threads / patch / 34785

v2, 5 partst6050-replace: add test to clean up all the replace refs

Subject: [PATCH v2 4/5] t6050-replace: add test to clean up all the replace refs

## tl;dr

13 messages between Aug 27, 2013 and Aug 30, 2013. Diffs are folded; open one to read it.

replies: 12people: 3as markdown or json

Christian Couder· Aug 27, 2013, 19:48 UTC · lore

[PATCH v2 0/5] Check replacement object type and minor updates

The only changes compared to the previous version are those suggested by Johannes:

- the error message is now:
	"Objects must be of the same type.\n"
	"'%s' points to a replaced object of type '%s'\n"
	"while '%s' points to a replacement object of type '%s'."
- the test uses "." instead of "'\''"
Only patchs 1/5 and 3/5 were changed.
Christian Couder (5):
  replace: forbid replacing an object with one of a different type
  Documentation/replace: state that objects must be of the same type
  t6050-replace: test that objects are of the same type
  t6050-replace: add test to clean up all the replace refs
  Documentation/replace: add Creating Replacement Objects section
 Documentation/git-replace.txt | 23 ++++++++++++++++++++---
 builtin/replace.c             | 10 ++++++++++
 t/t6050-replace.sh            | 19 +++++++++++++++++++
 3 files changed, 49 insertions(+), 3 deletions(-)
-- 
1.8.4.rc1.26.gdd51ee9
Christian Couder· Aug 27, 2013, 19:48 UTC · re: Christian Couder · lore

[PATCH v2 1/5] replace: forbid replacing an object with one of a different type

Users replacing an object with one of a different type were not prevented to do so, even if it was obvious, and stated in the doc, that bad things would result from doing that.

To avoid mistakes, it is better to just forbid that though.

There is no case where one object can be replaced with one of a different type while keeping the history valid, because:

* Annotated tags contain the type of the tagged object.
* The tree/parent lines in commits must be a tree and commits, resp.
* The object types referred to by trees are specified in the 'mode'
  field:
    100644 and 100755    blob
    160000               commit
    040000               tree
  (these are the only valid modes)
* Blobs don't point at anything.
The doc will be updated in a later patch.
Acked-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 builtin/replace.c | 10 ++++++++++
 1 file changed, 10 insertions(+)
Show changes to builtin/replace.c +10 −0
diff --git a/builtin/replace.c b/builtin/replace.c
index 59d3115..9a94769 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -85,6 +85,7 @@ static int replace_object(const char *object_ref, const char *replace_ref,
 			  int force)
 {
 	unsigned char object[20], prev[20], repl[20];
+	enum object_type obj_type, repl_type;
 	char ref[PATH_MAX];
 	struct ref_lock *lock;
 
@@ -100,6 +101,15 @@ static int replace_object(const char *object_ref, const char *replace_ref,
 	if (check_refname_format(ref, 0))
 		die("'%s' is not a valid ref name.", ref);
 
+	obj_type = sha1_object_info(object, NULL);
+	repl_type = sha1_object_info(repl, NULL);
+	if (obj_type != repl_type)
+		die("Objects must be of the same type.\n"
+		    "'%s' points to a replaced object of type '%s'\n"
+		    "while '%s' points to a replacement object of type '%s'.",
+		    object_ref, typename(obj_type),
+		    replace_ref, typename(repl_type));
+
 	if (read_ref(ref, prev))
 		hashclr(prev);
 	else if (!force)

-- 
1.8.4.rc1.26.gdd51ee9
Junio C Hamano· Aug 27, 2013, 20:30 UTC · re: Christian Couder · lore

Re: [PATCH v2 1/5] replace: forbid replacing an object with one of a different type

Christian Couder <chriscool@tuxfamily.org> writes:
Show 10 quoted lines
> Users replacing an object with one of a different type were not
> prevented to do so, even if it was obvious, and stated in the doc,
> that bad things would result from doing that.
>
> To avoid mistakes, it is better to just forbid that though.
>
> There is no case where one object can be replaced with one of a
> different type while keeping the history valid, because:
>
> * Annotated tags contain the type of the tagged object.

If you replace the tagged object and the tag at the same time, wouldn't that make the resulting history valid again?

Granted, there may not be a strong reason to reuse the object name of the tagged object in such a case, but this "there may not be" is merely "I do not think of offhand", so I am not sure what workflow of other people we are breaking with this change. A light-weight tag may already point at the tagged object (in other words, the object name of the tagged object is known to the outside world) and that could be a reason why you would need to reuse the object name of that object while changing its type.

