sha1_file: be paranoid when creating loose objects
We don't want the data being deflated and stored into loose objects to be different from what we expect. While the deflated data is protected by a CRC which is good enough for safe data retrieval operations, we still want to be doubly sure that the source data used at object creation time is still what we expected once that data has been deflated and its CRC32 computed. The most plausible data corruption may occur if the source file is modified while Git is deflating and writing it out in a loose object. Or Git itself could have a bug causing memory corruption. Or even bad RAM could cause trouble. So it is best to make sure everything is coherent and checksum protected from beginning to end. To do so we compute the SHA1 of the data being deflated _after_ the deflate operation has consumed that data, and make sure it matches with the expected SHA1. This way we can rely on the CRC32 checked by the inflate operation to provide a good indication that the data is still coherent with its SHA1 hash. One pathological case we ignore is when the data is modified before (or during) deflate call, but changed back before it is hashed. There is some overhead of course. Using 'git add' on a set of large files: Before: real 0m25.210s user 0m23.783s sys 0m1.408s After: real 0m26.537s user 0m25.175s sys 0m1.358s The overhead is around 5% for full data coherency guarantee. Signed-off-by: NNicolas Pitre <nico@fluxnic.net> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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