Merge branch 'mt/parallel-checkout-part-3'

The final part of "parallel checkout".

* mt/parallel-checkout-part-3:
  ci: run test round with parallel-checkout enabled
  parallel-checkout: add tests related to .gitattributes
  t0028: extract encoding helpers to lib-encoding.sh
  parallel-checkout: add tests related to path collisions
  parallel-checkout: add tests for basic operations
  checkout-index: add parallel checkout support
  builtin/checkout.c: complete parallel checkout support
  make_transient_cache_entry(): optionally alloc from mem_pool

16 files changed, 734 insertions, 49 deletions

diff --git a/builtin/checkout--worker.c b/builtin/checkout--worker.c
index 31e0de2..289a9b8 100644
--- a/builtin/checkout--worker.c
+++ b/builtin/checkout--worker.c
@@ -39,7 +39,7 @@ static void packet_to_pc_item(const char *buffer, int len,
 	}
 
 	memset(pc_item, 0, sizeof(*pc_item));
-	pc_item->ce = make_empty_transient_cache_entry(fixed_portion->name_len);
+	pc_item->ce = make_empty_transient_cache_entry(fixed_portion->name_len, NULL);
 	pc_item->ce->ce_namelen = fixed_portion->name_len;
 	pc_item->ce->ce_mode = fixed_portion->ce_mode;
 	memcpy(pc_item->ce->name, variant, pc_item->ce->ce_namelen);
diff --git a/builtin/checkout-index.c b/builtin/checkout-index.c
index c9a3c71..e21620d 100644
--- a/builtin/checkout-index.c
+++ b/builtin/checkout-index.c
@@ -12,6 +12,7 @@
 #include "cache-tree.h"
 #include "parse-options.h"
 #include "entry.h"
+#include "parallel-checkout.h"
 
 #define CHECKOUT_ALL 4
 static int nul_term_line;
@@ -115,7 +116,7 @@ static int checkout_file(const char *name, const char *prefix)
 	return -1;
 }
 
-static void checkout_all(const char *prefix, int prefix_length)
+static int checkout_all(const char *prefix, int prefix_length)
 {
 	int i, errs = 0;
 	struct cache_entry *last_ce = NULL;
@@ -144,11 +145,7 @@ static void checkout_all(const char *prefix, int prefix_length)
 	}
 	if (last_ce && to_tempfile)
 		write_tempfile_record(last_ce->name, prefix);
-	if (errs)
-		/* we have already done our error reporting.
-		 * exit with the same code as die().
-		 */
-		exit(128);
+	return !!errs;
 }
 
 static const char * const builtin_checkout_index_usage[] = {
@@ -184,6 +181,7 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 	int force = 0, quiet = 0, not_new = 0;
 	int index_opt = 0;
 	int err = 0;
+	int pc_workers, pc_threshold;
 	struct option builtin_checkout_index_options[] = {
 		OPT_BOOL('a', "all", &all,
 			N_("check out all files in the index")),
@@ -238,6 +236,10 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 		hold_locked_index(&lock_file, LOCK_DIE_ON_ERROR);
 	}
 
+	get_parallel_checkout_configs(&pc_workers, &pc_threshold);
+	if (pc_workers > 1)
+		init_parallel_checkout();
+
 	/* Check out named files first */
 	for (i = 0; i < argc; i++) {
 		const char *arg = argv[i];
@@ -277,12 +279,16 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 		strbuf_release(&buf);
 	}
 
+	if (all)
+		err |= checkout_all(prefix, prefix_length);
+
+	if (pc_workers > 1)
+		err |= run_parallel_checkout(&state, pc_workers, pc_threshold,
+					     NULL, NULL);
+
 	if (err)
 		return 1;
 
-	if (all)
-		checkout_all(prefix, prefix_length);
-
 	if (is_lock_file_locked(&lock_file) &&
 	    write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
 		die("Unable to write new index file");
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 0bf61e6..f4cd774 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -27,6 +27,7 @@
 #include "wt-status.h"
 #include "xdiff-interface.h"
 #include "entry.h"
+#include "parallel-checkout.h"
 
 static const char * const checkout_usage[] = {
 	N_("git checkout [<options>] <branch>"),
@@ -230,7 +231,8 @@ static int checkout_stage(int stage, const struct cache_entry *ce, int pos,
 		return error(_("path '%s' does not have their version"), ce->name);
 }
 
-static int checkout_merged(int pos, const struct checkout *state, int *nr_checkouts)
+static int checkout_merged(int pos, const struct checkout *state,
+			   int *nr_checkouts, struct mem_pool *ce_mem_pool)
 {
 	struct cache_entry *ce = active_cache[pos];
 	const char *path = ce->name;
@@ -291,11 +293,10 @@ static int checkout_merged(int pos, const struct checkout *state, int *nr_checko
 	if (write_object_file(result_buf.ptr, result_buf.size, blob_type, &oid))
 		die(_("Unable to add merge result for '%s'"), path);
 	free(result_buf.ptr);
-	ce = make_transient_cache_entry(mode, &oid, path, 2);
+	ce = make_transient_cache_entry(mode, &oid, path, 2, ce_mem_pool);
 	if (!ce)
 		die(_("make_cache_entry failed for path '%s'"), path);
 	status = checkout_entry(ce, state, NULL, nr_checkouts);
-	discard_cache_entry(ce);
 	return status;
 }
 
