unpack-trees: fix '--recurse-submodules' when switching from no submodules to nested submodules

Currently, using git checkout --recurse-submodules (or the submodule.recurse config) to switch from a commit with no submodules to a commit with initialized nested submodules fails because a child git process tries to cd to a yet non-existing nested submodule directory. reset and read-tree are also affected in the same way since they also use the unpack-trees machinery.

The 5th commit in this series fixes this bug. The first four are clean-up patches in tests and in unpack-trees that mostly remove outdated comments/dead code. The 6th commit adds a test for the reverse transition (nested submodules -> no submodules) as it was not being tested.

The commit message of the 5th commit is quite long, as I tried to describe clearly what is the cause of the bug.

Philippe Blain (6):
t7112: remove mention of KNOWN_FAILURE_SUBMODULE_RECURSIVE_NESTED t/lib-submodule-update: remove outdated test description t/lib-submodule-update: move a test to the right section
unpack-trees: remove outdated description for verify_clean_submodule
unpack-trees: check for missing submodule directory in merged_entry t/lib-submodule-update: add test removing nested submodules

 t/lib-submodule-update.sh  | 68 +++++++++++++++++++++++++++-----------
 t/t7112-reset-submodule.sh |  1 -
 unpack-trees.c             |  7 ++--
 3 files changed, 51 insertions(+), 25 deletions(-)

base-commit: d8437c57fa0752716dde2d3747e7c22bf7ce2e41
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-555%2Fphil-blain%2Fcheckout-recurse-nested-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-555/phil-blain/checkout-recurse-nested-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/555

gitgitgadget

From: Philippe Blain <[email protected]>

The known failure mode KNOWN_FAILURE_SUBMODULE_RECURSIVE_NESTED was removed from lib-submodule-update.sh in 218c883783 (submodule: properly recurse for read-tree and checkout, 2017-05-02) but at that time this change was not ported over to topic sb/reset-recurse-submodules, such that when this topic was merged in 5f074ca7e8 (Merge branch 'sb/reset-recurse-submodules', 2017-05-29), t7112-reset-submodules.sh kept a mention of this removed failure mode.

Remove it now, as it does not mean anything anymore.

---
 t/t7112-reset-submodule.sh | 1 -
 1 file changed, 1 deletion(-)

index a1cb9ff858e..67346424a53 100755
@@ -5,7 +5,6 @@ test_description='reset can handle submodules'
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-submodule-update.sh
 
-KNOWN_FAILURE_SUBMODULE_RECURSIVE_NESTED=1
 KNOWN_FAILURE_DIRECTORY_SUBMODULE_CONFLICTS=1
 KNOWN_FAILURE_SUBMODULE_OVERWRITE_IGNORED_UNTRACKED=1
 

gitgitgadget

Thanks for cleaning up.

From: Philippe Blain <[email protected]>

The commands in the unpack_trees machinery (checkout, reset, read-tree) were fixed in 218c883783 (submodule: properly recurse for read-tree and checkout, 2017-05-02) to correctly update nested submodules when called with the --recurse-submodules flag.

However, a comment in t/lib-submodule-update.sh mentions that this use case still doesn't work.

Remove this outdated comment.

---
 t/lib-submodule-update.sh | 1 -
 1 file changed, 1 deletion(-)

index 1dd17fc03e1..5f9eb682f6a 100755
@@ -908,7 +908,6 @@ test_submodule_switch_recursing_with_args () {
 		)
 	'
 
-	# recursing deeper than one level doesn't work yet.
 	test_expect_success "$command: modified submodule updates submodule recursively" '
 		prolog &&
 		reset_work_tree_to_interested add_nested_sub &&

gitgitgadget

From: Philippe Blain <[email protected]>

The test "$command: submodule branch is not changed, detach HEAD instead" is in the "Appearing submodule" section of test_submodule_recursing_with_args_common(), but this test updates a submodule; it does not test a transition from a state with no submodule to a state with a submodule.

