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The previous patch demonstrates a bug where a MIDX's auxiliary object
order can become out of sync with a MIDX bitmap.
This is because of two confounding factors:
- First, the object order is stored in a file which is named according
to the multi-pack index's checksum, and the MIDX does not store the
object order. This means that the object order can change without
altering the checksum.
- But the .rev file is moved into place with finalize_object_file(),
which link(2)'s the file into place instead of renaming it. For us,
that means that a modified .rev file will not be moved into place if
MIDX's checksum was unchanged.
This fix is to force the MIDX's checksum to change when the preferred
pack changes but the set of packs contained in the MIDX does not. In
other words, when the object order changes, the MIDX's checksum needs to
change with it (regardless of whether the MIDX is tracking the same or
different packs).
This prevents a race whereby changing the object order (but not the
packs themselves) enables a reader to see the new .rev file with the old
MIDX, or similarly seeing the new bitmap with the old object order.
But why can't we just stop hardlinking the .rev into place instead
adding additional data to the MIDX? Suppose that's what we did. Then
when we go to generate the new bitmap, we'll load the old MIDX bitmap,
along with the MIDX that it references. That's fine, since the new MIDX
isn't moved into place until after the new bitmap is generated. But the
new object order *has* been moved into place. So we'll read the old
bitmaps in the new order when generating the new bitmap file, meaning
that without this secondary change, bitmap generation itself would
become a victim of the race described here.
This can all be prevented by forcing the MIDX's checksum to change when
the object order does. By embedding the entire object order into the
MIDX, we do just that. That is, the MIDX's checksum will change in
response to any perturbation of the underlying object order. In t5326,
this will cause the MIDX's checksum to update (even without changing the
set of packs in the MIDX), preventing the stale read problem.
Note that this makes it safe to continue to link(2) the MIDX .rev file
into place, since it is now impossible to have a .rev file that is
out-of-sync with the MIDX whose checksum it references. (But we will do
away with MIDX .rev files later in this series anyway, so this is
somewhat of a moot point).
In theory, it is possible to store a "fingerprint" of the full object
order here, so long as that fingerprint changes at least as often as the
full object order does. Some possibilities here include storing the
identity of the preferred pack, along with the mtimes of the
non-preferred packs in a consistent order. But storing a limited part of
the information makes it difficult to reason about whether or not there
are gaps between the two that would cause us to get bitten by this bug
again.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Docfix.
* jt/midx-doc-fix:
Doc: no midx and partial clone relation
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Doc mark-up fix.
* tl/midx-docfix:
midx: fix a formatting issue in "multi-pack-index.txt"
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The multi-pack index treats promisor packfiles (that is, packfiles that
have an accompanying .promisor file) the same as other packfiles. Remove
a section in the documentation that seems to indicate otherwise.
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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There is a formatting issue in "multi-pack-index.html", corresponding
to the nesting bulleted list of a wrong usage in "multi-pack-index.txt"
and this commit fix the problem.
In ASCIIDOC, it doesn't treat an indented character as the
beginning of a sub-list. If we want to write a nested bulleted list, we
could just use ASTERISK without any DASH like:
"
* Level 1 list item
** Level 2 list item
*** Level 3 list item
** Level 2 list item
* Level 1 list item
** Level 2 list item
* Level 1 list item
"
The DASH can be used for bulleted list too, But the DASH is suggested
only to be used as the marker for the first level because the DASH
doesn’t work well or a best practice for nested lists,
like (dash is as level 2 below):
"
* Level 1 list item
- Level 2 list item
* Level 1 list item
"
ASTERISK is recommanded to use because it works intuitively and clearly
("marker length = nesting level") in nested lists, but the DASH can't.
However, when you want to write a non-nested bulleted lists, DASH works
too, like:
"
- Level 1 list item
- Level 1 list item
- Level 1 list item
"
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Doc tweak.
* ew/midx-doc-update:
doc/technical: update note about core.multiPackIndex
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MIDX files are used by default since commit 18e449f86b74
(midx: enable core.multiPackIndex by default, 2020-09-25)
Helped-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Update the technical documentation to describe the multi-pack bitmap
format. This patch merely introduces the new format, and describes its
high-level ideas. Git does not yet know how to read nor write these
multi-pack variants, and so the subsequent patches will:
- Introduce code to interpret multi-pack bitmaps, according to this
document.
- Then, introduce code to write multi-pack bitmaps from the 'git
multi-pack-index write' sub-command.
Finally, the implementation will gain tests in subsequent patches (as
opposed to inline with the patch teaching Git how to write multi-pack
bitmaps) to avoid a cyclic dependency.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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When multiple packs in the multi-pack index contain the same object, the
MIDX machinery must make a choice about which pack it associates with
that object. Prior to this patch, the lowest-ordered[1] pack was always
selected.
Pack selection for duplicate objects is relatively unimportant today,
but it will become important for multi-pack bitmaps. This is because we
can only invoke the pack-reuse mechanism when all of the bits for reused
objects come from the reuse pack (in order to ensure that all reused
deltas can find their base objects in the same pack).
To encourage the pack selection process to prefer one pack over another
(the pack to be preferred is the one a caller would like to later use as
a reuse pack), introduce the concept of a "preferred pack". When
provided, the MIDX code will always prefer an object found in a
preferred pack over any other.
No format changes are required to store the preferred pack, since it
will be able to be inferred with a corresponding MIDX bitmap, by looking
up the pack associated with the object in the first bit position (this
ordering is described in detail in a subsequent commit).
[1]: the ordering is specified by MIDX internals; for our purposes we
can consider the "lowest ordered" pack to be "the one with the
most-recent mtime.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Doc update.
* jb/midx-doc-update:
docs: multi-pack-index: remove note about future 'verify' work
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This was implemented in the 'git multi-pack-index' command and
merged in 468b3221 (Merge branch 'ds/multi-pack-verify',
2018-10-10).
And there's no 'git midx' command.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Typofix.
* jb/doc-multi-pack-idx-fix:
multi-pack-index: correct configuration in documentation
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It's core.multiPackIndex, not pack.multiIndex.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Since we're now recommending lore.kernel.org (and because the
public-inbox.org domain might eventually go away), let's update our
internal references to use it, too. That future-proofs our references,
and sets the example we want people to follow.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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