static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *d
 		return NULL;
 	}
 
-	if (!dir->untracked->root)
+	if (!dir->untracked->root) {
 		FLEX_ALLOC_STR(dir->untracked->root, name, "");
+		/*
+		 * If we've had to initialize the root, then what we had was an
+		 * empty uninitialized untracked cache structure. We will be
+		 * populating it now, so we should trigger an index write.
+		 */
+		istate->cache_changed |= UNTRACKED_CHANGED;
+	}
 
 	/* Validate $GIT_DIR/info/exclude and core.excludesfile */
 	root = dir->untracked->root;
@@ -2838,7 +2846,7 @@ int read_directory(struct dir_struct *dir, struct index_state *istate,
 		return dir->nr;
 	}
 
-	untracked = validate_untracked_cache(dir, len, pathspec);
+	untracked = validate_untracked_cache(dir, len, pathspec, istate);
 	if (!untracked)
 		/*
 		 * make sure untracked cache code path is disabled,
-- 
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