Crash when dumping tar.gz or tar format from snapshot

[cultpony]

name = "rustic-rs"
operating_system = "Ubuntu 24.4.0 (noble) [64-bit]"
crate_version = "0.9.5"
explanation = """
Panic occurred in file '/home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/rustic_core-0.7.2/src/vfs.rs' at line 540
"""
cause = "index out of bounds: the len is 1 but the index is 18446744073709551615"
method = "Panic"
backtrace = """
   0:     0x5584050da049 - <unresolved>
   1:     0x558405e2a077 - <unresolved>
   2:     0x558405f4eeaf - <unresolved>
   3:     0x558405f4ecc8 - <unresolved>
   4:     0x558405f4ec29 - <unresolved>
   5:     0x558405f4ec1c - <unresolved>
   6:     0x558404ecb31f - <unresolved>
   7:     0x558404ecb2f1 - <unresolved>
   8:     0x558405cdae02 - <unresolved>
   9:     0x558405cda893 - <unresolved>
  10:     0x558405f5c4eb - <unresolved>
  11:     0x558405cc4b1b - <unresolved>
  12:     0x558405e0ba04 - <unresolved>
  13:     0x558405e06744 - <unresolved>
  14:     0x558404f1e9f3 - <unresolved>
  15:     0x558404f213b8 - <unresolved>
  16:     0x558404f25463 - <unresolved>
  17:     0x558404f21ba3 - <unresolved>
  18:     0x558404f258e5 - <unresolved>
  19:     0x6ffed042a1ca - <unresolved>
  20:     0x6ffed042a28b - __libc_start_main
  21:     0x558404f152f5 - <unresolved>
  22:                0x0 - <unresolved>
"""

Reproducer:

A repository was created on a relatively fresh Ubuntu VM with no extra options on init other than password and local store.

A backup is created by configuring multiple snapshots with associated sources to backup the entire machine (one-file-system to restrict it to only disk data), make a mysql dump prior to one snapshot and backup the boot loader.

Forget is not configured.

When running rustic dump --archive tar --file dump.tar.gz <snapshot> (or with targz instead of tar), the command crashes, producing above crashdump;

[INFO] using config /root/.config/rustic/rustic.toml
[INFO] repository local:/var/backup: password is correct.
[INFO] using cache at /root/.cache/rustic/<id>
[00:00:00] reading index...  ████████████████████████████████████████          6/6
[00:00:00] getting snapshot...   ████████████████████████████████████████          0
[INFO] getting snapshot ...
Well, this is embarrassing.

rustic-rs had a problem and crashed. To help us diagnose the problem you can send us a crash report.

We have generated a report file at "/tmp/report-0161cd70-d755-4f45-b68e-5b861c7fea64.toml". Submit an issue or email with the subject of "rustic-rs Crash Report" and include the report as an attachment.

- Homepage: https://rustic.cli.rs/
- Authors: the rustic-rs team

We take privacy seriously, and do not perform any automated error collection. In order to improve the software, we rely on people to submit reports.

Thank you kindly!
Aborted (core dumped)

Disabling cache has no effect. The snapshot ID definitely exists and the mysql snapshot can be dumped this way, it's only the root filesystem snapshot that can't be dumped this way.

My goal with the configuration is to have a full restic repo locally on the machine, but due to some restrictions, I have to store the backup externally as a tar file, zips are only useful as a workaround.

Running rustic check --read-data doesn't indicate any corruption. This happens even when recreating the backup repository.

[aawsome]

Thanks @cultpony for opening this issue - you have discovered a bug which appears when reading empty (0 size) files from the repository. I'll make a fix for this.