On Sun, Nov 11 2018, Ævar Arnfjörð Bjarmason wrote:
> [CC-ing some of the people involved in recent threads about this]
>
> On Sun, Nov 11 2018, Nguyễn Thái Ngọc Duy wrote:
>
>> Since this topic has come up twice recently, I revisited this
>> "precious" thingy that I started four years ago and tried to see if I
>> could finally finish it. There are a couple things to be sorted out...
>>
>> A new attribute "precious" is added to indicate that certain files
>> have valuable content and should not be easily discarded even if they
>> are ignored or untracked (*).
>>
>> So far there are two parts of Git that are made aware of precious
>> files: "git clean" will leave precious files alone and unpack-trees.c
>> (i.e. merges and branch switches) will not overwrite
>> ignored-but-precious files.
>>
>> Is there any other parts of Git that should be made aware of this
>> "precious" attribute?
>>
>> Also while "precious" is a fun name, but it does not sound serious.
>> Any suggestions? Perhaps "valuable"?
>>
>> Very lightly tested. The patch is more to have something to discuss
>> than is bug free and ready to use.
>>
>> (*) Note that tracked files could be marked "precious" in the future
>> too although the exact semantics is not very clear since tracked
>> files are by default precious.
>>
>> But something like "index log" could use this to record all
>> changes to precious files instead of just "git add -p" changes,
>> for example. So these files are in a sense more precious than
>> other tracked files.
>>
>> Signed-off-by: Nguyễn Thái Ngọc Duy <pclo...@gmail.com>
>> ---
>> Documentation/git-clean.txt | 3 ++-
>> Documentation/gitattributes.txt | 13 +++++++++++++
>> attr.c | 9 +++++++++
>> attr.h | 2 ++
>> builtin/clean.c | 19 ++++++++++++++++---
>> unpack-trees.c | 3 ++-
>> 6 files changed, 44 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/git-clean.txt b/Documentation/git-clean.txt
>> index 03056dad0d..a9beadfb12 100644
>> --- a/Documentation/git-clean.txt
>> +++ b/Documentation/git-clean.txt
>> @@ -21,7 +21,8 @@ option is specified, ignored files are also removed. This
>> can, for
>> example, be useful to remove all build products.
>>
>> If any optional `<path>...` arguments are given, only those paths
>> -are affected.
>> +are affected. Ignored or untracked files with `precious` attributes
>> +are not removed.
>>
>> OPTIONS
>> -------
>> diff --git a/Documentation/gitattributes.txt
>> b/Documentation/gitattributes.txt
>> index b8392fc330..c722479bdc 100644
>> --- a/Documentation/gitattributes.txt
>> +++ b/Documentation/gitattributes.txt
>> @@ -1188,6 +1188,19 @@ If this attribute is not set or has an invalid value,
>> the value of the
>> (See linkgit:git-config[1]).
>>
>>
>> +Precious files
>> +~~~~~~~~~~~~~~~~~~~~~~~~
>> +
>> +`precious`
>> +^^^^^^^^^^
>> +
>> +This attribute is set on files to indicate that their content is
>> +valuable. Many commands will behave slightly different on precious
>> +files. linkgit:git-clean[1] will leave precious files alone. Merging
>> +and branch switching will not silently overwrite ignored files that
>> +are marked "precious".
>> +
>> +
>> USING MACRO ATTRIBUTES
>> ----------------------
>>
>> diff --git a/attr.c b/attr.c
>> index 60d284796d..d06ca0ae4b 100644
>> --- a/attr.c
>> +++ b/attr.c
>> @@ -1186,3 +1186,12 @@ void attr_start(void)
>> pthread_mutex_init(&check_vector.mutex, NULL);
>> #endif
>> }
>> +
>> +int is_precious_file(struct index_state *istate, const char *path)
>> +{
>> + static struct attr_check *check;
>> + if (!check)
>> + check = attr_check_initl("precious", NULL);
>> + git_check_attr(istate, path, check);
>> + return check && ATTR_TRUE(check->items[0].value);
>> +}
>
> If we merge two branches is this using the merged post-image of
> .gitattributes as a source?
