Archive corrupted when contents added after header creation

[elbe0046]

If additional contents have been appended to a file after the Header is created but before the file contents are written to the archive the archive can become corrupted. Particularly if the additional contents straddle a 512-byte boundary.

Such that a tar -tf on the resulting archive reveals the following errors:

OS X

bugreport-3011000408-20220302_222509/logs/file1
tar: Damaged tar archive
tar: Retrying...

Linux

bugreport-3011000408-20220303_011636/logs/file1
tar: Skipping to next header

For example, this can happen when building an archive from log files, where processes are continually writing to the file even during the archiving process.

Upon inspecting a corrupted archive we see for a file file1 that has had additional contents written since header creation:

• the header for file1 starts at 0x0032c400
• the header size for file is 0x001C991B, so the next record should start at 0x004F5E00 (because this calculation is rounded up to the nearest 512 byte alignment)
• And yet we see that the file1 contents exceed what the header specifies, ending at 0x00526A19
• file2's header begins at 0x00526c00