As such, for consistency, move it to the "Modified submodule" section of the same function. While at it, add a comment describing the test.

---
 t/lib-submodule-update.sh | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

index 5f9eb682f6a..417da3602ae 100755
@@ -658,22 +658,6 @@ test_submodule_recursing_with_args_common() {
 			test_submodule_content sub1 origin/add_sub1
 		)
 	'
-	test_expect_success "$command: submodule branch is not changed, detach HEAD instead" '
-		prolog &&
-		reset_work_tree_to_interested add_sub1 &&
-		(
-			cd submodule_update &&
-			git -C sub1 checkout -b keep_branch &&
-			git -C sub1 rev-parse HEAD >expect &&
-			git branch -t modify_sub1 origin/modify_sub1 &&
-			$command modify_sub1 &&
-			test_superproject_content origin/modify_sub1 &&
-			test_submodule_content sub1 origin/modify_sub1 &&
-			git -C sub1 rev-parse keep_branch >actual &&
-			test_cmp expect actual &&
-			test_must_fail git -C sub1 symbolic-ref HEAD
-		)
-	'
 
 	# Replacing a tracked file with a submodule produces a checked out submodule
 	test_expect_success "$command: replace tracked file with submodule checks out submodule" '

@@ -789,6 +773,23 @@ test_submodule_recursing_with_args_common() {
 			test_submodule_content sub1 origin/add_sub1
 		)
 	'
+	# Updating a submodule does not touch the currently checked out branch in the submodule
+	test_expect_success "$command: submodule branch is not changed, detach HEAD instead" '
+		prolog &&
+		reset_work_tree_to_interested add_sub1 &&
+		(
+			cd submodule_update &&
+			git -C sub1 checkout -b keep_branch &&
+			git -C sub1 rev-parse HEAD >expect &&
+			git branch -t modify_sub1 origin/modify_sub1 &&
+			$command modify_sub1 &&
+			test_superproject_content origin/modify_sub1 &&
+			test_submodule_content sub1 origin/modify_sub1 &&
+			git -C sub1 rev-parse keep_branch >actual &&
+			test_cmp expect actual &&
+			test_must_fail git -C sub1 symbolic-ref HEAD
+		)
+	'
 }
 
 # Declares and invokes several tests that, in various situations, checks that

gitgitgadget

From: Philippe Blain <[email protected]>

The function verify_clean_submodule() learned to verify if a submodule working tree is clean in a7bc845a9a (unpack-trees: check if we can perform the operation for submodules, 2017-03-14), but the commented description above it was not updated to reflect that, such that this description has been outdated since then.

Since Git has now learned to optionnally recursively check out submodules during a superproject checkout, remove this outdated description.

---
 unpack-trees.c | 3 ---
 1 file changed, 3 deletions(-)

index da4d6d4ec01..37eca3ede8b 100644
@@ -1815,9 +1815,6 @@ static void invalidate_ce_path(const struct cache_entry *ce,
 /*
  * Check that checking out ce->sha1 in subdir ce->name is not
  * going to overwrite any working files.
- *
- * Currently, git does not checkout subprojects during a superproject
- * checkout, so it is not going to overwrite anything.
  */
 static int verify_clean_submodule(const char *old_sha1,
 				  const struct cache_entry *ce,

gitgitgadget

From: Philippe Blain <[email protected]>

Using git checkout --recurse-submodules to switch between a branch with no submodules and a branch with initialized nested submodules currently causes a fatal error:

    $ git checkout --recurse-submodules branch-with-nested-submodules
    fatal: exec '--super-prefix=submodule/nested/': cd to 'nested'
failed: No such file or directory
    error: Submodule 'nested' could not be updated.
    error: Submodule 'submodule/nested' cannot checkout new HEAD.
    error: Submodule 'submodule' could not be updated.
    M	submodule
    Switched to branch 'branch-with-nested-submodules'

The checkout succeeds but the worktree and index of the first level submodule are left empty:

    $ cd submodule
    $ git -c status.submoduleSummary=1 status
    HEAD detached at b3ce885
    Changes to be committed:
      (use "git restore --staged <file>..." to unstage)
          deleted:    .gitmodules
          deleted:    first.t
          deleted:    nested

fatal: not a git repository: 'nested/.git'
Submodule changes to be committed:

* nested 1e96f59...0000000:

$ git ls-files -s
$ # empty
$ ls -A
.git

The reason for the fatal error during the checkout is that a child git process tries to cd into the yet unexisting nested submodule directory. The sequence is the following:

1. The main git process (the one running in the superproject) eventually reaches write_entry() in entry.c, which creates the first level submodule directory and then calls submodule_move_head() in submodule.c, which spawns git read-tree in the submodule directory.

2. The first child git process (the one in the submodule of the superproject) eventually calls check_submodule_move_head() at unpack_trees.c:2021, which calls submodule_move_head in dry-run mode, which spawns git read-tree in the nested submodule directory.

3. The second child git process tries to chdir() in the yet unexisting nested submodule directory in start_command() at run-command.c:829 and dies before exec'ing.

The reason why check_submodule_move_head() is reached in the first child and not in the main process is that it is inside an if(submodule_from_ce()) construct, and submodule_from_ce() returns a valid struct submodule pointer, whereas it returns a null pointer in the main git process.

The reason why submodule_from_ce() returns a null pointer in the main git process is because the call to cache_lookup_path() in config_from() (called from submodule_from_path() in submodule_from_ce()) returns a null pointer since the hashmap "for_path" in the submodule_cache of the_repository is not yet populated. It is not populated because both repo_get_oid(repo, GITMODULES_INDEX, &oid) and repo_get_oid(repo, GITMODULES_HEAD, &oid) in config_from_gitmodules() at submodule-config.c:639-640 return -1, as at this stage of the operation, neither the HEAD of the superproject nor its index contain any .gitmodules file.

In contrast, in the first child the hashmap is populated because repo_get_oid(repo, GITMODULES_HEAD, &oid) returns 0 as the HEAD of the first level submodule, i.e. .git/modules/submodule/HEAD, points to a commit where .gitmodules is present and records 'nested' as a submodule.

Fix this bug by checking that the submodule directory exists before calling check_submodule_move_head() in merged_entry() in the if(!old) branch, i.e. if going from a commit with no submodule to a commit with a submodule present.

Also protect the other call to check_submodule_move_head() in merged_entry() the same way as it is safer, even though the else if (!(old->ce_flags & CE_CONFLICTED)) branch of the code is not at play in the present bug.

The other calls to check_submodule_move_head() in other functions in unpack_trees.c are all already protected by calls to lstat() somewhere in
the program flow so we don't need additional protection for them.

All commands in the unpack_trees machinery are affected, i.e. checkout, reset and read-tree when called with the --recurse-submodules flag.

This bug was first reported in [1].

[1]
https://lore.kernel.org/git/[email protected]/

---
 t/lib-submodule-update.sh | 14 ++++++++++++++
 unpack-trees.c            |  4 ++--
 2 files changed, 16 insertions(+), 2 deletions(-)

index 417da3602ae..ab30b2da24f 100755
@@ -626,6 +626,7 @@ test_submodule_forced_switch () {
 # New test cases
 # - Removing a submodule with a git directory absorbs the submodules
 #   git directory first into the superproject.
+# - Switching from no submodule to nested submodules
 
 # Internal function; use test_submodule_switch_recursing_with_args() or
 # test_submodule_forced_switch_recursing_with_args() instead.

