new feature: Improve sftp error propagation so we don't get just "timeout"

[vt-alt]

Feature Description

We are users of rustic software which have opendal as backend to access sftp. Currently, rustic does not report much about sftp/ssh errors except "timeout", see, this makes debugging ssh/sftp issues extremely hard and confusing. Rustic author suggested us to report the issue to your project.

Could you please improve error reporting and propagation for sftp backend up to the caller. Thanks.

Problem and Solution

Hopefully, we get all low-level network and sftp/ssh protocol errors up with human readable message.

Additional Context

No response

Are you willing to contribute to the development of this feature?

• Yes, I am willing to contribute to the development of this feature.

[suyanhanx]

Thanks for reporting this.

I took a closer look at the SFTP path, and the main problem here seems to be that we currently apply a generic timeout around pooled connection acquisition.

Because pool.get() can cover multiple stages, different failures can get flattened into the same connection request: timeout error, including:

• waiting for a pooled connection
• establishing the SSH connection
• initializing the SFTP session

That matches the reports here and in rustic-rs/rustic#1488: some cases surface the underlying SFTP error correctly, while others only end up with the generic timeout, which makes debugging much harder than it should be.

I don't think timeout protection itself should be removed. The problem is that the current timeout is applied at the wrong layer.

The small patch I'm sending keeps the timeout behavior, but splits it by stage instead:

• preserve underlying SSH/SFTP errors whenever possible
• apply connect_timeout to the SSH connect stage
• keep a separate acquire_timeout for waiting on a saturated pooled connection
• avoid collapsing those paths back into the same generic timeout

This keeps the change focused on error propagation and timeout placement, without widening scope into broader SSH configuration changes.