reftable: avoid reading and writing empty keys

this series makes sure that the object record does not have to consider empty keys (and therefore, a NULL memcpy destination)

while we're at it add some more tests, and fix a naming mistake.

Han-Wen Nienhuys (7):
Documentation: object_id_len goes up to 31
reftable: reject 0 object_id_len
reftable: add a test that verifies that writing empty keys fails
reftable: avoid writing empty keys at the block layer
reftable: ensure that obj_id_len is >= 2 on writing
reftable: add test for length of disambiguating prefix
reftable: rename writer_stats to reftable_writer_stats

 Documentation/technical/reftable.txt |   2 +-
 reftable/block.c                     |  27 ++++---
 reftable/block_test.c                |   5 ++
 reftable/reader.c                    |   5 ++
 reftable/readwrite_test.c            | 105 ++++++++++++++++++++++++++-
 reftable/reftable-writer.h           |   2 +-
 reftable/writer.c                    |   9 ++-
 7 files changed, 136 insertions(+), 19 deletions(-)

base-commit: 45fe28c951c3e70666ee4ef8379772851a8e4d32
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1185%2Fhanwen%2Fobj-id-len-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1185/hanwen/obj-id-len-v3
Pull-Request: https://github.com/git/git/pull/1185

Range-diff vs v2:

 1:  80d29e8f269 = 1:  80d29e8f269 Documentation: object_id_len goes up to 31
 2:  4c1a19fc4ae ! 2:  68e7bc32ff8 reftable: reject 0 object_id_len

@@ Commit message
reftable: reject 0 object_id_len

The spec says 2 <= object_id_len <= 31. We are lenient and allow 1,
- but we forbid 0, so we can we can be sure that we never read a
- 0-length key.
+ but we forbid 0, so we can be sure that we never read a 0-length key.

 3:  600b115f8b1 = 3:  8b5aebdb07a reftable: add a test that verifies that writing empty keys fails
 4:  ba036ee8543 ! 4:  a9372cacd1b reftable: avoid writing empty keys at the block layer
     @@ reftable/block.c: int block_reader_first_key(struct block_reader *br, struct str
       	if (n < 0)
       		return n;
      +	if (!key->len)
     -+		return -1;
     ++		return REFTABLE_FORMAT_ERROR;
       
       	return 0;
       }
 5:  2bd3d44ba57 = 5:  0b8a42399dd reftable: ensure that obj_id_len is >= 2 on writing
 6:  82d36ee0e0d = 6:  bdccd969475 reftable: add test for length of disambiguating prefix
 7:  c6ffdb3471c = 7:  72499a14e38 reftable: rename writer_stats to reftable_writer_stats

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