@@ -359,19 +360,27 @@ static int checkout_worktree(const struct checkout_opts *opts,
 	int nr_checkouts = 0, nr_unmerged = 0;
 	int errs = 0;
 	int pos;
+	int pc_workers, pc_threshold;
+	struct mem_pool ce_mem_pool;
 
 	state.force = 1;
 	state.refresh_cache = 1;
 	state.istate = &the_index;
 
+	mem_pool_init(&ce_mem_pool, 0);
+	get_parallel_checkout_configs(&pc_workers, &pc_threshold);
 	init_checkout_metadata(&state.meta, info->refname,
 			       info->commit ? &info->commit->object.oid : &info->oid,
 			       NULL);
 
 	enable_delayed_checkout(&state);
 
+	if (pc_workers > 1)
+		init_parallel_checkout();
+
 	/* TODO: audit for interaction with sparse-index. */
 	ensure_full_index(&the_index);
+
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];
 		if (ce->ce_flags & CE_MATCHED) {
@@ -387,10 +396,15 @@ static int checkout_worktree(const struct checkout_opts *opts,
 						       &nr_checkouts, opts->overlay_mode);
 			else if (opts->merge)
 				errs |= checkout_merged(pos, &state,
-							&nr_unmerged);
+							&nr_unmerged,
+							&ce_mem_pool);
 			pos = skip_same_name(ce, pos) - 1;
 		}
 	}
+	if (pc_workers > 1)
+		errs |= run_parallel_checkout(&state, pc_workers, pc_threshold,
+					      NULL, NULL);
+	mem_pool_discard(&ce_mem_pool, should_validate_cache_entries());
 	remove_marked_cache_entries(&the_index, 1);
 	remove_scheduled_dirs();
 	errs |= finish_delayed_checkout(&state, &nr_checkouts);
diff --git a/builtin/difftool.c b/builtin/difftool.c
index 0202a43..89334b7 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -323,7 +323,7 @@ static int checkout_path(unsigned mode, struct object_id *oid,
 	struct cache_entry *ce;
 	int ret;
 
-	ce = make_transient_cache_entry(mode, oid, path, 0);
+	ce = make_transient_cache_entry(mode, oid, path, 0, NULL);
 	ret = checkout_entry(ce, state, NULL, NULL);
 
 	discard_cache_entry(ce);
diff --git a/cache.h b/cache.h
index abeaec2..ba04ff8 100644
--- a/cache.h
+++ b/cache.h
@@ -370,16 +370,20 @@ struct cache_entry *make_empty_cache_entry(struct index_state *istate,
 					   size_t name_len);
 
 /*
- * Create a cache_entry that is not intended to be added to an index.
- * Caller is responsible for discarding the cache_entry
- * with `discard_cache_entry`.
+ * Create a cache_entry that is not intended to be added to an index. If
+ * `ce_mem_pool` is not NULL, the entry is allocated within the given memory
+ * pool. Caller is responsible for discarding "loose" entries with
+ * `discard_cache_entry()` and the memory pool with
+ * `mem_pool_discard(ce_mem_pool, should_validate_cache_entries())`.
  */
 struct cache_entry *make_transient_cache_entry(unsigned int mode,
 					       const struct object_id *oid,
 					       const char *path,
-					       int stage);
+					       int stage,
+					       struct mem_pool *ce_mem_pool);
 
-struct cache_entry *make_empty_transient_cache_entry(size_t name_len);
+struct cache_entry *make_empty_transient_cache_entry(size_t len,
+						     struct mem_pool *ce_mem_pool);
 
 /*
  * Discard cache entry.
diff --git a/ci/run-build-and-tests.sh b/ci/run-build-and-tests.sh
index d19be40..3ce81ff 100755
--- a/ci/run-build-and-tests.sh
+++ b/ci/run-build-and-tests.sh
@@ -26,6 +26,7 @@ linux-gcc)
 	export GIT_TEST_ADD_I_USE_BUILTIN=1
 	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
 	export GIT_TEST_WRITE_REV_INDEX=1
+	export GIT_TEST_CHECKOUT_WORKERS=2
 	make test
 	;;
 linux-clang)
diff --git a/parallel-checkout.c b/parallel-checkout.c
index 09e8b10..6b1af32 100644
--- a/parallel-checkout.c
+++ b/parallel-checkout.c
@@ -8,6 +8,7 @@
 #include "sigchain.h"
 #include "streaming.h"
 #include "thread-utils.h"
+#include "trace2.h"
 
 struct pc_worker {
 	struct child_process cp;
@@ -34,6 +35,20 @@ static const int DEFAULT_NUM_WORKERS = 1;
 
