[Bug-tar] Appending file(s) to tar file may silently fail

[Hendrik Grewe]

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Hi!
If one tries to append a file to an existing .tar file through

  tar --append --file defective.tar fileToAdd.xxx

tar may return exit code 0 and terminate without error though the
fileToAdd.xxx is not added to the .tar file. However the tarfile has
been changed in some kinde since md5sums of defective.tar befor the
non working call and after the call differ.

If the above command is issued a second time the file `fileToAdd.xxx`
is correctly appendet to the defective.tar. (So defective.tar must
have been `repaired` in some weird way)

The expected result is either

  - the appending of fileToAdd.xxx to defective.tar _DID NOT_ work and
tar returns with an error (exit code !=0) indicating fileToAdd.xxx
could not be added

.OR.
  - the appending of fileToAdd.xxx to defective.tar _DID_ work and tar
exits with exit code 0 and fileToAdd.xxx is included into defective tar.

The tar file I am experiencing this issue is generated by
pg_basebackup, an official backup programm for Postgres Database. I
issued a bug report there, too since their generated .tar file is
somewhat defective. Since tar did not complain about this "defective"
tar it is a bug within tar also.

The defective tar file (generated by pg_basebackup) may be found here:

https://depot.tu-dortmund.de/get/8aq4g

If you need any further information feel free to contact me.
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