resolving a weird tar extract issue

ok, every so often,  I have been getting a strange failure on dataset install:

```
$ python -c 'import sys; from datasets import load_dataset; ds=load_dataset(sys.argv[1])' HuggingFaceM4/general-pmd-synthetic-testing
No config specified, defaulting to: general-pmd-synthetic-testing/100.unique
Downloading and preparing dataset general-pmd-synthetic-testing/100.unique (download: 3.21 KiB, generated: 16.01 MiB, post-processed: Unknown size, total: 16.02 MiB) to /home/stas/.cache/huggingface/datasets/HuggingFaceM4___general-pmd-synthetic-testing/100.unique/1.1.1/86bc445e3e48cb5ef79de109eb4e54ff85b318cd55c3835c4ee8f86eae33d9d2...
Extraction of data is blocked (illegal path)
Extraction of data/1 is blocked (illegal path)
Extraction of data/1/text.null is blocked (illegal path)
[...]
```

I had no idea what to do with that - what in the world does **illegal path** mean?

I started looking at the code in `TarExtractor` and added a debug print of `base` so that told me that there was a problem with the current directory - which was a clone of one of the hf repos.

This particular dataset extracts into a directory `data` and the current dir I was running the tests from already had `data` in it which was a symbolic link to another partition and somehow all that `badpath` code was blowing up there.

https://github.com/huggingface/datasets/blob/80eb8db74f49b7ee9c0f73a819c22177fabd61db/src/datasets/utils/extract.py#L113-L114

I tried hard to come up with a repro, but no matter what I tried it only fails in that particular clone directory that has a `data` symlink and not anywhere else.

In any case, in this PR I'm proposing to at least give a user a hint of what seems to be an issue. 

I'm not at all happy with the info I got with this proposed change, but at least it gave me a hint that `TarExtractor` tries to extract into the current directory without any respect to pre-existing files. Say what?

https://github.com/huggingface/datasets/blob/80eb8db74f49b7ee9c0f73a819c22177fabd61db/src/datasets/utils/extract.py#L110

why won't it use the `datasets` designated directory for that? There would never be a problem if it were to do that.

I had to look at all those `resolved`, `badpath` calls and see what it did and why it failed, since it was far from obvious. It appeared like it resolved a symlink and compared it to the original path which of course wasn't matching.

So perhaps you have a better solution than what I proposed in this PR. I think that code line I quoted is the one that should be fixed instead. 

But if you can't think of a better solution let's merge this at least so that the user will have a clue that the current dir is somehow involved.

p.s. I double checked that if I remove the pre-existing `data` symlink in the current dir I'm running the dataset install command from, the problem goes away too.

Thanks.