 void get_parallel_checkout_configs(int *num_workers, int *threshold)
 {
+	char *env_workers = getenv("GIT_TEST_CHECKOUT_WORKERS");
+
+	if (env_workers && *env_workers) {
+		if (strtol_i(env_workers, 10, num_workers)) {
+			die("invalid value for GIT_TEST_CHECKOUT_WORKERS: '%s'",
+			    env_workers);
+		}
+		if (*num_workers < 1)
+			*num_workers = online_cpus();
+
+		*threshold = 0;
+		return;
+	}
+
 	if (git_config_get_int("checkout.workers", num_workers))
 		*num_workers = DEFAULT_NUM_WORKERS;
 	else if (*num_workers < 1)
@@ -326,6 +341,7 @@ void write_pc_item(struct parallel_checkout_item *pc_item,
 	if (dir_sep && !has_dirs_only_path(path.buf, dir_sep - path.buf,
 					   state->base_dir_len)) {
 		pc_item->status = PC_ITEM_COLLIDED;
+		trace2_data_string("pcheckout", NULL, "collision/dirname", path.buf);
 		goto out;
 	}
 
@@ -341,6 +357,8 @@ void write_pc_item(struct parallel_checkout_item *pc_item,
 			 * call should have already caught these cases.
 			 */
 			pc_item->status = PC_ITEM_COLLIDED;
+			trace2_data_string("pcheckout", NULL,
+					   "collision/basename", path.buf);
 		} else {
 			error_errno("failed to open file '%s'", path.buf);
 			pc_item->status = PC_ITEM_FAILED;
diff --git a/read-cache.c b/read-cache.c
index fbf3a4c..1b3c2eb 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -839,8 +839,11 @@ struct cache_entry *make_empty_cache_entry(struct index_state *istate, size_t le
 	return mem_pool__ce_calloc(find_mem_pool(istate), len);
 }
 
-struct cache_entry *make_empty_transient_cache_entry(size_t len)
+struct cache_entry *make_empty_transient_cache_entry(size_t len,
+						     struct mem_pool *ce_mem_pool)
 {
+	if (ce_mem_pool)
+		return mem_pool__ce_calloc(ce_mem_pool, len);
 	return xcalloc(1, cache_entry_size(len));
 }
 
@@ -874,8 +877,11 @@ struct cache_entry *make_cache_entry(struct index_state *istate,
 	return ret;
 }
 
-struct cache_entry *make_transient_cache_entry(unsigned int mode, const struct object_id *oid,
-					       const char *path, int stage)
+struct cache_entry *make_transient_cache_entry(unsigned int mode,
+					       const struct object_id *oid,
+					       const char *path,
+					       int stage,
+					       struct mem_pool *ce_mem_pool)
 {
 	struct cache_entry *ce;
 	int len;
@@ -886,7 +892,7 @@ struct cache_entry *make_transient_cache_entry(unsigned int mode, const struct o
 	}
 
 	len = strlen(path);
-	ce = make_empty_transient_cache_entry(len);
+	ce = make_empty_transient_cache_entry(len, ce_mem_pool);
 
 	oidcpy(&ce->oid, oid);
 	memcpy(ce->name, path, len);
diff --git a/t/README b/t/README
index 8eb9e46..a8cfd37 100644
--- a/t/README
+++ b/t/README
@@ -439,6 +439,10 @@ GIT_TEST_WRITE_REV_INDEX=<boolean>, when true enables the
 GIT_TEST_SPARSE_INDEX=<boolean>, when true enables index writes to use the
 sparse-index format by default.
 
+GIT_TEST_CHECKOUT_WORKERS=<n> overrides the 'checkout.workers' setting
+to <n> and 'checkout.thresholdForParallelism' to 0, forcing the
+execution of the parallel-checkout code.
+
 Naming Tests
 ------------
 
