threads / patch / 63961

patchsubmodule: truncate the oid when fetchig commits

Subject: [PATCH] submodule: truncate the oid when fetchig commits

## tl;dr

6 messages between Aug 14, 2025 and Aug 19, 2025. Diffs are folded; open one to read it.

replies: 5people: 3as markdown or json

Michael Schroeder· Aug 14, 2025, 15:06 UTC · lore

If a submodule uses a different hash algorithm than used in the main repository, the recorded submodule commit is padded with zeros. This is usually not a problem as the default is to do submodule clones non-shallow and the commit can be found in the local objects.

But this is not true if the --shallow-submodules clone option is used (or the --depth option in the submodule update call). In this case, the commit is often not reachable and a fetch of the specific commit is done. But the fetch cannot deal with the zero padding and interprets the commit as a name. Because of this, the checkout will fail.

Implement truncation of the recorded commit to the correct size corresponding to the hash algorithm used in the submodule.

Signed-off-by: Michael Schroeder <mls@suse.de>
---
 builtin/submodule--helper.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

Show changes to builtin/submodule--helper.c +12 −6

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 07a1935cbe..ef21eb42b8 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -72,7 +72,7 @@ static char *resolve_relative_url(const char *rel_url, const char *up_path, int
 	return resolved_url;
 }
 
-static int get_default_remote_submodule(const char *module_path, char **default_remote)
+static int get_default_remote_submodule(const char *module_path, char **default_remote, const struct git_hash_algo **hash_algo)
 {
 	const struct submodule *sub;
 	struct repository subrepo;
@@ -106,6 +106,9 @@ static int get_default_remote_submodule(const char *module_path, char **default_
 
 	*default_remote = xstrdup(remote_name);
 
+	if (hash_algo)
+		*hash_algo = subrepo.hash_algo;
+
 	repo_clear(&subrepo);
 	free(url);
 
@@ -1272,7 +1275,7 @@ static void sync_submodule(const char *path, const char *prefix,
 		goto cleanup;
 
 	strbuf_reset(&sb);
-	code = get_default_remote_submodule(path, &default_remote);
+	code = get_default_remote_submodule(path, &default_remote, NULL);
 	if (code)
 		exit(code);
 
@@ -2319,16 +2322,19 @@ static int fetch_in_submodule(const char *module_path, int depth, int quiet,
 	if (depth)
 		strvec_pushf(&cp.args, "--depth=%d", depth);
 	if (oid) {
-		char *hex = oid_to_hex(oid);
+		char hexbuffer[GIT_MAX_HEXSZ + 1];
+		char *hex = oid_to_hex_r(hexbuffer, oid);
 		char *remote;
+		const struct git_hash_algo *hash_algo = NULL;
 		int code;
 
-		code = get_default_remote_submodule(module_path, &remote);
+		code = get_default_remote_submodule(module_path, &remote, &hash_algo);
 		if (code) {
 			child_process_clear(&cp);
 			return code;
 		}
-
+		if (hash_algo)
+			hex[hash_algo->hexsz] = 0;	/* truncate to correct size */
 		strvec_pushl(&cp.args, remote, hex, NULL);
 		free(remote);
 	}
@@ -2635,7 +2641,7 @@ static int update_submodule(struct update_data *update_data)
 		char *remote_ref;
 		int code;
 
-		code = get_default_remote_submodule(update_data->sm_path, &remote_name);
+		code = get_default_remote_submodule(update_data->sm_path, &remote_name, NULL);
 		if (code)
 			return code;
 		code = remote_submodule_branch(update_data->sm_path, &branch);

-- 
2.50.0

Junio C Hamano· Aug 14, 2025, 15:42 UTC · re: Michael Schroeder · lore

Re: [PATCH] submodule: truncate the oid when fetchig commits

Michael Schroeder <mls@suse.de> writes:

> If a submodule uses a different hash algorithm than used in
> the main repository, the recorded submodule commit is padded
> with zeros.

Hmph. I suspect that this "extra zero bits" would happen when your superproject uses a longer hash (e.g. SHA-256) than the hash used by the submodule you are fetching? If the arrangement is reversed, would we see a different and possibly an even severe problem?

I do not mean to say that you need to solve the problem in both direction at all. But perhaps

    ... uses a hash algorithm with shorter hash length than what is
    used in the main repository, ...

would clarify what problem you are solving in this patch better.

