Error in `Json(datasets.ArrowBasedBuilder)` class
Opened by JunShern (closed)

## Describe the bug
When a json file contains a `text` field that is larger than the block_size, the JSON dataset builder fails.

## Steps to reproduce the bug
Create a folder that contains the following:
```
.
├── testdata
│   └── mydata.json
└── test.py
```

Please download [this file](https://github.com/huggingface/datasets/files/7491797/mydata.txt) as `mydata.json`. (The error does not occur in JSON files with shorter text, but it is reproducible when the text is long as in the file I provide)
:exclamation: :exclamation: GitHub doesn't allow me to upload JSON so this file is a TXT, and you should rename it to `.json`!

`test.py` simply contains:
```python
from datasets import load_dataset
my_dataset = load_dataset("testdata")
```

To reproduce the error, simply run
```
python test.py
```

## Expected results
The data should load correctly without error.

## Actual results
The dataset builder fails with:
```
Using custom data configuration testdata-d490389b8ab4fd82
Downloading and preparing dataset json/testdata to /home/junshern.chan/.cache/huggingface/datasets/json/testdata-d490389b8ab4fd82/0.0.0/3333a8af0db9764dfcff43a42ff26228f0f2e267f0d8a0a294452d188beadb34...
100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 2264.74it/s]
100%|█████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 447.01it/s]
Failed to read file '/home/junshern.chan/hf-json-bug/testdata/mydata.json' with error <class 'pyarrow.lib.ArrowInvalid'>: JSON parse error: Missing a name for object member. in row 0
Traceback (most recent call last):
  File "test.py", line 28, in <module>
    my_dataset = load_dataset("testdata")
  File "/home/junshern.chan/.casio/miniconda/envs/hf-json-bug/lib/python3.8/site-packages/datasets/load.py", line 1632, in load_dataset
    builder_instance.download_and_prepare(
  File "/home/junshern.chan/.casio/miniconda/envs/hf-json-bug/lib/python3.8/site-packages/datasets/builder.py", line 607, in download_and_prepare
    self._download_and_prepare(
  File "/home/junshern.chan/.casio/miniconda/envs/hf-json-bug/lib/python3.8/site-packages/datasets/builder.py", line 697, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/junshern.chan/.casio/miniconda/envs/hf-json-bug/lib/python3.8/site-packages/datasets/builder.py", line 1156, in _prepare_split
    for key, table in utils.tqdm(
  File "/home/junshern.chan/.casio/miniconda/envs/hf-json-bug/lib/python3.8/site-packages/tqdm/std.py", line 1168, in __iter__
    for obj in iterable:
  File "/home/junshern.chan/.casio/miniconda/envs/hf-json-bug/lib/python3.8/site-packages/datasets/packaged_modules/json/json.py", line 146, in _generate_tables
    raise ValueError(
ValueError: Not able to read records in the JSON file at /home/junshern.chan/hf-json-bug/testdata/mydata.json. You should probably indicate the field of the JSON file containing your records. This JSON file contain the following fields: ['text']. Select the correct one and provide it as `field='XXX'` to the dataset loading method. 
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.15.1
- Platform: Linux-5.8.0-63-generic-x86_64-with-glibc2.17
- Python version: 3.8.12
- PyArrow version: 6.0.0


Comments:
I have additionally identified the source of the error, being that [this condition](https://github.com/huggingface/datasets/blob/fc46bba66ba4f432cc10501c16a677112e13984c/src/datasets/packaged_modules/json/json.py#L124-L126) in the file
`python3.8/site-packages/datasets/packaged_modules/json/json.py` is not being entered correctly:
```python
                                    if (
                                        isinstance(e, pa.ArrowInvalid)
                                        and "straddling" not in str(e)
                                        or block_size > len(batch)
                                    ):
```

From what I can tell, in my case the block_size simply needs to be increased, but the error message does not contain "straddling" so the condition does trigger correctly and we fail to reach [the line to increase block_size](https://github.com/huggingface/datasets/blob/fc46bba66ba4f432cc10501c16a677112e13984c/src/datasets/packaged_modules/json/json.py#L135).

Changing the condition above to simply
```python
                                    if (
                                        block_size > len(batch)
                                    ):
```

Fixes the error for me. I'm happy to create a PR containing this fix if the developers deem the other conditions unnecessary.

Hi ! I think the issue comes from the fact that your JSON file is not a valid JSON Lines file.
Each example should be on one single line.

Can you try fixing the format to have one line per example and try again ?

:open_mouth: you're right, that did it! I just put everything on a single line (my file only has a single example) and that fixed the error. Thank you so much!