On Tue, May 01 2018, Derrick Stolee wrote:
> On 5/1/2018 9:39 AM, Ævar Arnfjörð Bjarmason wrote:
>> On Tue, May 01 2018, Derrick Stolee wrote:
>>
>>> From: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
>>>
>>> Here is what I mean by sorting during for_each_abbrev(). This seems to work for
>>> me, so I don't know what the issue is with this one-pass approach.
>>> [...]
>>> +static int sort_ambiguous(const void *a, const void *b)
>>> +{
>>> + int a_type = oid_object_info(a, NULL);
>>> + int b_type = oid_object_info(b, NULL);
>>> + int a_type_sort;
>>> + int b_type_sort;
>>> +
>>> + /*
>>> + * Sorts by hash within the same object type, just as
>>> + * oid_array_for_each_unique() would do.
>>> + */
>>> + if (a_type == b_type)
>>> + return oidcmp(a, b);
>>> +
>>> + /*
>>> + * Between object types show tags, then commits, and finally
>>> + * trees and blobs.
>>> + *
>>> + * The object_type enum is commit, tree, blob, tag, but we
>>> + * want tag, commit, tree blob. Cleverly (perhaps too
>>> + * cleverly) do that with modulus, since the enum assigns 1 to
>>> + * commit, so tag becomes 0.
>>> + */
>>> + a_type_sort = a_type % 4;
>>> + b_type_sort = b_type % 4;
>>> + return a_type_sort > b_type_sort ? 1 : -1;
>>> +}
>>> +
>>> static int get_short_oid(const char *name, int len, struct object_id *oid,
>>> unsigned flags)
>>> {
>>> @@ -451,6 +479,9 @@ int for_each_abbrev(const char *prefix, each_abbrev_fn fn, void *cb_data)
>>> find_short_object_filename(&ds);
>>> find_short_packed_object(&ds);
>>>
>>> + QSORT(collect.oid, collect.nr, sort_ambiguous);
>>> + collect.sorted = 1;
>>> +
>> Yes this works. You're right. I wasn't trying to intentionally omit
>> stuff in my recent 878t93zh60.fsf@xxxxxxxxxxxxxxxxxxx, I'd just written
>> this code some days ago and forgotten why I did what I was doing (and
>> this is hard to test for), but it's all coming back to me now.
>>
>> The actual requirement for oid_array_for_each_unique() working properly
>> is that you've got to feed it in hash order,
>
> To work properly, duplicate entries must be consecutive. Since
> duplicate entries have the same type, our sort satisfies this
> condition.
>
>> but my new sort_ambiguous()
>> still does that (barring any SHA-1 collisions, at which point we have
>> bigger problems), so two passes aren't needed. So yes, this apporoach
>> works and is one-pass.
>>
>> But that's just an implementation detail of the current sort method,
>> when I wrote this I was initially playing with other sort orders,
>> e.g. sorting SHAs regardless of type by the mtime of the file I found
>> them in. With this approach I'd start printing duplicates if I changed
>> the internals of sort_ambiguous() like that.
>
> That makes sense.
>
>> But I think it's extremely implausible that we'll start sorting things
>> like that, so I'll just take this method of doing it and add some
>> comment saying we must hashcmp() the entries in our own sort function
>> for the de-duplication to work, I don't see us ever changing that.
>
> Sounds good.
Actually I'm having second thoughts about that and thinking I might keep
my original approach (with a better explanation).
A few more lines of code seems worthwhile in order to not break the
assumptions a documented API is making, no matter how briefly, so I set
about documenting this case and supporting it, since
e.g. oid_array_lookup() will completely fail with the hack of setting
the .sorted member, and came up with this:
diff --git a/Documentation/technical/api-oid-array.txt b/Documentation/technical/api-oid-array.txt
index b0c11f868d..ff87260220 100644
--- a/Documentation/technical/api-oid-array.txt
+++ b/Documentation/technical/api-oid-array.txt
@@ -16,6 +16,20 @@ Data Structures
the actual data. The `nr` member contains the number of items in
the set. The `alloc` and `sorted` members are used internally,
and should not be needed by API callers.
++
+Both the `oid_array_lookup` and `oid_array_for_each_unique` functions
+rely on the array being sorted. For the former it's an absolute
+requirenment that the internal `oid_array_sort` function has been
+called on it, bu for the latter it's enough that the elements are
+ordered in such a way as to guarantee that identical object IDs are
+adjacent in the array.
++
+This is useful e.g. to print output where commits, tags etc. are
+grouped together (barring a hash collision they won't have the same
+object ID), in such cases the `custom_sorted` member can be set to `1`
+before calling `oid_array_for_each_unique`, and it'll skip its own
+sorting. Once it's been set calling e.g. `oid_array_lookup` without it
+being cleared again will cause an internal panic, so use it carefully.
Functions
---------
diff --git a/sha1-array.c b/sha1-array.c
index 466a926aa3..cbae07ff78 100644
--- a/sha1-array.c
+++ b/sha1-array.c
@@ -18,6 +18,7 @@ static void oid_array_sort(struct oid_array *array)
{
QSORT(array->oid, array->nr, void_hashcmp);
array->sorted = 1;
+ array->custom_sorted = 0;
}
static const unsigned char *sha1_access(size_t index, void *table)
@@ -28,6 +29,13 @@ static const unsigned char *sha1_access(size_t index, void *table)
int oid_array_lookup(struct oid_array *array, const struct object_id *oid)
{
+ if (array->custom_sorted)
+ /*
+ * We could also just clear custom_sorted here, but if
+ * the caller is custom sorting and then calling this
+ * that's likely something they'd like to know about.
+ */
+ BUG("PANIC: Cannot lookup OIDs in arrays with a custom sort!");
if (!array->sorted)
oid_array_sort(array);
return sha1_pos(oid->hash, array->oid, array->nr, sha1_access);
@@ -39,6 +47,7 @@ void oid_array_clear(struct oid_array *array)
array->nr = 0;
array->alloc = 0;
array->sorted = 0;
+ array->custom_sorted = 0;
}
int oid_array_for_each_unique(struct oid_array *array,
@@ -47,7 +56,7 @@ int oid_array_for_each_unique(struct oid_array *array,
{
int i;
- if (!array->sorted)
+ if (!array->sorted && !array->custom_sorted)
oid_array_sort(array);
for (i = 0; i < array->nr; i++) {
diff --git a/sha1-array.h b/sha1-array.h
index 1e1d24b009..bfa77ba1e4 100644
--- a/sha1-array.h
+++ b/sha1-array.h
@@ -6,6 +6,7 @@ struct oid_array {
int nr;
int alloc;
int sorted;
+ int custom_sorted;
};
#define OID_ARRAY_INIT { NULL, 0, 0, 0 }
diff --git a/sha1-name.c b/sha1-name.c
index b81e07adbb..d190800db0 100644
--- a/sha1-name.c
+++ b/sha1-name.c
@@ -490,9 +490,11 @@ int for_each_abbrev(const char *prefix, each_abbrev_fn fn, void *cb_data)
find_short_packed_object(&ds);
QSORT(collect.oid, collect.nr, sort_ambiguous);
- collect.sorted = 1;
+ collect.custom_sorted = 1;
ret = oid_array_for_each_unique(&collect, fn, cb_data);
+ collect.custom_sorted = 0;
+
oid_array_clear(&collect);
return ret;
}
So maybe I should just stop worrying and YOLO it, it just seems wrong to
leave such a fragile setup in place where we set .sorted=1 and some
future refactoring reasonably tries to call oid_array_lookup() on it and
silently fails.
What do you think?