@@ -683,6 +684,19 @@ test_submodule_recursing_with_args_common() {
 			test_submodule_content sub1 origin/replace_directory_with_sub1
 		)
 	'
+	# Switching to a commit with nested submodules recursively checks them out
+	test_expect_success "$command: nested submodules are checked out" '
+		prolog &&
+		reset_work_tree_to_interested no_submodule &&
+		(
+			cd submodule_update &&
+			git branch -t modify_sub1_recursively origin/modify_sub1_recursively &&
+			$command modify_sub1_recursively &&
+			test_superproject_content origin/modify_sub1_recursively &&
+			test_submodule_content sub1 origin/modify_sub1_recursively &&
+			test_submodule_content -C sub1 sub2 origin/modify_sub1_recursively
+		)
+	'
 
 	######################## Disappearing submodule #######################
 	# Removing a submodule removes its work tree ...

index 37eca3ede8b..fc6ba19486d 100644
@@ -2064,7 +2064,7 @@ static int merged_entry(const struct cache_entry *ce,
 		}
 		invalidate_ce_path(merge, o);
 
-		if (submodule_from_ce(ce)) {
+		if (submodule_from_ce(ce) && file_exists(ce->name)) {
 			int ret = check_submodule_move_head(ce, NULL,
 							    oid_to_hex(&ce->oid),
 							    o);

@@ -2093,7 +2093,7 @@ static int merged_entry(const struct cache_entry *ce,
 			invalidate_ce_path(old, o);
 		}
 
-		if (submodule_from_ce(ce)) {
+		if (submodule_from_ce(ce) && file_exists(ce->name)) {
 			int ret = check_submodule_move_head(ce, oid_to_hex(&old->oid),
 							    oid_to_hex(&ce->oid),
 							    o);

gitgitgadget

Greate to see a detailed write-up. I'll read the surrounding codepath again later before commenting further.

Thanks.

From: Philippe Blain <[email protected]>

The previous commit fixed a bug with the (no submodule) -> (nested submodules) transition for commands in the unpack-trees machinery.

Let's add a test for the reverse transition (going from nested submodules to no submodule), as it is not being tested currently.

While at it, uniformize the capitalization in the list of tests.

---
 t/lib-submodule-update.sh | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

index ab30b2da24f..64fc6487dd9 100755
@@ -297,7 +297,7 @@ test_submodule_content () {
 # - Directory containing tracked files replaced by submodule
 # - Submodule replaced by tracked files in directory
 # - Submodule replaced by tracked file with the same name
-# - tracked file replaced by submodule
+# - Tracked file replaced by submodule
 #
 # The default is that submodule contents aren't changed until "git submodule
 # update" is run. And even then that command doesn't delete the work tree of

@@ -621,12 +621,13 @@ test_submodule_forced_switch () {
 # - Directory containing tracked files replaced by submodule
 # - Submodule replaced by tracked files in directory
 # - Submodule replaced by tracked file with the same name
-# - tracked file replaced by submodule
+# - Tracked file replaced by submodule
 #
 # New test cases
 # - Removing a submodule with a git directory absorbs the submodules
 #   git directory first into the superproject.
 # - Switching from no submodule to nested submodules
+# - Switching from nested submodules to no submodule
 
 # Internal function; use test_submodule_switch_recursing_with_args() or
 # test_submodule_forced_switch_recursing_with_args() instead.

@@ -760,6 +761,21 @@ test_submodule_recursing_with_args_common() {
 		)
 	'
 
+	# Switching to a commit without nested submodules removes their worktrees
+	test_expect_success "$command: worktrees of nested submodules are removed" '
+		prolog &&
+		reset_work_tree_to_interested add_nested_sub &&
+		(
+			cd submodule_update &&
+			git branch -t no_submodule origin/no_submodule &&
+			$command no_submodule &&
+			test_superproject_content origin/no_submodule &&
+			! test_path_is_dir sub1 &&
+			test_must_fail git config -f .git/modules/sub1/config core.worktree &&
+			test_must_fail git config -f .git/modules/sub1/modules/sub2/config core.worktree
+		)
+	'
+
 	########################## Modified submodule #########################
 	# Updating a submodule sha1 updates the submodule's work tree
 	test_expect_success "$command: modified submodule updates submodule work tree" '

gitgitgadget