Show 13 quoted lines

> This is usually not a problem as the default is to
> do submodule clones non-shallow and the commit can be found
> in the local objects.
>
> But this is not true if the --shallow-submodules clone option is
> used (or the --depth option in the submodule update call).
> In this case, the commit is often not reachable and a fetch of the
> specific commit is done. But the fetch cannot deal with the zero
> padding and interprets the commit as a name. Because of this,
> the checkout will fail.
>
> Implement truncation of the recorded commit to the correct size
> corresponding to the hash algorithm used in the submodule.

When we use the verb "truncate" in the context of this project, I think we always use the word to mean making it shorter than its natural length. But in the above, the word is used to remove excess so that we use it at its natural length. I cannot offhand offer a better phrase, so I'll let it go for now, but suggestions for better wording is very much welcome.

Show 48 quoted lines

>  builtin/submodule--helper.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
> index 07a1935cbe..ef21eb42b8 100644
> --- a/builtin/submodule--helper.c
> +++ b/builtin/submodule--helper.c
> @@ -72,7 +72,7 @@ static char *resolve_relative_url(const char *rel_url, const char *up_path, int
>  	return resolved_url;
>  }
>  
> -static int get_default_remote_submodule(const char *module_path, char **default_remote)
> +static int get_default_remote_submodule(const char *module_path, char **default_remote, const struct git_hash_algo **hash_algo)
>  {
>  	const struct submodule *sub;
>  	struct repository subrepo;
> @@ -106,6 +106,9 @@ static int get_default_remote_submodule(const char *module_path, char **default_
>  
>  	*default_remote = xstrdup(remote_name);
>  
> +	if (hash_algo)
> +		*hash_algo = subrepo.hash_algo;
> +
>  	repo_clear(&subrepo);
>  	free(url);
>  
> @@ -1272,7 +1275,7 @@ static void sync_submodule(const char *path, const char *prefix,
>  		goto cleanup;
>  
>  	strbuf_reset(&sb);
> -	code = get_default_remote_submodule(path, &default_remote);
> +	code = get_default_remote_submodule(path, &default_remote, NULL);
>  	if (code)
>  		exit(code);
>  
> @@ -2319,16 +2322,19 @@ static int fetch_in_submodule(const char *module_path, int depth, int quiet,
>  	if (depth)
>  		strvec_pushf(&cp.args, "--depth=%d", depth);
>  	if (oid) {
> -		char *hex = oid_to_hex(oid);
> +		char hexbuffer[GIT_MAX_HEXSZ + 1];
> +		char *hex = oid_to_hex_r(hexbuffer, oid);
>  		char *remote;
> +		const struct git_hash_algo *hash_algo = NULL;
>  		int code;
>  
> -		code = get_default_remote_submodule(module_path, &remote);
> +		code = get_default_remote_submodule(module_path, &remote, &hash_algo);

It feels highly unsatisfying to have oid_to_hex_r() blindly read and convert oid to hex (assuming the hash length of the current repository) without using the knowledge of how much of that object name is valid. We obtain that knowledge shortly after we call oid_to_hex_r() in hash_algo here. That is the only reason why we manually have to truncate below, isn't it?

In other words, shouldn't we be finding out hash_algo first, and then use something like hash_to_hex_algop_r() to convert only the valid bits of the oid into the buffer?. That way, we do not have to manually add '\0' in this function below.

Show 19 quoted lines

>  		if (code) {
>  			child_process_clear(&cp);
>  			return code;
>  		}
> -
> +		if (hash_algo)
> +			hex[hash_algo->hexsz] = 0;	/* truncate to correct size */
>  		strvec_pushl(&cp.args, remote, hex, NULL);
>  		free(remote);
>  	}
> @@ -2635,7 +2641,7 @@ static int update_submodule(struct update_data *update_data)
>  		char *remote_ref;
>  		int code;
>  
> -		code = get_default_remote_submodule(update_data->sm_path, &remote_name);
> +		code = get_default_remote_submodule(update_data->sm_path, &remote_name, NULL);
>  		if (code)
>  			return code;
>  		code = remote_submodule_branch(update_data->sm_path, &branch);

Michael Schroeder· Aug 18, 2025, 09:15 UTC · re: Junio C Hamano · lore

Re: [PATCH v2] submodule: truncate the oid when fetchig commits

Hi git team,

here's the new version of the patch based on your comments:

From 3962b9cb065504f52875f728f03ecbf8dc1f23e2 Mon Sep 17 00:00:00 2001
From: Michael Schroeder <mls@suse.de>
Date: Thu, 14 Aug 2025 16:12:53 +0200
Subject: [PATCH v2] submodule: use subrepo algo when fetchig commits

If a submodule uses a hash algorithm with shorter hash length than what is used in the main repository, the recorded submodule commit is padded with zeros. This is usually not a problem as the default is to do submodule clones non-shallow and the commit can be found in the local objects.