diff --git a/t/lib-encoding.sh b/t/lib-encoding.sh
new file mode 100644
index 0000000..2dabc8c
--- /dev/null
+++ b/t/lib-encoding.sh
@@ -0,0 +1,25 @@
+# Encoding helpers
+
+test_lazy_prereq NO_UTF16_BOM '
+	test $(printf abc | iconv -f UTF-8 -t UTF-16 | wc -c) = 6
+'
+
+test_lazy_prereq NO_UTF32_BOM '
+	test $(printf abc | iconv -f UTF-8 -t UTF-32 | wc -c) = 12
+'
+
+write_utf16 () {
+	if test_have_prereq NO_UTF16_BOM
+	then
+		printf '\376\377'
+	fi &&
+	iconv -f UTF-8 -t UTF-16
+}
+
+write_utf32 () {
+	if test_have_prereq NO_UTF32_BOM
+	then
+		printf '\0\0\376\377'
+	fi &&
+	iconv -f UTF-8 -t UTF-32
+}
diff --git a/t/lib-parallel-checkout.sh b/t/lib-parallel-checkout.sh
new file mode 100644
index 0000000..21f5759
--- /dev/null
+++ b/t/lib-parallel-checkout.sh
@@ -0,0 +1,45 @@
+# Helpers for tests invoking parallel-checkout
+
+# Parallel checkout tests need full control of the number of workers
+unset GIT_TEST_CHECKOUT_WORKERS
+
+set_checkout_config () {
+	if test $# -ne 2
+	then
+		BUG "usage: set_checkout_config <workers> <threshold>"
+	fi &&
+
+	test_config_global checkout.workers $1 &&
+	test_config_global checkout.thresholdForParallelism $2
+}
+
+# Run "${@:2}" and check that $1 checkout workers were used
+test_checkout_workers () {
+	if test $# -lt 2
+	then
+		BUG "too few arguments to test_checkout_workers"
+	fi &&
+
+	local expected_workers=$1 &&
+	shift &&
+
+	local trace_file=trace-test-checkout-workers &&
+	rm -f "$trace_file" &&
+	GIT_TRACE2="$(pwd)/$trace_file" "$@" 2>&8 &&
+
+	local workers=$(grep "child_start\[..*\] git checkout--worker" "$trace_file" | wc -l) &&
+	test $workers -eq $expected_workers &&
+	rm "$trace_file"
+} 8>&2 2>&4
+
+# Verify that both the working tree and the index were created correctly
+verify_checkout () {
+	if test $# -ne 1
+	then
+		BUG "usage: verify_checkout <repository path>"
+	fi &&
+
+	git -C "$1" diff-index --ignore-submodules=none --exit-code HEAD -- &&
+	git -C "$1" status --porcelain >"$1".status &&
+	test_must_be_empty "$1".status
+}
diff --git a/t/t0028-working-tree-encoding.sh b/t/t0028-working-tree-encoding.sh
index f970a98..82905a2 100755
--- a/t/t0028-working-tree-encoding.sh
+++ b/t/t0028-working-tree-encoding.sh
@@ -6,33 +6,10 @@ GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY/lib-encoding.sh"
 
 GIT_TRACE_WORKING_TREE_ENCODING=1 && export GIT_TRACE_WORKING_TREE_ENCODING
 
-test_lazy_prereq NO_UTF16_BOM '
-	test $(printf abc | iconv -f UTF-8 -t UTF-16 | wc -c) = 6
-'
-
-test_lazy_prereq NO_UTF32_BOM '
-	test $(printf abc | iconv -f UTF-8 -t UTF-32 | wc -c) = 12
-'
-
-write_utf16 () {
-	if test_have_prereq NO_UTF16_BOM
-	then
-		printf '\376\377'
-	fi &&
-	iconv -f UTF-8 -t UTF-16
-}
-
-write_utf32 () {
-	if test_have_prereq NO_UTF32_BOM
-	then
-		printf '\0\0\376\377'
-	fi &&
-	iconv -f UTF-8 -t UTF-32
-}
-
 test_expect_success 'setup test files' '
 	git config core.eol lf &&
 