I dunno.
Thomas Rast· Aug 28, 2013, 07:31 UTC · re: Junio C Hamano · lore

Re: [PATCH v2 1/5] replace: forbid replacing an object with one of a different type

Junio C Hamano <gitster@pobox.com> writes:
Show 26 quoted lines
> Christian Couder <chriscool@tuxfamily.org> writes:
>
>> Users replacing an object with one of a different type were not
>> prevented to do so, even if it was obvious, and stated in the doc,
>> that bad things would result from doing that.
>>
>> To avoid mistakes, it is better to just forbid that though.
>>
>> There is no case where one object can be replaced with one of a
>> different type while keeping the history valid, because:
>>
>> * Annotated tags contain the type of the tagged object.
>
> If you replace the tagged object and the tag at the same time,
> wouldn't that make the resulting history valid again?
>
> Granted, there may not be a strong reason to reuse the object name
> of the tagged object in such a case, but this "there may not be" is
> merely "I do not think of offhand", so I am not sure what workflow
> of other people we are breaking with this change.  A light-weight
> tag may already point at the tagged object (in other words, the
> object name of the tagged object is known to the outside world) and
> that could be a reason why you would need to reuse the object name
> of that object while changing its type.
>
> I dunno.

Hrm, you're right, that's a flaw in my logic. You could do the same in all other cases too, e.g. replace a tree so that an entry is of a different type and at the same time change the type of the object itself. You however have to carefully go through all objects that refer to the one that was replaced, and fix the type in all of them.

It still seems an extremely unsafe thing to do with trees: especially for small trees there is a small probability that you will generate the same tree again in the future (by having the same blobs in the directory again) and record it as a subtree or the 'tree' field of a commit. The history would then again be invalid.

Should we add a --force flag of some sort to allow the user to do this, while keeping the normal safety checks?

-- 
Thomas Rast
trast@{inf,student}.ethz.ch
Junio C Hamano· Aug 28, 2013, 19:32 UTC · re: Thomas Rast · lore

Re: [PATCH v2 1/5] replace: forbid replacing an object with one of a different type

Thomas Rast <trast@inf.ethz.ch> writes:
Show 10 quoted lines
> Hrm, you're right, that's a flaw in my logic.  You could do the same in
> all other cases too, e.g. replace a tree so that an entry is of a
> different type and at the same time change the type of the object
> itself.  You however have to carefully go through all objects that refer
> to the one that was replaced, and fix the type in all of them.
>
> It still seems an extremely unsafe thing to do with trees...
>  ...
> Should we add a --force flag of some sort to allow the user to do this,
> while keeping the normal safety checks?

As long as we do not forbid such an unusual replacement on the reading side, we won't break people who are more inventive than we are (I am not convinced that we know people's workflow well enough to definitively say that no sane workflow, which benefits from being able to replace an object with another from a different type, exists).

Preventing "git replace" wrapper from creating such a replacement by default will make it harder to do and may reduce mistakes, without breaking them too much, I think.

Christian Couder· Aug 29, 2013, 06:29 UTC · re: Junio C Hamano · lore

Re: [PATCH v2 1/5] replace: forbid replacing an object with one of a different type

From: Junio C Hamano <gitster@pobox.com>
Show 24 quoted lines
>
> Thomas Rast <trast@inf.ethz.ch> writes:
> 
>> Hrm, you're right, that's a flaw in my logic.  You could do the same in
>> all other cases too, e.g. replace a tree so that an entry is of a
>> different type and at the same time change the type of the object
>> itself.  You however have to carefully go through all objects that refer
>> to the one that was replaced, and fix the type in all of them.
>>
>> It still seems an extremely unsafe thing to do with trees...
>>  ...
>> Should we add a --force flag of some sort to allow the user to do this,
>> while keeping the normal safety checks?
> 
> As long as we do not forbid such an unusual replacement on the
> reading side, we won't break people who are more inventive than we
> are (I am not convinced that we know people's workflow well enough
> to definitively say that no sane workflow, which benefits from being
> able to replace an object with another from a different type,
> exists).
> 
> Preventing "git replace" wrapper from creating such a replacement by
> default will make it harder to do and may reduce mistakes, without
> breaking them too much, I think.

I agree. It is always possible to create replacement refs using "git update-ref" if one really wants to.

What about using the following in the commit message or in the documentation:

------------------------------------------
DISCUSSION

If one object is replaced with one of a different type, the only way to keep the history valid is to also replace all the other objects that point to the replaced object. That's because:

* Annotated tags contain the type of the tagged object.
* The tree/parent lines in commits must be a tree and commits, resp.
* The object types referred to by trees are specified in the 'mode'
  field:
    100644 and 100755    blob
    160000               commit
    040000               tree
  (these are the only valid modes)
* Blobs don't point at anything.

But if all the objects that point to an object, called O, are to be replaced, then in most cases object O probably doesn't need to be replaced. It's probably sufficient to create the new object, called O2, that would replace object O and to replace all the objects pointing to object O with objects pointing to O2.

