Re: [PATCH v12 06/12] reftable: define version 2 of the spec to accomodate SHA256
From: Junio C Hamano <hidden>
Date: 2020-05-08 20:00:03
"Han-Wen Nienhuys via GitGitGadget" [off-list ref] writes:
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From: Han-Wen Nienhuys <redacted> Signed-off-by: Han-Wen Nienhuys <redacted> --- Documentation/technical/reftable.txt | 50 ++++++++++++++++------------ 1 file changed, 28 insertions(+), 22 deletions(-)diff --git a/Documentation/technical/reftable.txt b/Documentation/technical/reftable.txt index 9fa4657d9ff..ee3f36ea851 100644 --- a/Documentation/technical/reftable.txt +++ b/Documentation/technical/reftable.txt@@ -193,8 +193,8 @@ and non-aligned files. Very small files (e.g. 1 only ref block) may omit `padding` and the ref
Hmph, I am seeing nbsp before '1' and am wondering where it came from.
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index to reduce total file size. -Header -^^^^^^ +Header (version 1) +^^^^^^^^^^^^^^^^^^ A 24-byte header appears at the beginning of the file:@@ -215,6 +215,24 @@ used in a stack for link:#Update-transactions[transactions], these fields can order the files such that the prior file’s `max_update_index + 1` is the next file’s `min_update_index`.
Am I correct to assume that we do not plan to support a repository with mixed set of refs, some referring to a commit with its SHA-1 object name while others using SHA-256 object name?
+Header (version 2) +^^^^^^^^^^^^^^^^^^ + +A 28-byte header appears at the beginning of the file: + +.... +'REFT' +uint8( version_number = 1 )
Shouldn't this be 2 instead, as v1 lacked the Hash-id field?
+uint24( block_size )
+uint64( min_update_index )
+uint64( max_update_index )
+uint32( hash_id )
+....
+
+The header is identical to `version_number=1`, with the 4-byte hash ID
+("sha1" for SHA1 and "s256" for SHA-256) append to the header.Am I correct to assume that SHA-1 repositories are encouraged to use version 2 when the code becomes available?
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First ref block ^^^^^^^^^^^^^^^@@ -671,14 +689,8 @@ Footer After the last block of the file, a file footer is written. It begins like the file header, but is extended with additional data. -A 68-byte footer appears at the end: - .... - 'REFT' - uint8( version_number = 1 ) - uint24( block_size ) - uint64( min_update_index ) - uint64( max_update_index ) + HEADER uint64( ref_index_position ) uint64( (obj_position << 5) | obj_id_len )@@ -701,12 +713,16 @@ obj blocks. * `obj_index_position`: byte position for the start of the obj index. * `log_index_position`: byte position for the start of the log index. +The size of the footer is 68 bytes for version 1, and 72 bytes for +version 2. + Reading the footer ++++++++++++++++++ -Readers must seek to `file_length - 68` to access the footer. A trusted -external source (such as `stat(2)`) is necessary to obtain -`file_length`. When reading the footer, readers must verify: +Readers must first read the file start to determine the version +number. Then they seek to `file_length - FOOTER_LENGTH` to access the +footer. A trusted external source (such as `stat(2)`) is necessary to +obtain `file_length`. When reading the footer, readers must verify:
In any case, the size of this patch is pleasant to see---it must be a sign that the previous step was done well not to hardcode the "hash size is 20 bytes" assumption all over the place and instead used "this field holds N+m bytes where N is the size of the hash described in the REFT header" consistently. Nicely done.