>
>> if (o->dir &&
>> - is_excluded(o->dir, o->src_index, name, &dtype))
>> + is_excluded(o->dir, o->src_index, name, &dtype) &&
>> + !is_precious_file(o->src_index, name))
>> /*
>> * ce->name is explicitly excluded, so it is Ok to
>> * overwrite it.
>
> I wonder if instead we should just be reverting c81935348b ("Fix
> switching to a branch with D/F when current branch has file D.",
> 2007-03-15), which these days (haven't dug deeply) would just be this,
> right?:
>
>> diff --git a/unpack-trees.c b/unpack-trees.c
> index 7570df481b..b3efaddd4f 100644
> --- a/unpack-trees.c
> +++ b/unpack-trees.c
> @@ -1894,13 +1894,6 @@ static int check_ok_to_remove(const char *name,
> int len, int dtype,
> if (ignore_case && icase_exists(o, name, len, st))
> return 0;
>
> - if (o->dir &&
> - is_excluded(o->dir, o->src_index, name, &dtype))
> - /*
> - * ce->name is explicitly excluded, so it is Ok to
> - * overwrite it.
> - */
> - return 0;
> if (S_ISDIR(st->st_mode)) {
> /*
> * We are checking out path "foo" and
>
> Something like the approach you're taking will absolutely work from a
> technical standpoint, but I fear that it's going to be useless in
> practice.
>
> The users who need protection against git deleting their files the most
> are exactly the sort of users who aren't expert-level enough to
> understand the nuances of how the semantics of .gitignore and "precious"
> are going to interact before git eats their data.
>
> This is pretty apparent from the bug reports we're getting about
> this. None of them are:
>
> "Hey, I 100% understood .gitignore semantics including this one part
> of the docs where you say you'll do this, but just forgot one day
> and deleted my work. Can we get some more safety?"
>
> But rather (with some hyperbole for effect):
>
> "ZOMG git deleted my file! Is this a bug??"
>
> So I think we should have the inverse of this "precious"
> attribute". Just a change to the docs to say that .gitignore doesn't
> imply these eager deletion semantics on tree unpacking anymore, and if
> users want it back they can define a "garbage" attribute
> (s/precious/garbage/).
>
> That will lose no data, and in the very rare cases where a checkout of
> tracked files would overwrite an ignored pattern, we can just error out
> (as we do with the "Ok to overwrite" branch removed) and tell the user
> to delete the files to proceed.
>
> Three tests in our test suite fail with that patch applied, and they're
> explicitly testing for exactly the sort of scenario where users are likely to
> lose data. I.e.:
>
> 1. Open a tracked file in an editor
> 2. Save it
> 3. Switch to a topic branch, that has different .gitignore semantics
> (e.g. let's say a build/ dir exists there)
> 4. Have their work deleted
>
> So actually in writing this out I've become convinced that this
> "precious" approach can't work either, because *even if* you're an
> expert who manages to perfectly define their .gitignore and "precious"
> rules in advance to avoid data deletion, those rules will *also* need to
> take into account switching between branches (or even different
> histories) where you have other sorts of rules.
>
> So really, if there's ambiguity let's just not delete stuff by default
> and ask the user to resolve it.
Here's a patch to implement that (which borrows from some of yours). It
passes all of our tests:
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index b8392fc330..a6cad17899 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -1188,6 +1188,17 @@ If this attribute is not set or has an invalid value,
the value of the
(See linkgit:git-config[1]).
+Trashable files
+~~~~~~~~~~~~~~~
+
+`trashable`
+^^^^^^^^^^
+
+Provides an escape hatch for re-enabling a potentially data destroying
+feature which was enabled by default between Git versions 1.5.2 and
+2.20. See the `NOTES` section of linkgit:gitignore[5] for details.
+
+
USING MACRO ATTRIBUTES
----------------------
diff --git a/Documentation/gitignore.txt b/Documentation/gitignore.txt
index d107daaffd..39c6d5955a 100644
--- a/Documentation/gitignore.txt
+++ b/Documentation/gitignore.txt
@@ -140,6 +140,13 @@ not tracked by Git remain untracked.
To stop tracking a file that is currently tracked, use
'git rm --cached'.