<details>
<summary>Show benchmarks</summary>

PyArrow==6.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.011815 / 0.011353 (0.000463) | 0.006407 / 0.011008 (-0.004601) | 0.132937 / 0.038508 (0.094429) | 0.040634 / 0.023109 (0.017525) | 0.398049 / 0.275898 (0.122151) | 0.498207 / 0.323480 (0.174727) | 0.010111 / 0.007986 (0.002126) | 0.007282 / 0.004328 (0.002954) | 0.103661 / 0.004250 (0.099411) | 0.046223 / 0.037052 (0.009171) | 0.411490 / 0.258489 (0.153001) | 0.480973 / 0.293841 (0.187132) | 0.058397 / 0.128546 (-0.070149) | 0.019952 / 0.075646 (-0.055695) | 0.440734 / 0.419271 (0.021463) | 0.064585 / 0.043533 (0.021052) | 0.392556 / 0.255139 (0.137417) | 0.437842 / 0.283200 (0.154643) | 0.130684 / 0.141683 (-0.010999) | 1.910552 / 1.452155 (0.458397) | 1.984644 / 1.492716 (0.491927) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.264417 / 0.018006 (0.246411) | 0.676519 / 0.000490 (0.676030) | 0.003369 / 0.000200 (0.003169) | 0.000125 / 0.000054 (0.000071) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.034558 / 0.037411 (-0.002854) | 0.126561 / 0.014526 (0.112035) | 0.134478 / 0.176557 (-0.042079) | 0.202125 / 0.737135 (-0.535010) | 0.143273 / 0.296338 (-0.153066) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.618592 / 0.215209 (0.403383) | 6.224435 / 2.077655 (4.146780) | 2.636689 / 1.504120 (1.132569) | 2.243507 / 1.541195 (0.702313) | 2.312449 / 1.468490 (0.843959) | 1.188499 / 4.584777 (-3.396277) | 5.738347 / 3.745712 (1.992635) | 4.891933 / 5.269862 (-0.377929) | 2.697631 / 4.565676 (-1.868046) | 0.140200 / 0.424275 (-0.284076) | 0.015484 / 0.007607 (0.007877) | 0.781947 / 0.226044 (0.555903) | 7.946600 / 2.268929 (5.677671) | 3.365574 / 55.444624 (-52.079050) | 2.783443 / 6.876477 (-4.093034) | 2.738634 / 2.142072 (0.596561) | 1.487247 / 4.805227 (-3.317980) | 0.255681 / 6.500664 (-6.244983) | 0.084607 / 0.075469 (0.009138) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.717846 / 1.841788 (-0.123941) | 18.405566 / 8.074308 (10.331258) | 20.508578 / 10.191392 (10.317186) | 0.262364 / 0.680424 (-0.418060) | 0.050881 / 0.534201 (-0.483319) | 0.587516 / 0.579283 (0.008232) | 0.650900 / 0.434364 (0.216536) | 0.656168 / 0.540337 (0.115830) | 0.778876 / 1.386936 (-0.608061) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.010817 / 0.011353 (-0.000536) | 0.007338 / 0.011008 (-0.003670) | 0.131949 / 0.038508 (0.093441) | 0.037244 / 0.023109 (0.014135) | 0.565994 / 0.275898 (0.290096) | 0.567434 / 0.323480 (0.243954) | 0.007733 / 0.007986 (-0.000252) | 0.005216 / 0.004328 (0.000887) | 0.096578 / 0.004250 (0.092328) | 0.056001 / 0.037052 (0.018949) | 0.538209 / 0.258489 (0.279720) | 0.580385 / 0.293841 (0.286544) | 0.053654 / 0.128546 (-0.074892) | 0.019471 / 0.075646 (-0.056176) | 0.448781 / 0.419271 (0.029509) | 0.064774 / 0.043533 (0.021241) | 0.540222 / 0.255139 (0.285083) | 0.563058 / 0.283200 (0.279858) | 0.122716 / 0.141683 (-0.018967) | 1.839402 / 1.452155 (0.387247) | 1.915523 / 1.492716 (0.422806) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.310448 / 0.018006 (0.292442) | 0.603664 / 0.000490 (0.603175) | 0.004833 / 0.000200 (0.004633) | 0.000145 / 0.000054 (0.000090) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.032340 / 0.037411 (-0.005072) | 0.130115 / 0.014526 (0.115589) | 0.154192 / 0.176557 (-0.022364) | 0.200655 / 0.737135 (-0.536480) | 0.144961 / 0.296338 (-0.151377) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.671588 / 0.215209 (0.456379) | 6.691642 / 2.077655 (4.613988) | 2.915230 / 1.504120 (1.411110) | 2.573337 / 1.541195 (1.032143) | 2.578204 / 1.468490 (1.109714) | 1.249028 / 4.584777 (-3.335749) | 5.808539 / 3.745712 (2.062827) | 3.079317 / 5.269862 (-2.190545) | 2.033308 / 4.565676 (-2.532369) | 0.142411 / 0.424275 (-0.281864) | 0.015525 / 0.007607 (0.007918) | 0.800389 / 0.226044 (0.574345) | 8.228236 / 2.268929 (5.959308) | 3.660207 / 55.444624 (-51.784417) | 3.021033 / 6.876477 (-3.855444) | 3.088335 / 2.142072 (0.946263) | 1.380137 / 4.805227 (-3.425091) | 0.252065 / 6.500664 (-6.248599) | 0.084302 / 0.075469 (0.008833) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.709429 / 1.841788 (-0.132359) | 18.358770 / 8.074308 (10.284462) | 21.109844 / 10.191392 (10.918452) | 0.231549 / 0.680424 (-0.448875) | 0.029251 / 0.534201 (-0.504950) | 0.560719 / 0.579283 (-0.018564) | 0.610125 / 0.434364 (0.175761) | 0.630015 / 0.540337 (0.089678) | 0.751656 / 1.386936 (-0.635280) |

</details>
</details>

![](https://cml.dev/watermark.png#18baf4eebf71c0db1d9980f7ee164f1272ff8f26 "CML watermark")


The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5441). All of your documentation changes will be reflected on that endpoint.

