fix: surface connection failures and clean failed mounts

[uhs-robert]

Summary

Fixes #10 by surfacing native SSH/SSHFS failures and cleaning up only mount directories owned by the failed connection attempt.

Changes

• preserve the first non-empty SSH process output when stderr is empty
• surface SSHFS exit codes while preserving stdout and stderr separately
• fail fast only for unresolved-host errors instead of opening interactive authentication
• preserve the existing interactive-auth fallback for all other batch failures
• return mount-directory ownership from MountPoint.get_or_create()
• make mount-directory ownership race-safe across concurrent Neovim processes
• clear PRE_MOUNT_DIRS state after failed setup
• remove only empty, inactive mount directories created by the failed attempt
• make failed-directory cleanup best-effort so cleanup errors do not mask the connection error
• report a connections.socket_dir that exceeds the Unix socket path limit from :checkhealth

Socket path length

ControlMaster sockets are Unix domain sockets, so the kernel caps their path at sun_path: 108 bytes on Linux, 104 on macOS and the BSDs. SSH appends /%C (a 40 character hash) plus a temporary suffix while creating one, which leaves 49 characters for connections.socket_dir on Linux and 45 on macOS.

Past that, every connection fails with:

unix_listener: path "/…/sockets/280c4fee….yp8WhV7cdz7iQXdQ" too long for Unix domain socket

That message names neither the setting at fault nor the limit involved, and it surfaces as an authentication failure rather than a configuration error. :checkhealth sshfs now reports it directly: an error past the budget, a warning within ten characters of it, and the measured budget either way.

Testing

Regression coverage is included, on top of the shared harness from #25. Run it with make test.

• unresolved-host diagnostics in both the Linux and macOS resolver wordings, skipping interactive authentication while other batch failures still reach it
• authentication failures reporting both the batch and interactive exit codes
• SSHFS failures preserving the exit code, preferring stderr over stdout, ignoring whitespace-only output, and still reporting a code when the process says nothing
• mount-directory ownership from get_or_create, including the concurrent-creation race and the non-raising failure path
• cleanup removing only a directory this attempt created and left empty, never one that pre-existed, holds contents, or turns out to be an active mount
• a cleanup error not masking the connection error, and a failed attempt never reaching the lockfile
• the successful connection path
• the socket path length check: short paths, the exact limit, the warning band, the smaller macOS budget, and that a missing ~/.ssh does not hide a broken socket path

Closes #10

[Copilot]

🟡 Changes recommended

The PR description states it must have regression coverage delivered via #24 before it is considered ready to merge, and those tests are not included here.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR improves sshfs.nvim’s connection failure diagnostics (surfacing native SSH/SSHFS exit codes and output) and tightens failure cleanup to only remove mount directories created by the failed connection attempt.

Changes:

• Preserve the first non-empty SSH process output (stderr preferred, otherwise stdout) to avoid losing diagnostics when stderr is an empty string.
• Surface SSHFS non-zero exit codes and preserve stdout/stderr separately in failure results.
• Track whether a mount directory was created by the current attempt and, on failure, best-effort remove only empty/inactive directories created by that attempt while clearing PRE_MOUNT_DIRS state.

File summaries

File	Description
lua/sshfs/session.lua	Clears pre-mount bookkeeping on failures and removes only empty/inactive mount dirs created by the failed attempt.
lua/sshfs/lib/sshfs.lua	Adds unresolved-host fast-fail and enriches mount/auth failure results with exit codes and separated output.
lua/sshfs/lib/ssh.lua	Introduces a helper to select the first non-empty trimmed process output (fixing empty-stderr masking).
lua/sshfs/lib/mount_point.lua	Extends get_or_create() to return both readiness and whether the directory was created by this call.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

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