[I] bug: WebDAV writer rejects streaming writes larger than one buffer [opendal]

["KiSetsuFu-PuLiN (via GitHub)" <gi...@apache.org>]

KiSetsuFu-PuLiN opened a new issue, #8291:
URL: https://github.com/apache/opendal/issues/8291

   ### OpenDAL version or commit
   
   v0.58.1 (the same one-shot writer is still present on main)
   
   ### Affected service or component
   
   services/webdav against a dav-server WebDAV endpoint
   
   ### Describe the bug
   
   The WebDAV writer is a one-shot writer (`oio::OneShotWriter`). One writer only accepts a single `write` call, so any streaming write that exceeds one internal buffer (256 KiB) fails with:
   
   ```
   Unsupported (permanent) at write => OneShotWriter doesn't support multiple write
   Context:
     service: webdav
     path: ...
     size: 262144
     written: 262144
   ```
   
   `FuturesAsyncWriter` (returned by `Operator::writer(...).into_futures_async_write()`) flushes its 256 KiB buffer by calling `write` again, which the one-shot writer rejects. As a result, a single writer cannot upload files larger than 256 KiB.
   
   ### Steps to Reproduce
   
   ```rust
   use futures::AsyncWriteExt;
   
   let mut writer = op.writer("big.bin").await?.into_futures_async_write();
   writer.write_all(&vec![0u8; 600 * 1024]).await?; // fails with the error above
   writer.close().await?;
   ```
   
   Reproduced against a real `dav-server` WebDAV endpoint (`dav_server::localfs::LocalFs`) through `opendal::services::Webdav`.
   
   ### Actual Behavior
   
   The second `write` on the same writer fails with `Unsupported (permanent) at write => OneShotWriter doesn't support multiple write`.
   
   ### Expected Behavior
   
   A single writer accepts multiple writes and uploads the data in ordered chunks (for example, the first chunk with a regular PUT and the following chunks with partial PUTs carrying `Content-Range`), and the service reports `write_can_multi = true`.
   
   ### Additional Context
   
   `write_can_multi` is currently `false` for webdav even though `Operator::writer` hands users a writer they can write to multiple times. `write_total_max_size` / server support for partial PUTs may need to be documented as part of the fix.
   
   ### Reproduction confirmation
   
   - [x] I reproduced this issue against the affected component. For a service-specific report, I used the actual service rather than only a mock, emulator, or source-code analysis.
   
   ### Are you willing to submit a PR to fix this bug?
   
   - [x] Yes, I would like to submit a PR.
   


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