clone: ignore invalid local refs in remote

From: Derrick Stolee <[email protected]>

When cloning directly from a local repository, we load a list of refs based on scanning the $GIT_DIR/refs/ directory of the "server" repository. If files exist in that directory that do not parse as hexadecimal hashes, then the ref array used by write_remote_refs() ends up with some entries with null OIDs. This causes us to hit a BUG() statement in ref_transaction_create():

  BUG: create called without valid new_oid

This BUG() call used to be a die() until 033abf97f (Replace all die("BUG: ...") calls by BUG() ones, 2018-05-02). Before that, the die() was added by f04c5b552 (ref_transaction_create(): check that new_sha1 is valid, 2015-02-17).

The original report for this bug [1] mentioned that this problem did not exist in Git 2.27.0. The failure bisects unsurprisingly to 968f12fda (refs: turn on GIT_REF_PARANOIA by default, 2021-09-24). When GIT_REF_PARANOIA is enabled, this case always fails as far back as I am able to successfully compile and test the Git codebase.

[1] https://github.com/git-for-windows/git/issues/3781

There are two approaches to consider here. One would be to remove this BUG() statement in favor of returning with an error. There are only two callers to ref_transaction_create(), so this would have a limited impact.

The other approach would be to add special casing in 'git clone' to avoid this faulty input to the method.

While I originally started with changing 'git clone', I decided that modifying ref_transaction_create() was a more complete solution. This prevents failing with a BUG() statement when we already have a good way to report an error (including a reason for that error) within the method. Both callers properly check the return value and die() with the error message, so this is an appropriate direction.

The added test helps check against a regression, but does check that our intended error message is handled correctly.

---
clone: ignore invalid local refs in remote

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1214%2Fderrickstolee%2Frefs-bug-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1214/derrickstolee/refs-bug-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1214

 refs.c                 | 6 ++++--
 t/t5605-clone-local.sh | 9 +++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

index 1a964505f92..f300f83e4d4 100644
@@ -1111,8 +1111,10 @@ int ref_transaction_create(struct ref_transaction *transaction,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err)
 {
-	if (!new_oid || is_null_oid(new_oid))
-		BUG("create called without valid new_oid");
+	if (!new_oid || is_null_oid(new_oid)) {
+		strbuf_addf(err, "'%s' has a null OID", refname);
+		return 1;
+	}
 	return ref_transaction_update(transaction, refname, new_oid,
 				      null_oid(), flags, msg, err);
 }

index 7d63365f93a..21ab6192839 100755
@@ -141,4 +141,13 @@ test_expect_success 'cloning locally respects "-u" for fetching refs' '
 	test_must_fail git clone --bare -u false a should_not_work.git
 '
 
+test_expect_success 'local clone from repo with corrupt refs fails gracefully' '
+	git init corrupt &&
+	test_commit -C corrupt one &&
+	echo a >corrupt/.git/refs/heads/topic &&
+
+	test_must_fail git clone corrupt working 2>err &&
+	grep "has a null OID" err
+'
+
 test_done

base-commit: 11cfe552610386954886543f5de87dcc49ad5735

gitgitgadget

If I change this test to clone from "$PWD/corrupt" instead we get as far as dying in "not our ref" in upload-pack", after the "client" said "want 0000....".

I don't see anything wrong with this narrow fix, but I wonder how close we are to just doing something useful here for the user. I.e. if we can't parse_object() what we're about to "copy" to e.g. error() on that, but carry on cloning the rest if possible.

We should probably always return a non-zero at the end, suggesting the user omit the bad ref with a refspec. But if we did all that we could usefully recover partially corrupt-repos in this state...

After seeing the title, I expected that cloning from such a repository with cruft in .git/refs/ directory would issue a warning and succeed without these non-ref files.

But that is not what is happening here?

Sorry, this title of the commit message is stale from a version where I started making the clones succeed (but without these bad refs). I changed my mind to only switch BUG() to die() to avoid giving the impression that we have a "matching" repo after the clone.