Use the subrepo algorithm when converting the stored commit to hex so that the result matches the expected oid length of the submodule.

Signed-off-by: Michael Schroeder <mls@suse.de>
---
 builtin/submodule--helper.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

Show changes to builtin/submodule--helper.c +10 −5

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 07a1935cbe..5c2e96b517 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -72,7 +72,7 @@ static char *resolve_relative_url(const char *rel_url, const char *up_path, int
 	return resolved_url;
 }
 
-static int get_default_remote_submodule(const char *module_path, char **default_remote)
+static int get_default_remote_submodule(const char *module_path, char **default_remote, const struct git_hash_algo **hash_algo)
 {
 	const struct submodule *sub;
 	struct repository subrepo;
@@ -106,6 +106,9 @@ static int get_default_remote_submodule(const char *module_path, char **default_
 
 	*default_remote = xstrdup(remote_name);
 
+	if (hash_algo)
+		*hash_algo = subrepo.hash_algo;
+
 	repo_clear(&subrepo);
 	free(url);
 
@@ -1272,7 +1275,7 @@ static void sync_submodule(const char *path, const char *prefix,
 		goto cleanup;
 
 	strbuf_reset(&sb);
-	code = get_default_remote_submodule(path, &default_remote);
+	code = get_default_remote_submodule(path, &default_remote, NULL);
 	if (code)
 		exit(code);
 
@@ -2319,16 +2322,18 @@ static int fetch_in_submodule(const char *module_path, int depth, int quiet,
 	if (depth)
 		strvec_pushf(&cp.args, "--depth=%d", depth);
 	if (oid) {
-		char *hex = oid_to_hex(oid);
+		const struct git_hash_algo *hash_algo = NULL;
+		char *hex;
 		char *remote;
 		int code;
 
-		code = get_default_remote_submodule(module_path, &remote);
+		code = get_default_remote_submodule(module_path, &remote, &hash_algo);
 		if (code) {
 			child_process_clear(&cp);
 			return code;
 		}
 
+		hex = hash_to_hex_algop(oid->hash, hash_algo);
 		strvec_pushl(&cp.args, remote, hex, NULL);
 		free(remote);
 	}
@@ -2635,7 +2640,7 @@ static int update_submodule(struct update_data *update_data)
 		char *remote_ref;
 		int code;
 
-		code = get_default_remote_submodule(update_data->sm_path, &remote_name);
+		code = get_default_remote_submodule(update_data->sm_path, &remote_name, NULL);
 		if (code)
 			return code;
 		code = remote_submodule_branch(update_data->sm_path, &branch);

-- 
2.50.0

brian m. carlson· Aug 14, 2025, 22:16 UTC · re: Michael Schroeder · lore

Re: [PATCH] submodule: truncate the oid when fetchig commits

On 2025-08-14 at 15:06:32, Michael Schroeder wrote:

Show 5 quoted lines

> If a submodule uses a different hash algorithm than used in
> the main repository, the recorded submodule commit is padded
> with zeros. This is usually not a problem as the default is to
> do submodule clones non-shallow and the commit can be found
> in the local objects.

This should not even work at all. It may currently behave as you suggest when the main repository is SHA-256 and the submodule is SHA-1, but it will corrupt the data if the submodule is SHA-256 and the main repository is SHA-1, since then the data will be truncated.

The proper way for this to work is that the SHA-1 version of the repository stores submodules in their SHA-1 states and the SHA-256 version of the repository stores submodules in their SHA-256 states. Yes, this means that you have to convert submodules to the same algorithm, but that's required because trees are binary and not text.

-- 
brian m. carlson (they/them)
Toronto, Ontario, CA

Michael Schroeder· Aug 18, 2025, 09:30 UTC · re: brian m. carlson · lore

Re: [PATCH] submodule: truncate the oid when fetchig commits

On Thu, Aug 14, 2025 at 10:16:24PM +0000, brian m. carlson wrote:

Show 11 quoted lines

> On 2025-08-14 at 15:06:32, Michael Schroeder wrote:
> > If a submodule uses a different hash algorithm than used in
> > the main repository, the recorded submodule commit is padded
> > with zeros. This is usually not a problem as the default is to
> > do submodule clones non-shallow and the commit can be found
> > in the local objects.
> 
> This should not even work at all.  It may currently behave as you
> suggest when the main repository is SHA-256 and the submodule is SHA-1,
> but it will corrupt the data if the submodule is SHA-256 and the main
> repository is SHA-1, since then the data will be truncated.