+Between Git versions 1.5.2 and 2.20 untracked files or directories
+which were ignored and conflicted with a file about to be checked out
+(e.g. during linkgit:git-checkout[1] or linkgit:git-merge[1]) would be
+deleted. This could lead to loss of user data and is no longer the
+default, See `trashable` in linkgit:gitattributes[5]. for how to
+selectively enable this behavior.
+
EXAMPLES
--------
diff --git a/attr.c b/attr.c
index 60d284796d..930af78650 100644
--- a/attr.c
+++ b/attr.c
@@ -1186,3 +1186,12 @@ void attr_start(void)
pthread_mutex_init(&check_vector.mutex, NULL);
#endif
}
+
+int is_trashable_file(struct index_state *istate, const char *path)
+{
+ static struct attr_check *check;
+ if (!check)
+ check = attr_check_initl("trashable", NULL);
+ git_check_attr(istate, path, check);
+ return check && ATTR_TRUE(check->items[0].value);
+}
diff --git a/attr.h b/attr.h
index b0378bfe5f..ccf4d4e6b5 100644
--- a/attr.h
+++ b/attr.h
@@ -82,4 +82,6 @@ void git_attr_set_direction(enum git_attr_direction
new_direction);
void attr_start(void);
+int is_trashable_file(struct index_state *istate, const char *path);
+
#endif /* ATTR_H */
diff --git a/t/lib-submodule-update.sh b/t/lib-submodule-update.sh
index 016391723c..d2ceee33d2 100755
--- a/t/lib-submodule-update.sh
+++ b/t/lib-submodule-update.sh
@@ -844,6 +844,8 @@ test_submodule_switch_recursing_with_args () {
git branch -t add_sub1 origin/add_sub1 &&
: >sub1 &&
echo sub1 >.git/info/exclude &&
+ test_must_fail $command add_sub1 &&
+ echo sub1 trashable >.gitattributes &&
$command add_sub1 &&
test_superproject_content origin/add_sub1 &&
test_submodule_content sub1 origin/add_sub1
diff --git a/t/t1004-read-tree-m-u-wf.sh b/t/t1004-read-tree-m-u-wf.sh
index c13578a635..2243cd955e 100755
--- a/t/t1004-read-tree-m-u-wf.sh
+++ b/t/t1004-read-tree-m-u-wf.sh
@@ -63,8 +63,10 @@ test_expect_success 'two-way with incorrect
--exclude-per-directory (2)' '
fi
'
-test_expect_success 'two-way clobbering a ignored file' '
+test_expect_success 'two-way keeping a ignored file, trashing a trashable
file' '
+ read_tree_u_must_fail -m -u --exclude-per-directory=.gitignore master
side &&
+ echo file2 trashable >.gitattributes &&
read_tree_u_must_succeed -m -u --exclude-per-directory=.gitignore
master side
'
@@ -106,7 +108,7 @@ test_expect_success 'three-way not clobbering a working
tree file' '
echo >.gitignore file3
-test_expect_success 'three-way not complaining on an untracked file' '
+test_expect_success 'three-way complaining on an untracked file, trashing a
trashable file' '
git reset --hard &&
rm -f file2 subdir/file2 file3 subdir/file3 &&
@@ -114,6 +116,8 @@ test_expect_success 'three-way not complaining on an
untracked file' '
echo >file3 file three created in master, untracked &&
echo >subdir/file3 file three created in master, untracked &&
+ read_tree_u_must_fail -m -u --exclude-per-directory=.gitignore
branch-point master side &&
+ echo file3 trashable >.gitattributes &&
read_tree_u_must_succeed -m -u --exclude-per-directory=.gitignore
branch-point master side
'
diff --git a/unpack-trees.c b/unpack-trees.c
index 7570df481b..e9a7fb6583 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1895,9 +1895,10 @@ static int check_ok_to_remove(const char *name, int len,
int dtype,
return 0;
if (o->dir &&
- is_excluded(o->dir, o->src_index, name, &dtype))
+ is_excluded(o->dir, o->src_index, name, &dtype) &&
+ is_trashable_file(o->src_index, name))
/*
- * ce->name is explicitly excluded, so it is Ok to
+ * ce->name is explicitly trashable, so it is Ok to
* overwrite it.
*/
return 0;
