entry: flush fscache after creating directories and writing files (#6250)

## Problem

`git checkout <tree> -- <pathspec>` with `checkout.workers > 1` and
`core.fscache=true` fails when restoring files into directories that do
not yet exist on disk. Two failure modes:

1. `fatal: cannot create directory at '...': Directory not empty` (exit
128)
2. `error: unable to stat just-written file '...'` (exit 255)

100% reproducible when two or more files share a not-yet-created parent
directory.

## Root Cause

The Windows fscache caches directory listings that become stale when
`create_directories()` creates new parent directories via `mkdir()` or
when `write_pc_item()` writes new files. With `workers=1`,
`write_entry()` calls `flush_fscache()` after each file, keeping the
cache in sync. With `workers>1`, `enqueue_checkout()` defers the write
(and the flush), leaving the cache stale for subsequent entries.

## Fix

Add `flush_fscache()` calls:
- In `create_directories()` after each successful `mkdir()`, so
`has_dirs_only_path()` sees the new directory
- In `write_pc_item()` before `lstat()` of the just-written file

On non-Windows platforms `flush_fscache()` is a no-op.

## Test

Adds a regression test to `t2080-parallel-checkout-basics.sh` (`MINGW`
prereq) that deterministically reproduces the bug: two files sharing a
nested parent directory, deleted in a second commit, then restored via
`git checkout <tree> -- <pathspec>` with `workers=2`.

entry.c

@@ -49,10 +49,23 @@ static void create_directories(const char *path, int path_len,
 		 */
 		if (mkdir(buf, 0777)) {
 			if (errno == EEXIST && state->force &&
-			    !unlink_or_warn(buf) && !mkdir(buf, 0777))
+			    !unlink_or_warn(buf) && !mkdir(buf, 0777)) {
+				flush_fscache();
 				continue;
+			}
 			die_errno("cannot create directory at '%s'", buf);
 		}
+
+		/*
+		 * Flush the lstat cache of directory listings so that
+		 * subsequent has_dirs_only_path() calls see the
+		 * just-created directory. Without this, the Windows
+		 * fscache returns stale ENOENT for the new directory,
+		 * causing the next entry sharing this parent to
+		 * incorrectly hit the mkdir/unlink recovery path
+		 * above, which then fails with "Directory not empty".
+		 */
+		flush_fscache();
 	}
 	free(buf);
 }

parallel-checkout.c

@@ -395,6 +395,13 @@ void write_pc_item(struct parallel_checkout_item *pc_item,
 		goto out;
 	}
 
+	/*
+	 * Flush the Windows fscache so that the lstat() below sees the
+	 * file we just wrote. Without this, the cached parent directory
+	 * listing may not yet include the new file entry.
+	 */
+	flush_fscache();
+
 	if (state->refresh_cache && !fstat_done && lstat(path.buf, &pc_item->st) < 0) {
 		error_errno("unable to stat just-written file '%s'",  path.buf);
 		pc_item->status = PC_ITEM_FAILED;

t/t2080-parallel-checkout-basics.sh

@@ -274,4 +274,50 @@ test_expect_success '"git checkout ." report should not include failed entries'
 	)
 '
 
+# Regression test: parallel checkout + fscache stale directory listing.
+#
+# When checkout.workers > 1, checkout_entry_ca() enqueues files for deferred
+# writing instead of writing them inline. The inline write_entry() path calls
+# flush_fscache() after each file, keeping the Windows fscache in sync with
+# newly-created directories. The deferred path skips this flush, so
+# has_dirs_only_path() sees stale ENOENT for directories that mkdir() just
+# created. The recovery path in create_directories() then tries to unlink+
+# recreate the directory, which fails because it already has children.
+#
+# The trigger is: two files sharing a parent directory that does not yet exist
+# on disk when `git checkout <tree> -- <pathspec>` runs.
+test_expect_success MINGW 'parallel checkout with fscache does not fail on new directories' '
+	git init fscache-pc &&
+	(
+		cd fscache-pc &&
+		git config core.fscache true &&
+
+		# Commit B1: files in a nested directory
+		mkdir -p sub/deep/dir &&
+		echo one >sub/deep/dir/file1.txt &&
+		echo two >sub/deep/dir/file2.txt &&
+		git add sub &&
+		git commit -m "B1: with sub/deep/dir" &&
+		git tag B1 &&
+
+		# Commit B2: the directory is gone
+		git rm -rf sub &&
+		git commit -m "B2: without sub" &&
+
+		# Now restore both files from B1 with parallel checkout.
+		# This is the pathspec checkout path (checkout_worktree in
+		# builtin/checkout.c), which defers writes via enqueue_checkout
+		# when workers > 1 and does not flush fscache between entries.
+		git -c checkout.workers=2 \
+		    -c checkout.thresholdForParallelism=0 \
+		    checkout B1 -- sub/deep/dir/file1.txt sub/deep/dir/file2.txt &&
+
+		# Verify both files are correctly restored
+		echo one >expect1 &&
+		echo two >expect2 &&
+		test_cmp expect1 sub/deep/dir/file1.txt &&
+		test_cmp expect2 sub/deep/dir/file2.txt
+	)
+'
+
 test_done