Perhaps the commit could instead start with

clone: die() instead of BUG() on bad refs

?

Thanks,
-Stolee

Ah, that figures ;-)

Yeah, it would be the title I would have been happy with.

From: Derrick Stolee <[email protected]>

When cloning directly from a local repository, we load a list of refs based on scanning the $GIT_DIR/refs/ directory of the "server" repository. If files exist in that directory that do not parse as hexadecimal hashes, then the ref array used by write_remote_refs() ends up with some entries with null OIDs. This causes us to hit a BUG() statement in ref_transaction_create():

  BUG: create called without valid new_oid

This BUG() call used to be a die() until 033abf97f (Replace all die("BUG: ...") calls by BUG() ones, 2018-05-02). Before that, the die() was added by f04c5b552 (ref_transaction_create(): check that new_sha1 is valid, 2015-02-17).

The original report for this bug [1] mentioned that this problem did not exist in Git 2.27.0. The failure bisects unsurprisingly to 968f12fda (refs: turn on GIT_REF_PARANOIA by default, 2021-09-24). When GIT_REF_PARANOIA is enabled, this case always fails as far back as I am able to successfully compile and test the Git codebase.

[1] https://github.com/git-for-windows/git/issues/3781

There are two approaches to consider here. One would be to remove this BUG() statement in favor of returning with an error. There are only two callers to ref_transaction_create(), so this would have a limited impact.

The other approach would be to add special casing in 'git clone' to avoid this faulty input to the method.

While I originally started with changing 'git clone', I decided that modifying ref_transaction_create() was a more complete solution. This prevents failing with a BUG() statement when we already have a good way to report an error (including a reason for that error) within the method. Both callers properly check the return value and die() with the error message, so this is an appropriate direction.

The added test helps check against a regression, but does check that our intended error message is handled correctly.

---
clone: ignore invalid local refs in remote

Update in v2:

* Subject line is improved to better match the message and fix.
* I did not explore Ævar's idea to optimistically continue the clone as much as possible. That seems like a bigger change that could be investigated independently.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1214%2Fderrickstolee%2Frefs-bug-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1214/derrickstolee/refs-bug-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1214

Range-diff vs v1:

1: 6c9e85c5a98 ! 1: 9801123c534 clone: ignore invalid local refs in remote
@@ Metadata
Author: Derrick Stolee <[email protected]>

## Commit message ##
- clone: ignore invalid local refs in remote
+ clone: die() instead of BUG() on bad refs

When cloning directly from a local repository, we load a list of refs based on scanning the $GIT_DIR/refs/ directory of the "server"

 refs.c                 | 6 ++++--
 t/t5605-clone-local.sh | 9 +++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

index 1a964505f92..f300f83e4d4 100644
@@ -1111,8 +1111,10 @@ int ref_transaction_create(struct ref_transaction *transaction,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err)
 {
-	if (!new_oid || is_null_oid(new_oid))
-		BUG("create called without valid new_oid");
+	if (!new_oid || is_null_oid(new_oid)) {
+		strbuf_addf(err, "'%s' has a null OID", refname);
+		return 1;
+	}
 	return ref_transaction_update(transaction, refname, new_oid,
 				      null_oid(), flags, msg, err);
 }

index 7d63365f93a..21ab6192839 100755
@@ -141,4 +141,13 @@ test_expect_success 'cloning locally respects "-u" for fetching refs' '
 	test_must_fail git clone --bare -u false a should_not_work.git
 '
 
+test_expect_success 'local clone from repo with corrupt refs fails gracefully' '
+	git init corrupt &&
+	test_commit -C corrupt one &&
+	echo a >corrupt/.git/refs/heads/topic &&
+
+	test_must_fail git clone corrupt working 2>err &&
+	grep "has a null OID" err
+'
+
 test_done

base-commit: 11cfe552610386954886543f5de87dcc49ad5735

gitgitgadget