diff --git a/t/t2080-parallel-checkout-basics.sh b/t/t2080-parallel-checkout-basics.sh
new file mode 100755
index 0000000..7087818
--- /dev/null
+++ b/t/t2080-parallel-checkout-basics.sh
@@ -0,0 +1,229 @@
+#!/bin/sh
+
+test_description='parallel-checkout basics
+
+Ensure that parallel-checkout basically works on clone and checkout, spawning
+the required number of workers and correctly populating both the index and the
+working tree.
+'
+
+TEST_NO_CREATE_REPO=1
+. ./test-lib.sh
+. "$TEST_DIRECTORY/lib-parallel-checkout.sh"
+
+# Test parallel-checkout with a branch switch containing a variety of file
+# creations, deletions, and modifications, involving different entry types.
+# The branches B1 and B2 have the following paths:
+#
+#      B1                 B2
+#  a/a (file)         a   (file)
+#  b   (file)         b/b (file)
+#
+#  c/c (file)         c   (symlink)
+#  d   (symlink)      d/d (file)
+#
+#  e/e (file)         e   (submodule)
+#  f   (submodule)    f/f (file)
+#
+#  g   (submodule)    g   (symlink)
+#  h   (symlink)      h   (submodule)
+#
+# Additionally, the following paths are present on both branches, but with
+# different contents:
+#
+#  i   (file)         i   (file)
+#  j   (symlink)      j   (symlink)
+#  k   (submodule)    k   (submodule)
+#
+# And the following paths are only present in one of the branches:
+#
+#  l/l (file)         -
+#  -                  m/m (file)
+#
+test_expect_success 'setup repo for checkout with various types of changes' '
+	git init sub &&
+	(
+		cd sub &&
+		git checkout -b B2 &&
+		echo B2 >file &&
+		git add file &&
+		git commit -m file &&
+
+		git checkout -b B1 &&
+		echo B1 >file &&
+		git add file &&
+		git commit -m file
+	) &&
+
+	git init various &&
+	(
+		cd various &&
+
+		git checkout -b B1 &&
+		mkdir a c e &&
+		echo a/a >a/a &&
+		echo b >b &&
+		echo c/c >c/c &&
+		test_ln_s_add c d &&
+		echo e/e >e/e &&
+		git submodule add ../sub f &&
+		git submodule add ../sub g &&
+		test_ln_s_add c h &&
+
+		echo "B1 i" >i &&
+		test_ln_s_add c j &&
+		git submodule add -b B1 ../sub k &&
+		mkdir l &&
+		echo l/l >l/l &&
+
+		git add . &&
+		git commit -m B1 &&
+
+		git checkout -b B2 &&
+		git rm -rf :^.gitmodules :^k &&
+		mkdir b d f &&
+		echo a >a &&
+		echo b/b >b/b &&
+		test_ln_s_add b c &&
+		echo d/d >d/d &&
+		git submodule add ../sub e &&
+		echo f/f >f/f &&
+		test_ln_s_add b g &&
+		git submodule add ../sub h &&
+
+		echo "B2 i" >i &&
+		test_ln_s_add b j &&
+		git -C k checkout B2 &&
+		mkdir m &&
+		echo m/m >m/m &&
+
+		git add . &&
+		git commit -m B2 &&
+
+		git checkout --recurse-submodules B1
+	)
+'
+
+for mode in sequential parallel sequential-fallback
+do
+	case $mode in
+	sequential)          workers=1 threshold=0 expected_workers=0 ;;
+	parallel)            workers=2 threshold=0 expected_workers=2 ;;
+	sequential-fallback) workers=2 threshold=100 expected_workers=0 ;;
+	esac
+
+	test_expect_success "$mode checkout" '
+		repo=various_$mode &&
+		cp -R various $repo &&
+
+		# The just copied files have more recent timestamps than their
+		# associated index entries. So refresh the cached timestamps
+		# to avoid an "entry not up-to-date" error from `git checkout`.
+		# We only have to do this for the submodules as `git checkout`
+		# will already refresh the superproject index before performing
+		# the up-to-date check.
+		#
+		git -C $repo submodule foreach "git update-index --refresh" &&
+
+		set_checkout_config $workers $threshold &&
+		test_checkout_workers $expected_workers \
+			git -C $repo checkout --recurse-submodules B2 &&
+		verify_checkout $repo
+	'
+done
+
+for mode in parallel sequential-fallback
+do
+	case $mode in
+	parallel)            workers=2 threshold=0 expected_workers=2 ;;
+	sequential-fallback) workers=2 threshold=100 expected_workers=0 ;;
+	esac
+
+	test_expect_success "$mode checkout on clone" '
+		repo=various_${mode}_clone &&
+		set_checkout_config $workers $threshold &&
+		test_checkout_workers $expected_workers \
+			git clone --recurse-submodules --branch B2 various $repo &&
+		verify_checkout $repo
+	'
+done
+
+# Just to be paranoid, actually compare the working trees' contents directly.
+test_expect_success 'compare the working trees' '
+	rm -rf various_*/.git &&
+	rm -rf various_*/*/.git &&
+
+	# We use `git diff` instead of `diff -r` because the latter would
+	# follow symlinks, and not all `diff` implementations support the
+	# `--no-dereference` option.
+	#
+	git diff --no-index various_sequential various_parallel &&
+	git diff --no-index various_sequential various_parallel_clone &&
