fix: Fix read result not full

[jiaoew1991]

After actual testing, it was found that using self.inner.read will read data based on the size of internal bytes. By using self.inner.read_exact, can read the input buffer size directly.

[xyjixyjixyji]

LGTM, thanks

[Xuanwo]

This PR is wrong, we can't guarantee that the source has enough data.

[xyjixyjixyji]

This PR is wrong, we can't guarantee that the source has enough data.

I thought this function is supposed to read 'len' bytes?

[xyjixyjixyji]

This PR is wrong, we can't guarantee that the source has enough data.

/// ```Rust
/// # futures::executor::block_on(async {
/// use futures::io::{self, AsyncReadExt, Cursor};
///
/// let mut reader = Cursor::new([1, 2, 3, 4]);
/// let mut output = [0u8; 5];
///
/// let result = reader.read_exact(&mut output).await;
///
/// assert_eq!(result.unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
/// # });
/// ```

This is the documentation of read_exact, which shows reader.read_exact() returns an error if the source does not have enough data.

Is there anything different between opendal_reader_read and this?

[Xuanwo]

This is the documentation of read_exact, which shows reader.read_exact() returns an error if the source does not have enough data.

Please note that read() and read_exact() are different APIs that have different semantic.

• For read(buf, len) -> Result<read_size>, users will input a buffer with the maximum number of bytes to read and we will return read_size by which 0 indicates we have reached EOF.
• For read_exact(buf, len) -> Result<()>, users will input a buffer with expected number of bytes to read and we will return UnexpectedEof when source doesn't have enough data to read.

It's ok to provide an API called opendal_reader_read_exact() but we can't implement like this for opendal_reader_read(). Users must repeatedly call opendal_reader_read() until it returns 0, indicating that the buffer is full or the source has reached EOF.

[Xuanwo]

I thought this function is supposed to read 'len' bytes?

len represents the size of this buffer. However, there's no guarantee that we'll need to fill it completely. It's possible for underlying storage to read only 1 bytes every time.

[xyjixyjixyji]

I thought this function is supposed to read 'len' bytes?

len represents the size of this buffer. However, there's no guarantee that we'll need to fill it completely. It's possible for underlying storage to read only 1 bytes every time.

I see, I will raise a PR to revert this.

[Xuanwo]

I see, I will raise a PR to revert this.

Fixed in #3282