fix(serial): stop holding ports after deploy; name the holder on EBUSY (#1429)

[zackees]

Partial fix for #1429.

• Timeout errors now name the process holding the port (scans /proc/*/fd) instead of always blaming the driver.
• open_port closes and reopens a session whose reader thread exited (it kept the OS fd, so later opens got EBUSY).
• WebSocket cleanup always releases the client's writer so the idle close can run.

Not fixed: espflash thread outliving the deploy hard deadline (unconfirmed). Refs #1429.

Summary by CodeRabbit

• Bug Fixes
  • Reopening a serial port now recovers from stale sessions whose background reader has stopped, including when a subsequent open attempt times out.
  • Port-opening timeouts now identify processes holding the port when contention is detected. If no holder is found, the serial-driver diagnosis is retained and the timeout cause is noted as unknown.
  • WebSocket serial sessions now attempt to release the port during cleanup, helping prevent ports from remaining unavailable after a session ends.

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Reviewing files that changed from the base of the PR and between c01175b and 251e7ee.

📒 Files selected for processing (5)

• crates/fbuild-daemon/src/handlers/operations/monitor.rs
• crates/fbuild-daemon/src/handlers/websockets.rs
• crates/fbuild-serial/src/manager.rs
• crates/fbuild-serial/src/manager/tests.rs
• crates/fbuild-serial/src/port_holders.rs

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📝 Walkthrough

Walkthrough

The serial library now detects processes holding a port and closes open sessions whose reader task has exited. The daemon includes holder details in open-port timeout messages when available. Websocket cleanup always calls release_writer.

Changes

Serial port handling

Layer / File(s)	Summary
Detect serial-port holders crates/fbuild-serial/src/lib.rs, crates/fbuild-serial/src/port_holders.rs	The serial library exposes APIs that scan process file descriptors for the port path or its canonical path. Matching processes are sorted by PID, and their names and PIDs can be formatted as a contention hint.
Close sessions with exited readers crates/fbuild-serial/src/manager.rs, crates/fbuild-serial/src/manager/tests.rs	Before reusing an open session, open_port checks whether its reader task has exited. If so, it closes the stale session, restores aliases, and retries with the physical session key. Regression tests cover failed reopening and alias preservation.
Report port contention and release websocket writers crates/fbuild-daemon/src/handlers/operations/monitor.rs, crates/fbuild-daemon/src/handlers/websockets.rs	On open-port timeout, the daemon reports a detected holder or a possible serial-driver wedge. Websocket cleanup drops the writer_acquired parameter and always calls release_writer.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 251e7

The recovery path preserves recovered device paths, diagnostics avoid blocking async workers during descriptor scans, and timeout messages no longer claim a proven cause. No actionable merge-blocking risk remains after normal checks.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to 251e7

The cleanup improvements address stale port handles, but simultaneous recovery requests can tear down a newly recovered connection. Timeout responses also reveal host process identities to clients that can reach the daemon. These risks are bounded by the selected port, operating-system permissions, and network access to the daemon.

Retained concerns

• Medium · reliability · inferred: Stale detection and session removal are separate operations. One opener can observe a finished reader, pause while another opener recovers the port, and then remove the replacement session through close_port. The new recovery path can therefore discard a live connection and its writer/subscriber state. Its close operation does not verify that it is removing the session originally inspected; the impact is conditional on concurrent operations addressing the same physical session.
• Low · security · observed: Timeout responses newly disclose daemon-visible process names and PIDs matching a client-selected path. The inspected HTTP and WebSocket routes do not authenticate callers, and the existing listener binds all IPv4 interfaces. Anyone able to reach those routes can receive the diagnostic when an open times out and the scan finds a holder. This is additional host metadata exposure, not disclosure of file contents or credentials; timeout, scan visibility, and operating-system permissions constrain it.

Security review details

Security Blast Radius

• inferred — A recovery race affects the selected physical session and clients sharing it, not every session atomically. Requests can address multiple paths over time. Diagnostic exposure extends to matching process identities whose descriptor links the daemon can inspect; it does not return process memory, file contents, or credentials.

Security Findings and Attack Paths

• inferred — A reachable client supplies a path through monitor input or WebSocket Attach. If opening times out, the daemon resolves that path, scans accessible process descriptors, and returns matching names and PIDs. This introduces a conditional host-metadata disclosure through routes that previously returned generic timeout text.

Trust Boundaries and Controls

• inferred — WebSocket client IDs are supplied by the caller, so matching IDs coordinate writer ownership rather than establish authenticated identity. Existing reset handling already preempts serial sessions. Those preexisting capabilities prevent attributing general device-control authority to this PR, while leaving the new diagnostic disclosure and recovery race as distinct changes.

Resilience and Maintainability Implications

• observed — The descriptor scan runs on a blocking worker with a 500 ms wait budget, and failed or empty scans retain generic error text. Canonicalization occurs before that budget starts; the timeout bounds waiting for the scan, not the entire diagnostic operation or completion of an already-running blocking task.

Hardening Proposals

• proposed — Make stale recovery conditional on the same session identity or generation that was inspected, with coordination covering removal and publication. Explicitly define how existing writer and subscriber ownership terminates or transfers during replacement.
• proposed — Keep detailed holder identities in a trusted diagnostic channel, or authorize their disclosure separately from ordinary monitor access. Untrusted callers can retain a generic contention hint without process names and PIDs.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main changes: preventing lingering serial-port holders after deployment and identifying the process that holds a busy port. It is specific, concise, and related to the…

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Actionable comments posted: 3

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🤖 Prompt to fix review comments

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Inline comments:
Review comments at @crates/fbuild-daemon/src/handlers/websockets.rs:
- Line 141: Update the timeout message in websockets.rs at line 141 to say the
timeout cause is unknown, removing the unsupported EBUSY and driver-wedge claims
while retaining holder details. Apply the same wording correction in monitor.rs
at line 287, preserving its holder information.

Review comments at @crates/fbuild-serial/src/manager.rs:
- Line 128: Update the stale-session cleanup around close_port to retain the
physical session.port before closing it, then reopen that endpoint and restore
the logical alias so subsequent operations through original resolve to the
recovered session.

Review comments at @crates/fbuild-serial/src/port_holders.rs:
- Line 67: Update the diagnostic path around read_proc_tree to run the scan via
tokio::task::spawn_blocking with an owned proc_root, and bound how long callers
wait for its result. If the diagnostic wait expires, return the generic message;
preserve this behavior for both daemon timeout handlers.

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Configuration used: Repository: FastLED/fbuild/.coderabbit.yaml

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📥 Commits

Reviewing files that changed from the base of the PR and between 449695b and 8d2567b.

📒 Files selected for processing (6)

• crates/fbuild-daemon/src/handlers/operations/monitor.rs
• crates/fbuild-daemon/src/handlers/websockets.rs
• crates/fbuild-serial/src/lib.rs
• crates/fbuild-serial/src/manager.rs
• crates/fbuild-serial/src/manager/tests.rs
• crates/fbuild-serial/src/port_holders.rs

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[zackees]

@coderabbitai review

Addressed in the latest push: (1) timeout messages now say timeout cause unknown and no longer claim EBUSY; (2) stale-session reopen in open_port now reopens the physical endpoint and restores the logical alias (test: stale_session_cleanup_keeps_logical_alias_to_physical_port); (3) the /proc scan runs via spawn_blocking with a 500 ms bound.

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[zackees]

@coderabbitai resolve

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