+	git diff --no-index various_sequential various_sequential-fallback &&
+	git diff --no-index various_sequential various_sequential-fallback_clone
+'
+
+# Currently, each submodule is checked out in a separated child process, but
+# these subprocesses must also be able to use parallel checkout workers to
+# write the submodules' entries.
+test_expect_success 'submodules can use parallel checkout' '
+	set_checkout_config 2 0 &&
+	git init super &&
+	(
+		cd super &&
+		git init sub &&
+		test_commit -C sub A &&
+		test_commit -C sub B &&
+		git submodule add ./sub &&
+		git commit -m sub &&
+		rm sub/* &&
+		test_checkout_workers 2 git checkout --recurse-submodules .
+	)
+'
+
+test_expect_success 'parallel checkout respects --[no]-force' '
+	set_checkout_config 2 0 &&
+	git init dirty &&
+	(
+		cd dirty &&
+		mkdir D &&
+		test_commit D/F &&
+		test_commit F &&
+
+		rm -rf D &&
+		echo changed >D &&
+		echo changed >F.t &&
+
+		# We expect 0 workers because there is nothing to be done
+		test_checkout_workers 0 git checkout HEAD &&
+		test_path_is_file D &&
+		grep changed D &&
+		grep changed F.t &&
+
+		test_checkout_workers 2 git checkout --force HEAD &&
+		test_path_is_dir D &&
+		grep D/F D/F.t &&
+		grep F F.t
+	)
+'
+
+test_expect_success SYMLINKS 'parallel checkout checks for symlinks in leading dirs' '
+	set_checkout_config 2 0 &&
+	git init symlinks &&
+	(
+		cd symlinks &&
+		mkdir D untracked &&
+		# Commit 2 files to have enough work for 2 parallel workers
+		test_commit D/A &&
+		test_commit D/B &&
+		rm -rf D &&
+		ln -s untracked D &&
+
+		test_checkout_workers 2 git checkout --force HEAD &&
+		! test -h D &&
+		grep D/A D/A.t &&
+		grep D/B D/B.t
+	)
+'
+
+test_done
diff --git a/t/t2081-parallel-checkout-collisions.sh b/t/t2081-parallel-checkout-collisions.sh
new file mode 100755
index 0000000..f6fcfc0
--- /dev/null
+++ b/t/t2081-parallel-checkout-collisions.sh
@@ -0,0 +1,162 @@
+#!/bin/sh
+
+test_description="path collisions during parallel checkout
+
+Parallel checkout must detect path collisions to:
+
+1) Avoid racily writing to different paths that represent the same file on disk.
+2) Report the colliding entries on clone.
+
+The tests in this file exercise parallel checkout's collision detection code in
+both these mechanics.
+"
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY/lib-parallel-checkout.sh"
+
+TEST_ROOT="$PWD"
+
+test_expect_success CASE_INSENSITIVE_FS 'setup' '
+	empty_oid=$(git hash-object -w --stdin </dev/null) &&
+	cat >objs <<-EOF &&
+	100644 $empty_oid	FILE_X
+	100644 $empty_oid	FILE_x
+	100644 $empty_oid	file_X
+	100644 $empty_oid	file_x
+	EOF
+	git update-index --index-info <objs &&
+	git commit -m "colliding files" &&
+	git tag basename_collision &&
+
+	write_script "$TEST_ROOT"/logger_script <<-\EOF
+	echo "$@" >>filter.log
+	EOF
+'
+
+test_workers_in_event_trace ()
+{
+	test $1 -eq $(grep ".event.:.child_start..*checkout--worker" $2 | wc -l)
+}
+
+test_expect_success CASE_INSENSITIVE_FS 'worker detects basename collision' '
+	GIT_TRACE2_EVENT="$(pwd)/trace" git \
+		-c checkout.workers=2 -c checkout.thresholdForParallelism=0 \
+		checkout . &&
+
+	test_workers_in_event_trace 2 trace &&
+	collisions=$(grep -i "category.:.pcheckout.,.key.:.collision/basename.,.value.:.file_x.}" trace | wc -l) &&
+	test $collisions -eq 3
+'
+
+test_expect_success CASE_INSENSITIVE_FS 'worker detects dirname collision' '
+	test_config filter.logger.smudge "\"$TEST_ROOT/logger_script\" %f" &&
+	empty_oid=$(git hash-object -w --stdin </dev/null) &&
+
+	# By setting a filter command to "a", we make it ineligible for parallel
+	# checkout, and thus it is checked out *first*. This way we can ensure
+	# that "A/B" and "A/C" will both collide with the regular file "a".
+	#
+	attr_oid=$(echo "a filter=logger" | git hash-object -w --stdin) &&
+
+	cat >objs <<-EOF &&
+	100644 $empty_oid	A/B
+	100644 $empty_oid	A/C
+	100644 $empty_oid	a
+	100644 $attr_oid	.gitattributes
+	EOF
+	git rm -rf . &&
+	git update-index --index-info <objs &&
+
+	rm -f trace filter.log &&
+	GIT_TRACE2_EVENT="$(pwd)/trace" git \
+		-c checkout.workers=2 -c checkout.thresholdForParallelism=0 \
+		checkout . &&
+
+	# Check that "a" (and only "a") was filtered
+	echo a >expected.log &&
+	test_cmp filter.log expected.log &&
+
+	# Check that it used the right number of workers and detected the collisions
+	test_workers_in_event_trace 2 trace &&
+	grep "category.:.pcheckout.,.key.:.collision/dirname.,.value.:.A/B.}" trace &&
+	grep "category.:.pcheckout.,.key.:.collision/dirname.,.value.:.A/C.}" trace