The only case where someone might really want to replace object 0, with an object O2 of a different type, and all the objects pointing to it, is if it's really important, perhaps for external reasons, to have object O's SHA1 point to O2.

And anyway, if one really wants to do that, it can still be done using "git update-ref".

------------------------------------------

Thanks, Christian.

Junio C Hamano· Aug 29, 2013, 16:04 UTC · re: Christian Couder · lore

Re: [PATCH v2 1/5] replace: forbid replacing an object with one of a different type

Christian Couder <chriscool@tuxfamily.org> writes:
Show 5 quoted lines
> But if all the objects that point to an object, called O, are to be
> replaced, then in most cases object O probably doesn't need to be
> replaced. It's probably sufficient to create the new object, called
> O2, that would replace object O and to replace all the objects
> pointing to object O with objects pointing to O2.
Hmmmm.

What the above says, with "probably" and "most cases", can easily inferred by anybody remotely intelligent, and the only reason to have something like the above paragraph would be if it assures that the statement holds without these qualifications to weaken it, which it doesn't. I am not sure this paragraph adds much value.

> The only case where someone might really want to replace object 0,
> with an object O2 of a different type, and all the objects pointing to
> it, is if it's really important, perhaps for external reasons, to have
> object O's SHA1 point to O2.
The same comment applies here.
> And anyway, if one really wants to do that, it can still be done using
> "git update-ref".

And I really do not think this sentence is right---you can justify with the same sentence to remove "git replace" wrapper.

The earlier suggestion to bypass the new hand-holding with "--force" is more sensible, I think.

Christian Couder· Aug 30, 2013, 05:41 UTC · re: Junio C Hamano · lore

Re: [PATCH v2 1/5] replace: forbid replacing an object with one of a different type

From: Junio C Hamano <gitster@pobox.com>
Show 29 quoted lines
>
> Christian Couder <chriscool@tuxfamily.org> writes:
> 
>> But if all the objects that point to an object, called O, are to be
>> replaced, then in most cases object O probably doesn't need to be
>> replaced. It's probably sufficient to create the new object, called
>> O2, that would replace object O and to replace all the objects
>> pointing to object O with objects pointing to O2.
> 
> Hmmmm.
> 
> What the above says, with "probably" and "most cases", can easily
> inferred by anybody remotely intelligent, and the only reason to
> have something like the above paragraph would be if it assures that
> the statement holds without these qualifications to weaken it, which
> it doesn't.  I am not sure this paragraph adds much value.
> 
>> The only case where someone might really want to replace object 0,
>> with an object O2 of a different type, and all the objects pointing to
>> it, is if it's really important, perhaps for external reasons, to have
>> object O's SHA1 point to O2.
> 
> The same comment applies here.
> 
>> And anyway, if one really wants to do that, it can still be done using
>> "git update-ref".
> 
> And I really do not think this sentence is right---you can justify
> with the same sentence to remove "git replace" wrapper.

Ok, so the commit message (excluding the Acked-by and Signed-off-by) will be only:

------------------------------------------

Users replacing an object with one of a different type were not prevented to do so, even if it was obvious, and stated in the doc, that bad things would result from doing that.

To avoid mistakes, it is better to just forbid that though.

If one object is replaced with one of a different type, the only way to keep the history valid is to also replace all the other objects that point to the replaced object. That's because:

* Annotated tags contain the type of the tagged object.
* The tree/parent lines in commits must be a tree and commits, resp.
* The object types referred to by trees are specified in the 'mode'
  field:
    100644 and 100755    blob
    160000               commit
    040000               tree
  (these are the only valid modes)
* Blobs don't point at anything.
The doc will be updated in a later patch.
------------------------------------------
> The earlier suggestion to bypass the new hand-holding with "--force"
> is more sensible, I think.

There is already a --force option, but I can add a --force-type in a another patch.

Thanks, Christian.

Junio C Hamano· Aug 30, 2013, 16:52 UTC · re: Christian Couder · lore

Re: [PATCH v2 1/5] replace: forbid replacing an object with one of a different type

Christian Couder <chriscool@tuxfamily.org> writes:
> There is already a --force option, but I can add a --force-type in a
> another patch.

Oh, that was not what I meant. As long as existing --force override this check, that should be sufficient and more preferrable than yet another kind of "force".

Thanks.
Christian Couder· Aug 27, 2013, 19:48 UTC · re: Christian Couder · lore

[PATCH v2 2/5] Documentation/replace: state that objects must be of the same type

A previous patch ensures that both the replaced and the replacement objects passed to git replace must be of the same type.

While at it state that there is no other restriction on both objects.
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 Documentation/git-replace.txt | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
Show changes to Documentation/git-replace.txt +4 −3
diff --git a/Documentation/git-replace.txt b/Documentation/git-replace.txt
index e0b4057..aa66d27 100644
--- a/Documentation/git-replace.txt
+++ b/Documentation/git-replace.txt
@@ -20,6 +20,9 @@ The name of the 'replace' reference is the SHA-1 of the object that is
 replaced. The content of the 'replace' reference is the SHA-1 of the
 replacement object.
 