But it works, and I'm pretty sure people already use it. If you have a sha1 main repo and a sha256 submodule, git will truncate the commit when recording the gitlink. The checkout done by git submodule update will work as it does the normal prefix matching.

If you have a sha256 main repo and a sha submodule, the recorded commit is padded with zero. The checkout will also work as git ignores the extra data since commit 52fca06db2 (object-names: support input of oids in any supported hash, 2023-10-01).

What doesn't work is if a shallow clone is done for the submodule. In that case the commit is not reachable and git tries a direct fetch. This fetch can be made to work if the commit was padded with zeros. If the commit was truncated, we would probably need some protocol extension to make the server do a prefix match for a "want" request.

> The proper way for this to work is that the SHA-1 version of the
> repository stores submodules in their SHA-1 states and the SHA-256
> version of the repository stores submodules in their SHA-256 states.

You mean by using "compatObjectFormat"? I couldn't make that work, but maybe I missed something. Anyway, I think this also will not work for shallow clones.

Cheers,
  Michael.

-- 
Michael Schroeder          SUSE Software Solutions Germany GmbH
mls@suse.de               GF: Ivo Totev HRB 36809, AG Nuernberg
main(_){while(_=~getchar())putchar(~_-1/(~(_|32)/13*2-11)*13);}

brian m. carlson· Aug 19, 2025, 00:45 UTC · re: Michael Schroeder · lore

Re: [PATCH] submodule: truncate the oid when fetchig commits

On 2025-08-18 at 09:30:51, Michael Schroeder wrote:

Show 17 quoted lines

> On Thu, Aug 14, 2025 at 10:16:24PM +0000, brian m. carlson wrote:
> > On 2025-08-14 at 15:06:32, Michael Schroeder wrote:
> > > If a submodule uses a different hash algorithm than used in
> > > the main repository, the recorded submodule commit is padded
> > > with zeros. This is usually not a problem as the default is to
> > > do submodule clones non-shallow and the commit can be found
> > > in the local objects.
> > 
> > This should not even work at all.  It may currently behave as you
> > suggest when the main repository is SHA-256 and the submodule is SHA-1,
> > but it will corrupt the data if the submodule is SHA-256 and the main
> > repository is SHA-1, since then the data will be truncated.
> 
> But it works, and I'm pretty sure people already use it. If you
> have a sha1 main repo and a sha256 submodule, git will truncate
> the commit when recording the gitlink. The checkout done by
> git submodule update will work as it does the normal prefix matching.

Unfortunately, that will break with the interoperability work. The protocol will learn to convert the object ID on the server side by announcing the mapping and when the object ID doesn't exist, the client will die because it can't remap the object and the process will fail. By doing that, you'll end up with a repository that you can never use interoperability code on, ever, without rewriting history. There's no way around this problem because we don't keep the object format in trees, so we can't distinguish between a SHA-256 submodule that happens to end in 24 zeros and a SHA-1 submodule.

The entire hash function transition has mandated exactly one object format on disk and in data structures from the very beginning:

    This affects both object names and object content -- both the names
    of objects and all references to other objects within an object are
    switched to the new hash function.

I apologize that I didn't think about this problem and make the code die on this case earlier, but it's not a supported configuration and it will absolutely break in the future. Sorry to be the bearer of bad news.

Show 7 quoted lines

> > The proper way for this to work is that the SHA-1 version of the
> > repository stores submodules in their SHA-1 states and the SHA-256
> > version of the repository stores submodules in their SHA-256 states.
> 
> You mean by using "compatObjectFormat"? I couldn't make that work,
> but maybe I missed something. Anyway, I think this also will not
> work for shallow clones.

There is interoperability code only for loose objects now. The code that handles packs and interoperability between repositories is in a branch on my remote. It's work that I'm doing for a talk at Git Merge and I will send it upstream when it's ready.

Right now, I'm working on shallow clones at the moment and then submodules are next.

-- 
brian m. carlson (they/them)
Toronto, Ontario, CA