Zip: omit Disk Start Number from the Zip64 Central Directory Entry #6
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Clause 4.5.3 of the PKWare spec clearly states that it MUST only appear in the Extra block when the corresponding field in the CDE (or LDE) was set to -1 (0xFFFF in the case of the Disk Start Number.) Previously, this would only be the case when Zip64 was presumed or the disk number was actually outside the range for the CDE. In every other case, the Disk Start Number would go into both locations, violating the spec. Some tools (such as 7-Zip) show warnings in this case, but often work as expected. Others (such as older versions of System.IO.Compression) follow the spec more stringently and will refuse to use the values from the Zip64 Extra block when the related values do not match their expectation. However, fixing this to always write a conformant CDE that has its Disk Start Number as 0xFFFF with the actual value in the Extra block breaks other tools (such as marmelroy/Zip on iOS) that do not fully support Zip64. The spec does allow the Extra block to omit the Disk Start Number (and the Relativ Header Offset) when they fit in the CDE, so we do just that for general compatibility. Fixes haf#260
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Clause 4.5.3 of the PKWare spec clearly states that it MUST only appear in the Extra block when the corresponding field in the CDE (or LDE) was set to -1 (0xFFFF in the case of the Disk Start Number.)
Previously, this would only be the case when Zip64 was presumed or the disk number was actually outside the range for the CDE. In every other case, the Disk Start Number would go into both locations, violating the spec.
Some tools (such as 7-Zip) show warnings in this case, but often work as expected. Others (such as older versions of System.IO.Compression) follow the spec more stringently and will refuse to use the values from the Zip64 Extra block when the related values do not match their expectation.
However, fixing this to always write a conformant CDE that has its Disk Start Number as 0xFFFF with the actual value in the Extra block breaks other tools (such as marmelroy/Zip on iOS) that do not fully support Zip64.
The spec does allow the Extra block to omit the Disk Start Number (and the Relativ Header Offset) when they fit in the CDE, so we do just that for general compatibility.
Fixes haf#260