tape_archive's reconstruct operation can build an incorrect table and make data unreadable

[Eric Swenson]

The reconstruct operation of tape archive rebuilds an online table from the files on a volume set, for use when no table copy on the tape can be loaded with load table. The table it builds is wrong in several ways, and one of them can destroy data on the tape.

On a tape written by tape\_archive with 55 components, of which ten are longer than two pages:

ta reconstruct foo tatest
ta x foo
ta go foo
Mounting volume "tatest" with no write ring
Mounted ANSI volume "TATEST" (recorded at 800 BPI), on device tapa_04
tape_archive: End of information reached. Unexpected error while processing >udd>...>multics_unsqueeze.pl1
tape_archive: End of information reached. multics_unsqueeze.pl1 could not be extracted .
   ... (the same for the other nine components longer than two pages) ...
tape_archive: Invalid I/O operation. Unexpected error while processing tape copy of online table
tape_archive: Invalid I/O operation.

ta load_table of the same tape, followed by the x and go operations, extracts all 55 components without error.

[Eric Swenson]

​https://s3.amazonaws.com/eswenson-multics/public/mcrs/MCR10197.pdf