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35079f50
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2017-04-21T07:31:56
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odb: add option to turn off hash verification
Verifying hashsums of objects we are reading from the ODB may be costly
as we have to perform an additional hashsum calculation on the object.
Especially when reading large objects, the penalty can be as high as
35%, as can be seen when executing the equivalent of `git cat-file` with
and without verification enabled. To mitigate for this, we add a global
option for libgit2 which enables the developer to turn off the
verification, e.g. when he can be reasonably sure that the objects on
disk won't be corrupted.
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28a0741f
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2017-04-10T09:30:08
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odb: verify object hashes
The upstream git.git project verifies objects when looking them up from
disk. This avoids scenarios where objects have somehow become corrupt on
disk, e.g. due to hardware failures or bit flips. While our mantra is
usually to follow upstream behavior, we do not do so in this case, as we
never check hashes of objects we have just read from disk.
To fix this, we create a new error class `GIT_EMISMATCH` which denotes
that we have looked up an object with a hashsum mismatch. `odb_read_1`
will then, after having read the object from its backend, hash the
object and compare the resulting hash to the expected hash. If hashes do
not match, it will return an error.
This obviously introduces another computation of checksums and could
potentially impact performance. Note though that we usually perform I/O
operations directly before doing this computation, and as such the
actual overhead should be drowned out by I/O. Running our test suite
seems to confirm this guess. On a Linux system with best-of-five
timings, we had 21.592s with the check enabled and 21.590s with the
ckeck disabled. Note though that our test suite mostly contains very
small blobs only. It is expected that repositories with bigger blobs may
notice an increased hit by this check.
In addition to a new test, we also had to change the
odb::backend::nonrefreshing test suite, which now triggers a hashsum
mismatch when looking up the commit "deadbeef...". This is expected, as
the fake backend allocated inside of the test will return an empty
object for the OID "deadbeef...", which will obviously not hash back to
"deadbeef..." again. We can simply adjust the hash to equal the hash of
the empty object here to fix this test.
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d59dabe5
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2017-04-10T09:00:51
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tests: object: test looking up corrupted objects
We currently have no tests which check whether we fail reading corrupted
objects. Add one which modifies contents of an object stored on disk and
then tries to read the object.
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86c03552
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2017-04-10T09:27:04
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tests: object: create sandbox
The object::lookup tests do use the "testrepo.git" repository in a
read-only way, so we do not set up the repository as a sandbox but
simply open it. But in a future commit, we will want to test looking up
objects which are corrupted in some way, which requires us to modify the
on-disk data. Doing this in a repository without creating the sandbox
will modify contents of our libgit2 repository, though.
Create the repository in a sandbox to avoid this.
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17820381
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2013-11-14T14:05:52
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Rename tests-clar to tests
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