Running tests with GIT_TEST_SPLIT_INDEX=1 is supposed to turn on the
split index feature and trigger index splitting (mostly) randomly.
Alas, this has been broken since 6e37c8ed3c (read-cache.c: fix writing
"link" index ext with null base oid, 2019-02-13), and
GIT_TEST_SPLIT_INDEX=1 hasn't triggered any index splitting since
then.
This patch makes GIT_TEST_SPLIT_INDEX work again, though it doesn't
restore the pre-6e37c8ed3c behavior. To understand the bug, the fix,
and the behavior change we first have to look at how
GIT_TEST_SPLIT_INDEX used to work before 6e37c8ed3c:
There are two places where we check the value of
GIT_TEST_SPLIT_INDEX, and before 6e37c8ed3c they worked like this:
1) In the lower-level do_write_index(), where, if
GIT_TEST_SPLIT_INDEX is enabled, we call init_split_index().
This call merely allocates and zero-initializes
'istate->split_index', but does nothing else (i.e. doesn't fill
the base/shared index with cache entries, doesn't actually
write a shared index file, etc.). Pertinent to this issue, the
hash of the base index remains all zeroed out.
2) In the higher-level write_locked_index(), but only when
'istate->split_index' has already been initialized. Then, if
GIT_TEST_SPLIT_INDEX is enabled, it randomly sets the flag that
triggers index splitting later in this function. This
randomness comes from the first byte of the hash of the base
index via an 'if ((first_byte & 15) < 6)' condition.
However, if 'istate->split_index' hasn't been initialized (i.e.
it is still NULL), then write_locked_index() just calls
do_write_locked_index(), which internally calls the above
mentioned do_write_index().
This means that while GIT_TEST_SPLIT_INDEX=1 usually triggered index
splitting randomly, the first two index writes were always
deterministic (though I suspect this was unintentional):
- The initial index write never splits the index.
During the first index write write_locked_index() is called with
'istate->split_index' still uninitialized, so the check in 2) is
not executed. It still calls do_write_index(), though, which
then executes the check in 1). The resulting all zero base
index hash then leads to the 'link' extension being written to
'.git/index', though a shared index file is not written:
$ rm .git/index
$ GIT_TEST_SPLIT_INDEX=1 git update-index --add file
$ test-tool dump-split-index .git/index
own c6ef71168597caec8553c83d9d0048f1ef416170
base 0000000000000000000000000000000000000000
100644 d00491fd7e5bb6fa28c517a0bb32b8b506539d4d 0 file
replacements:
deletions:
$ ls -l .git/sharedindex.*
ls: cannot access '.git/sharedindex.*': No such file or directory
- The second index write always splits the index.
When the index written in the previous point is read,
'istate->split_index' is initialized because of the presence of
the 'link' extension. So during the second write
write_locked_index() does run the check in 2), and the first
byte of the all zero base index hash always fulfills the
randomness condition, which in turn always triggers the index
splitting.
- Subsequent index writes will find the 'link' extension with a
real non-zero base index hash, so from then on the check in 2)
is executed and the first byte of the base index hash is as
random as it gets (coming from the SHA-1 of index data including
timestamps and inodes...).
All this worked until 6e37c8ed3c came along, and stopped writing the
'link' extension if the hash of the base index was all zero:
$ rm .git/index
$ GIT_TEST_SPLIT_INDEX=1 git update-index --add file
$ test-tool dump-split-index .git/index
own abbd6f6458d5dee73ae8e210ca15a68a390c6fd7
not a split index
$ ls -l .git/sharedindex.*
ls: cannot access '.git/sharedindex.*': No such file or directory
So, since the first index write with GIT_TEST_SPLIT_INDEX=1 doesn't
write a 'link' extension, in the second index write
'istate->split_index' remains uninitialized, and the check in 2) is
not executed, and ultimately the index is never split.
Fix this by modifying write_locked_index() to make sure to check
GIT_TEST_SPLIT_INDEX even if 'istate->split_index' is still
uninitialized, and initialize it if necessary. The check for
GIT_TEST_SPLIT_INDEX and separate init_split_index() call in
do_write_index() thus becomes unnecessary, so remove it. Furthermore,
add a test to 't1700-split-index.sh' to make sure that
GIT_TEST_SPLIT_INDEX=1 will keep working (though only check the
index splitting on the first index write, because after that it will
be random).
Note that this change does not restore the pre-6e37c8ed3c behaviour,
as it will deterministically split the index already on the first
index write. Since GIT_TEST_SPLIT_INDEX is purely a developer aid,
there is no backwards compatibility issue here. The new behaviour did
trigger test failures in 't0003-attributes.sh' and 't1600-index.sh',
though, which have been fixed in preparatory patches in this series.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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The name "git" was given by Linus Torvalds when he wrote the very
first version. He described the tool as "the stupid content tracker"
and the name as (depending on your mood):
random three-letter combination that is pronounceable, and not
actually used by any common UNIX command. The fact that it is a
mispronunciation of "get" may or may not be relevant.
stupid. contemptible and despicable. simple. Take your pick from the
dictionary of slang.
"global information tracker": you're in a good mood, and it actually
works for you. Angels sing, and a light suddenly fills the room.
"goddamn idiotic truckload of sh*t": when it breaks
e8ffd034c8
