zip: find preambles for zip files written by archive/zip
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chrisnovakovic
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zip files written by `archive/zip`'s `Writer` correctly report the byte offsets of their central directory file header and end of central directory entries, but this means the offset of the zip data within the file as calculated by `archive/zip`'s `Reader` will always be 0 - in other words, `Reader` thinks that any non-zip data prepended to the file is actually part of the first file's local file header. This prevents any zip file written by `archive/zip` from being used as a source for the `zip` command's `--preamble_from` option, because the non-zip data doesn't appear to be non-zip data at all. In cases where `Reader` identifies that the byte offset of the zip data within the file is 0, check whether the byte offset of the header for the first local file header is also 0. If it isn't, assume the byte offset of the first local file header is the true starting position of the zip data within the file, and that anything before it is in fact a preamble.
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chrisnovakovic
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Oct 24, 2024
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| { | ||
| ZipFile: "arcat_preamble.zip", | ||
| PreambleChecksum: "46533b2dfa35ad537d3561ebee0c7af8941bc65363c1b188e1be6eaf79e9138c", // shebang_twolines.txt | ||
| }, |
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This zip file was written by arcat (i.e. by archive/zip), whereas the other test zip files here were simply catted onto another file acting as the preamble. This meant that the byte offsets of the CDFH and EOCD entries in those zip files were wrong, so the presence of the preamble was easily detectable by the previous version of Preamble. The main use case for the --preamble_from option is in fact to write the shebang from one pex file into another pex file in the python-rules plugin, so it's important that preambles can be read correctly from zip files written by arcat.
peterebden
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Oct 25, 2024
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zip files written by
archive/zip'sWritercorrectly report the byte offsets of their central directory file header and end of central directory entries, but this means the offset of the zip data within the file as calculated byarchive/zip'sReaderwill always be 0 - in other words,Readerthinks that any non-zip data prepended to the file is actually part of the first file's local file header. This prevents any zip file written byarchive/zipfrom being used as a source for thezipcommand's--preamble_fromoption, because the non-zip data doesn't appear to be non-zip data at all.In cases where
Readeridentifies that the byte offset of the zip data within the file is 0, check whether the byte offset of the header for the first local file header is also 0. If it isn't, assume the byte offset of the first local file header is the true starting position of the zip data within the file, and that anything before it is in fact a preamble.