+'
+
+test_expect_success SYMLINKS,CASE_INSENSITIVE_FS 'do not follow symlinks colliding with leading dir' '
+	empty_oid=$(git hash-object -w --stdin </dev/null) &&
+	symlink_oid=$(echo "./e" | git hash-object -w --stdin) &&
+	mkdir e &&
+
+	cat >objs <<-EOF &&
+	120000 $symlink_oid	D
+	100644 $empty_oid	d/x
+	100644 $empty_oid	e/y
+	EOF
+	git rm -rf . &&
+	git update-index --index-info <objs &&
+
+	set_checkout_config 2 0 &&
+	test_checkout_workers 2 git checkout . &&
+	test_path_is_dir e &&
+	test_path_is_missing e/x
+'
+
+# The two following tests check that parallel checkout correctly reports
+# colliding entries on clone. The sequential code detects a collision by
+# calling lstat() before trying to open(O_CREAT) a file. (Note that this only
+# works for clone.) Then, to find the pair of a colliding item k, it searches
+# cache_entry[0, k-1]. This is not sufficient in parallel checkout because:
+#
+# - A colliding file may be created between the lstat() and open() calls;
+# - A colliding entry might appear in the second half of the cache_entry array.
+#
+test_expect_success CASE_INSENSITIVE_FS 'collision report on clone (w/ racy file creation)' '
+	git reset --hard basename_collision &&
+	set_checkout_config 2 0 &&
+	test_checkout_workers 2 git clone . clone-repo 2>stderr &&
+
+	grep FILE_X stderr &&
+	grep FILE_x stderr &&
+	grep file_X stderr &&
+	grep file_x stderr &&
+	grep "the following paths have collided" stderr
+'
+
+# This test ensures that the collision report code is correctly looking for
+# colliding peers in the second half of the cache_entry array. This is done by
+# defining a smudge command for the *last* array entry, which makes it
+# non-eligible for parallel-checkout. Thus, it is checked out *first*, before
+# spawning the workers.
+#
+# Note: this test doesn't work on Windows because, on this system, the
+# collision report code uses strcmp() to find the colliding pairs when
+# core.ignoreCase is false. And we need this setting for this test so that only
+# 'file_x' matches the pattern of the filter attribute. But the test works on
+# OSX, where the colliding pairs are found using inode.
+#
+test_expect_success CASE_INSENSITIVE_FS,!MINGW,!CYGWIN \
+	'collision report on clone (w/ colliding peer after the detected entry)' '
+
+	test_config_global filter.logger.smudge "\"$TEST_ROOT/logger_script\" %f" &&
+	git reset --hard basename_collision &&
+	echo "file_x filter=logger" >.gitattributes &&
+	git add .gitattributes &&
+	git commit -m "filter for file_x" &&
+
+	rm -rf clone-repo &&
+	set_checkout_config 2 0 &&
+	test_checkout_workers 2 \
+		git -c core.ignoreCase=false clone . clone-repo 2>stderr &&
+
+	grep FILE_X stderr &&
+	grep FILE_x stderr &&
+	grep file_X stderr &&
+	grep file_x stderr &&
+	grep "the following paths have collided" stderr &&
+
+	# Check that only "file_x" was filtered
+	echo file_x >expected.log &&
+	test_cmp clone-repo/filter.log expected.log
+'
+
+test_done
diff --git a/t/t2082-parallel-checkout-attributes.sh b/t/t2082-parallel-checkout-attributes.sh
new file mode 100755
index 0000000..2525457
--- /dev/null
+++ b/t/t2082-parallel-checkout-attributes.sh
@@ -0,0 +1,194 @@
+#!/bin/sh
+
+test_description='parallel-checkout: attributes
+
+Verify that parallel-checkout correctly creates files that require
+conversions, as specified in .gitattributes. The main point here is
+to check that the conv_attr data is correctly sent to the workers
+and that it contains sufficient information to smudge files
+properly (without access to the index or attribute stack).
+'
+
+TEST_NO_CREATE_REPO=1
+. ./test-lib.sh
+. "$TEST_DIRECTORY/lib-parallel-checkout.sh"
+. "$TEST_DIRECTORY/lib-encoding.sh"
+
+test_expect_success 'parallel-checkout with ident' '
+	set_checkout_config 2 0 &&
+	git init ident &&
+	(
+		cd ident &&
+		echo "A ident" >.gitattributes &&
+		echo "\$Id\$" >A &&
+		echo "\$Id\$" >B &&
+		git add -A &&
+		git commit -m id &&
+
+		rm A B &&
+		test_checkout_workers 2 git reset --hard &&
+		hexsz=$(test_oid hexsz) &&
+		grep -E "\\\$Id: [0-9a-f]{$hexsz} \\\$" A &&
+		grep "\\\$Id\\\$" B
+	)
+'
+
+test_expect_success 'parallel-checkout with re-encoding' '
+	set_checkout_config 2 0 &&
+	git init encoding &&
+	(
+		cd encoding &&
+		echo text >utf8-text &&
+		write_utf16 <utf8-text >utf16-text &&
+
+		echo "A working-tree-encoding=UTF-16" >.gitattributes &&
+		cp utf16-text A &&
+		cp utf8-text B &&
+		git add A B .gitattributes &&
+		git commit -m encoding &&
+
+		# Check that A is stored in UTF-8
+		git cat-file -p :A >A.internal &&