I think I managed to reproduce it:

```
rm -rf ~/.cache/huggingface/datasets/HuggingFaceM4___cm4-synthetic-testing
mkdir -p /tmp/xxx/hf-data
sudo ln -s /tmp/xxx /test
mkdir -p /tmp/yyy
ln -sf /test/hf-data /tmp/yyy/data
cd /tmp/yyy
python -c 'import sys; from datasets import load_dataset; ds=load_dataset(sys.argv[1])' HuggingFaceM4/cm4-synthetic-testing
```

Please note it includes a creation of a symlink from the `/` (so `sudo`) - may be there is a simpler way but I'm just trying to replicate the real setup. Of course please be careful - it's mostly under `/tmp` not to destroy anything if you try to run this.

this fails with:

```
No config specified, defaulting to: cm4-synthetic-testing/100.unique
Downloading and preparing dataset cm4-synthetic-testing/100.unique (download: 20.71 KiB, generated: 49.99 MiB, post-processed: Unknown size, total: 50.01 MiB) to /home/stas/.cache/huggingface/datasets/HuggingFaceM4___cm4-synthetic-testing/100.unique/1.1.1/2e33dcc086c7209b8ccff4b19e44f1d41b5be53262e7d793142b96c2e984602b...
Extraction of data is blocked (illegal path: /tmp/yyy)
[...]
Extraction of data/115/texts_03.txt is blocked (illegal path: /tmp/yyy)
Generating 100.unique split:   0%|                                                                         | 0/100 [00:00<?, ? examples/s]Generating 100-long unique records split

Traceback (most recent call last):
  File "/mnt/nvme0/code/huggingface/datasets-master/src/datasets/builder.py", line 1571, in _prepare_split_single
    for key, record in generator:
  File "/home/stas/.cache/huggingface/modules/datasets_modules/datasets/HuggingFaceM4--cm4-synthetic-testing/2e33dcc086c7209b8ccff4b19e44f1d41b5be53262e7d793142b96c2e984602b/cm4-synthetic-testing.py", line 190, in _generate_examples
    raise ValueError(f"can't find any data - check {data_path}")
ValueError: can't find any data - check /home/stas/.cache/huggingface/datasets/downloads/extracted/134227b9b94c4eccf19b205bf3021d4492d0227b9be6c2ddb6bf517d8d55a8cb/data

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/mnt/nvme0/code/huggingface/datasets-master/src/datasets/load.py", line 1757, in load_dataset
    builder_instance.download_and_prepare(
  File "/mnt/nvme0/code/huggingface/datasets-master/src/datasets/builder.py", line 860, in download_and_prepare
    self._download_and_prepare(
  File "/mnt/nvme0/code/huggingface/datasets-master/src/datasets/builder.py", line 1612, in _download_and_prepare
    super()._download_and_prepare(
  File "/mnt/nvme0/code/huggingface/datasets-master/src/datasets/builder.py", line 953, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/mnt/nvme0/code/huggingface/datasets-master/src/datasets/builder.py", line 1450, in _prepare_split
    for job_id, done, content in self._prepare_split_single(
  File "/mnt/nvme0/code/huggingface/datasets-master/src/datasets/builder.py", line 1607, in _prepare_split_single
    raise DatasetGenerationError("An error occurred while generating the dataset") from e
datasets.builder.DatasetGenerationError: An error occurred while generating the dataset
```

note that `illegal path: /tmp/yyy` is now with the mods of this PR.

----------------------

Also I think the whole thing should have failed at the first `illegal path` and not continue running. But as it continued and gave:


> ValueError: can't find any data - check /home/stas/.cache/huggingface/datasets/downloads/extracted/134227b9b94c4eccf19b205bf3021d4492d0227b9be6c2ddb6bf517d8d55a8cb/data

what can a user do with that other than confirming that that dir is indeed empty, but no clue is given to why and it's far from obvious that one needs to scroll up and discover earlier issues. Most users won't do that.

(my apologies for writing out so much - was trying to make the situation clear)

Thank you, Albert, for the explanation.

To summarize I think what's needed is:

1. add a comment in the code to why this is done for someone being puzzled over the odd code
2. and to use an actionable by the user error message
3. perform an untrapped assert on that tar extract error and not continue, so that the user will not get a later misleading error that the folder is empty and is completely not actionable and it's is far from obvious that one needs to scroll up to find earlier errors, which were trapped.

After reading the advisory I'm still not sure why `cwd` is used and not a designated `~/.cache/huggingface/datasets/downloads/extracted`, I can't see what difference does it make since I could `chdir` to the designated directory and it would be `cwd`. The security solution is trying to ensure that `/etc/passwd` won't get overriden. So why is the check done in `.` and not the real target base directory, since the extraction isn't done in the current working dir. By not using `.` you lower the chances that the user will have all sorts of local symlinks that could trigger the issue since `datasets` typically is the only one managing it's `~/.cache/huggingface/datasets` domain and 99.9% of the time the user won't manually create files in it.

thank you!