[PATCH 6/6] read-cache: fix GIT_TEST_SPLIT_INDEX

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>
---
 read-cache.c           | 23 ++++++++++++++---------
 t/t1700-split-index.sh | 11 +++++++++++
 2 files changed, 25 insertions(+), 9 deletions(-)