+		test_cmp_bin utf8-text A.internal &&
+
+		rm A B &&
+		test_checkout_workers 2 git checkout A B &&
+
+		# Check that A (and only A) is re-encoded during checkout
+		test_cmp_bin utf16-text A &&
+		test_cmp_bin utf8-text B
+	)
+'
+
+test_expect_success 'parallel-checkout with eol conversions' '
+	set_checkout_config 2 0 &&
+	git init eol &&
+	(
+		cd eol &&
+		printf "multi\r\nline\r\ntext" >crlf-text &&
+		printf "multi\nline\ntext" >lf-text &&
+
+		git config core.autocrlf false &&
+		echo "A eol=crlf" >.gitattributes &&
+		cp crlf-text A &&
+		cp lf-text B &&
+		git add A B .gitattributes &&
+		git commit -m eol &&
+
+		# Check that A is stored with LF format
+		git cat-file -p :A >A.internal &&
+		test_cmp_bin lf-text A.internal &&
+
+		rm A B &&
+		test_checkout_workers 2 git checkout A B &&
+
+		# Check that A (and only A) is converted to CRLF during checkout
+		test_cmp_bin crlf-text A &&
+		test_cmp_bin lf-text B
+	)
+'
+
+# Entries that require an external filter are not eligible for parallel
+# checkout. Check that both the parallel-eligible and non-eligible entries are
+# properly writen in a single checkout operation.
+#
+test_expect_success 'parallel-checkout and external filter' '
+	set_checkout_config 2 0 &&
+	git init filter &&
+	(
+		cd filter &&
+		write_script <<-\EOF rot13.sh &&
+		tr \
+		  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" \
+		  "nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM"
+		EOF
+
+		git config filter.rot13.clean "\"$(pwd)/rot13.sh\"" &&
+		git config filter.rot13.smudge "\"$(pwd)/rot13.sh\"" &&
+		git config filter.rot13.required true &&
+
+		echo abcd >original &&
+		echo nopq >rot13 &&
+
+		echo "A filter=rot13" >.gitattributes &&
+		cp original A &&
+		cp original B &&
+		cp original C &&
+		git add A B C .gitattributes &&
+		git commit -m filter &&
+
+		# Check that A (and only A) was cleaned
+		git cat-file -p :A >A.internal &&
+		test_cmp rot13 A.internal &&
+		git cat-file -p :B >B.internal &&
+		test_cmp original B.internal &&
+		git cat-file -p :C >C.internal &&
+		test_cmp original C.internal &&
+
+		rm A B C *.internal &&
+		test_checkout_workers 2 git checkout A B C &&
+
+		# Check that A (and only A) was smudged during checkout
+		test_cmp original A &&
+		test_cmp original B &&
+		test_cmp original C
+	)
+'
+
+# The delayed queue is independent from the parallel queue, and they should be
+# able to work together in the same checkout process.
+#
+test_expect_success PERL 'parallel-checkout and delayed checkout' '
+	write_script rot13-filter.pl "$PERL_PATH" \
+		<"$TEST_DIRECTORY"/t0021/rot13-filter.pl &&
+
+	test_config_global filter.delay.process \
+		"\"$(pwd)/rot13-filter.pl\" --always-delay \"$(pwd)/delayed.log\" clean smudge delay" &&
+	test_config_global filter.delay.required true &&
+
+	echo "abcd" >original &&
+	echo "nopq" >rot13 &&
+
+	git init delayed &&
+	(
+		cd delayed &&
+		echo "*.d filter=delay" >.gitattributes &&
+		cp ../original W.d &&
+		cp ../original X.d &&
+		cp ../original Y &&
+		cp ../original Z &&
+		git add -A &&
+		git commit -m delayed &&
+
+		# Check that *.d files were cleaned
+		git cat-file -p :W.d >W.d.internal &&
+		test_cmp W.d.internal ../rot13 &&
+		git cat-file -p :X.d >X.d.internal &&
+		test_cmp X.d.internal ../rot13 &&
+		git cat-file -p :Y >Y.internal &&
+		test_cmp Y.internal ../original &&
+		git cat-file -p :Z >Z.internal &&
+		test_cmp Z.internal ../original &&
+
+		rm *
+	) &&
+
+	set_checkout_config 2 0 &&
+	test_checkout_workers 2 git -C delayed checkout -f &&
+	verify_checkout delayed &&
+
+	# Check that the *.d files got to the delay queue and were filtered
+	grep "smudge W.d .* \[DELAYED\]" delayed.log &&
+	grep "smudge X.d .* \[DELAYED\]" delayed.log &&
+	test_cmp delayed/W.d original &&
+	test_cmp delayed/X.d original &&
+
+	# Check that the parallel-eligible entries went to the right queue and
+	# were not filtered
+	! grep "smudge Y .* \[DELAYED\]" delayed.log &&
+	! grep "smudge Z .* \[DELAYED\]" delayed.log &&
+	test_cmp delayed/Y original &&
+	test_cmp delayed/Z original
+'
+
+test_done
diff --git a/unpack-trees.c b/unpack-trees.c
index 7a1804c..f88a69f 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1038,7 +1038,7 @@ static struct cache_entry *create_ce_entry(const struct traverse_info *info,
 	size_t len = traverse_path_len(info, tree_entry_len(n));
 	struct cache_entry *ce =
 		is_transient ?
-		make_empty_transient_cache_entry(len) :
+		make_empty_transient_cache_entry(len, NULL) :
 		make_empty_cache_entry(istate, len);
 
 	ce->ce_mode = create_ce_mode(n->mode);