| Message ID | f449a72877de97d89e31a56724b4f65be2f33f20.1721250704.git.me@ttaylorr.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | midx: incremental multi-pack indexes, part one | expand |
On Wed, Jul 17, 2024 at 05:12:04PM -0400, Taylor Blau wrote: > [^1]: As a reminder, this means that the object is identified among the > objects contained in all layers of the incremental MIDX chain, not any > particular layer. For example, consider MIDX chain with two individual > MIDXs, one with 4 objects and another with 3 objects. If the MIDX with > 4 objects appears earlier in the chain, then asking for pack "6" would > return the second object in the MIDX with 3 objects. I think this is "object 6" in the final sentence? Otherwise, the explanation lays things out pretty well. Let's look at the code. > +static uint32_t midx_for_object(struct multi_pack_index **_m, uint32_t pos) > +{ > + struct multi_pack_index *m = *_m; > + while (m && pos < m->num_objects_in_base) > + m = m->base_midx; OK, so given a global position, we walk backwards until we find the correct midx... > + if (!m) > + BUG("NULL multi-pack-index for object position: %"PRIu32, pos); > + > + if (pos >= m->num_objects + m->num_objects_in_base) > + die(_("invalid MIDX object position, MIDX is likely corrupt")); ...and we double check that the given base claims to have that position. Seems obvious. > + *_m = m; > + > + return pos - m->num_objects_in_base; And then we adjust it into a per-midx position. > @@ -334,8 +351,10 @@ off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos) > > uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos) > { > - return get_be32(m->chunk_object_offsets + > - (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH); > + pos = midx_for_object(&m, pos); > + > + return m->num_packs_in_base + get_be32(m->chunk_object_offsets + > + (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH); > } OK, so now this function translates a global position into a local one, and then we get the pack id for the local midx/pos, and then turn it back into a global pack id. That all makes sense, but you definitely have to read carefully to make sure which positions/ids are global within the chain and which are local to a midx. I wonder if the type system can help us annotate them, but I suspect it becomes awkward. Just typedef-ing them to uint32_t means the compiler won't warn us when we use one in the wrong spot. Sticking them in structs would solve that, but then using them is painful. Let's keep reading and see if it's even an issue in practice. -Peff
On Thu, Aug 01, 2024 at 05:30:06AM -0400, Jeff King wrote: > On Wed, Jul 17, 2024 at 05:12:04PM -0400, Taylor Blau wrote: > > > [^1]: As a reminder, this means that the object is identified among the > > objects contained in all layers of the incremental MIDX chain, not any > > particular layer. For example, consider MIDX chain with two individual > > MIDXs, one with 4 objects and another with 3 objects. If the MIDX with > > 4 objects appears earlier in the chain, then asking for pack "6" would > > return the second object in the MIDX with 3 objects. > > I think this is "object 6" in the final sentence? Oops, yes. Thanks for spotting, this was as easy as s/pack/object on the second to last line in the paragraph quoted above. > OK, so now this function translates a global position into a local one, > and then we get the pack id for the local midx/pos, and then turn it > back into a global pack id. > > That all makes sense, but you definitely have to read carefully to make > sure which positions/ids are global within the chain and which are local > to a midx. Exactly. Thanks, Taylor
diff --git a/midx.c b/midx.c index 3992b05465..39d358da20 100644 --- a/midx.c +++ b/midx.c @@ -242,6 +242,23 @@ void close_midx(struct multi_pack_index *m) free(m); } +static uint32_t midx_for_object(struct multi_pack_index **_m, uint32_t pos) +{ + struct multi_pack_index *m = *_m; + while (m && pos < m->num_objects_in_base) + m = m->base_midx; + + if (!m) + BUG("NULL multi-pack-index for object position: %"PRIu32, pos); + + if (pos >= m->num_objects + m->num_objects_in_base) + die(_("invalid MIDX object position, MIDX is likely corrupt")); + + *_m = m; + + return pos - m->num_objects_in_base; +} + int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t pack_int_id) { struct strbuf pack_name = STRBUF_INIT; @@ -334,8 +351,10 @@ off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos) uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos) { - return get_be32(m->chunk_object_offsets + - (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH); + pos = midx_for_object(&m, pos); + + return m->num_packs_in_base + get_be32(m->chunk_object_offsets + + (off_t)pos * MIDX_CHUNK_OFFSET_WIDTH); } int fill_midx_entry(struct repository *r,
The function `nth_midxed_pack_int_id()` takes in a object position in MIDX lexicographic order and returns an identifier of the pack from which that object was selected in the MIDX. Currently, the given object position is an index into the lexicographic order of objects in a single MIDX. Change this position to instead refer into the concatenated lexicographic order of all MIDXs in a MIDX chain. This has two visible effects within the implementation of `prepare_midx_pack()`: - First, the given position is now an index into the concatenated lexicographic order of all MIDXs in the order in which they appear in the MIDX chain. - Second the pack ID returned from this function is now also in the concatenated order of packs among all layers of the MIDX chain in the same order that they appear in the MIDX chain. To do this, introduce the first of two general purpose helpers, this one being `midx_for_object()`. `midx_for_object()` takes a double pointer to a `struct multi_pack_index` as well as an object `pos` in terms of the entire MIDX chain[^1]. The function chases down the '->base_midx' field until it finds the MIDX layer within the chain that contains the given object. It then: - modifies the double pointer to point to the containing MIDX, instead of the tip of the chain, and - returns the MIDX-local position[^2] at which the given object can be found. Use this function within `nth_midxed_pack_int_id()` so that the `pos` it expects is now relative to the entire MIDX chain, and that it returns the appropriate pack position for that object. [^1]: As a reminder, this means that the object is identified among the objects contained in all layers of the incremental MIDX chain, not any particular layer. For example, consider MIDX chain with two individual MIDXs, one with 4 objects and another with 3 objects. If the MIDX with 4 objects appears earlier in the chain, then asking for pack "6" would return the second object in the MIDX with 3 objects. [^2]: Building on the previous example, asking for object 6 in a MIDX chain with (4, 3) objects, respectively, this would set the double pointer to point at the MIDX containing three objects, and would return an index to the second object within that MIDX. Signed-off-by: Taylor Blau <me@ttaylorr.com> --- midx.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-)