+The replaced object and the replacement object must be of the same type.
+There is no other restriction on them.
+
 Unless `-f` is given, the 'replace' reference must not yet exist.
 
 Replacement references will be used by default by all Git commands
@@ -69,9 +72,7 @@ go back to a replaced commit will move the branch to the replacement
 commit instead of the replaced commit.
 
 There may be other problems when using 'git rev-list' related to
-pending objects. And of course things may break if an object of one
-type is replaced by an object of another type (for example a blob
-replaced by a commit).
+pending objects.
 
 SEE ALSO
 --------

-- 
1.8.4.rc1.26.gdd51ee9
Christian Couder· Aug 27, 2013, 19:48 UTC · re: Christian Couder · lore

[PATCH v2 3/5] t6050-replace: test that objects are of the same type

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 t/t6050-replace.sh | 13 +++++++++++++
 1 file changed, 13 insertions(+)
Show changes to t/t6050-replace.sh +13 −0
diff --git a/t/t6050-replace.sh b/t/t6050-replace.sh
index decdc33..5c352c4 100755
--- a/t/t6050-replace.sh
+++ b/t/t6050-replace.sh
@@ -263,4 +263,17 @@ test_expect_success 'not just commits' '
 	test_cmp file.replaced file
 '
 
+test_expect_success 'replaced and replacement objects must be of the same type' '
+	test_must_fail git replace mytag $HASH1 2>err &&
+	grep "mytag. points to a replaced object of type .tag" err &&
+	grep "$HASH1. points to a replacement object of type .commit" err &&
+	test_must_fail git replace HEAD^{tree} HEAD~1 2>err &&
+	grep "HEAD^{tree}. points to a replaced object of type .tree" err &&
+	grep "HEAD~1. points to a replacement object of type .commit" err &&
+	BLOB=$(git rev-parse :file) &&
+	test_must_fail git replace HEAD^ $BLOB 2>err &&
+	grep "HEAD^. points to a replaced object of type .commit" err &&
+	grep "$BLOB. points to a replacement object of type .blob" err
+'
+
 test_done

-- 
1.8.4.rc1.26.gdd51ee9
Christian Couder· Aug 27, 2013, 19:48 UTC · re: Christian Couder · lore
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 t/t6050-replace.sh | 6 ++++++
 1 file changed, 6 insertions(+)
Show changes to t/t6050-replace.sh +6 −0
diff --git a/t/t6050-replace.sh b/t/t6050-replace.sh
index 5c352c4..05be228 100755
--- a/t/t6050-replace.sh
+++ b/t/t6050-replace.sh
@@ -276,4 +276,10 @@ test_expect_success 'replaced and replacement objects must be of the same type'
 	grep "$BLOB. points to a replacement object of type .blob" err
 '
 
+test_expect_success 'replace ref cleanup' '
+	test -n "$(git replace)" &&
+	git replace -d $(git replace) &&
+	test -z "$(git replace)"
+'
+
 test_done

-- 
1.8.4.rc1.26.gdd51ee9
Christian Couder· Aug 27, 2013, 19:48 UTC · re: Christian Couder · lore

[PATCH v2 5/5] Documentation/replace: add Creating Replacement Objects section

There were no hints in the documentation about how to create replacement objects.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 Documentation/git-replace.txt | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
Show changes to Documentation/git-replace.txt +16 −0
diff --git a/Documentation/git-replace.txt b/Documentation/git-replace.txt
index aa66d27..736b48c 100644
--- a/Documentation/git-replace.txt
+++ b/Documentation/git-replace.txt
@@ -64,6 +64,19 @@ OPTIONS
 	Typing "git replace" without arguments, also lists all replace
 	refs.
 
+CREATING REPLACEMENT OBJECTS
+----------------------------
+
+linkgit:git-filter-branch[1], linkgit:git-hash-object[1] and
+linkgit:git-rebase[1], among other git commands, can be used to create
+replacement objects from existing objects.
+
+If you want to replace many blobs, trees or commits that are part of a
+string of commits, you may just want to create a replacement string of
+commits and then only replace the commit at the tip of the target
+string of commits with the commit at the tip of the replacement string
+of commits.
+
 BUGS
 ----
 Comparing blobs or trees that have been replaced with those that
@@ -76,6 +89,9 @@ pending objects.
 
 SEE ALSO
 --------
+linkgit:git-hash-object[1]
+linkgit:git-filter-branch[1]
+linkgit:git-rebase[1]
 linkgit:git-tag[1]
 linkgit:git-branch[1]
 linkgit:git[1]

-- 
1.8.4.rc1.26.gdd51ee9

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