14 KiB
Durable design constraints and decisions
Only constraints supported by implementation or current project documentation belong here. When original rationale is unknown, the entry describes the observable constraint without inventing intent.
One broker mediates all production serial transports
Decision: USB CDC, WebSocket, and role-user SSH access UART1 through session_broker; transports do not independently own the serial service.
Rationale/evidence: The broker is initialized after the serial service and all transport implementations connect broker clients. It is the normal serial RX consumer and TX gate. Project documentation requires one writer and multiple observers.
Consequence for future changes: New serial transports must become broker clients. Do not bypass writer checks or consume serial_service RX directly. serial_service_start() is not idempotent, so admission code must reconcile check/start races as the existing transports do. Broker paths enter serial-service APIs while holding the broker mutex; preserve that lock order and do not call back into the broker while holding the serial state mutex. Preserve binary transparency and avoid in-band ownership control.
Relevant files: src/session_broker.{h,c}, src/serial_service.{h,c}, src/usb_cdc_transport.c, src/web_serial_transport.c, src/ssh_transport.c
Slow clients are isolated by bounded per-client storage
Decision: UART RX is drained and copied into independent bounded broker output streams; a full observer loses only its own copy.
Rationale/evidence: session_broker accounts per-client dropped bytes instead of blocking fan-out. The roadmap records slow-client isolation as a project-wide constraint.
Consequence for future changes: Do not replace fan-out with a blocking shared queue. Any added transport must tolerate partial/no-progress reads and expose drop/backpressure counters.
Relevant files: src/session_broker.c, src/session_broker.h, docs/roadmap.md
Physical UART ownership and logical writer ownership remain separate
Decision: rs232_port_owner controls whether diagnostics or the serial service may manipulate UART/MAX3243 hardware; the broker separately controls which connected client may write.
Rationale/evidence: The code has explicit NONE, PHASE0, SERVICE, and FAULT hardware states plus broker client/writer IDs.
Consequence for future changes: A writer lease never authorizes direct UART/GPIO access. Active hardware tests must claim PHASE0; the production service must claim SERVICE. Boot-time static-safe GPIO setup and service-owned static-mode restoration are explicit exceptions to this cooperative gate. Ambiguous cleanup must keep the transceiver safe and require reboot rather than clearing fault casually.
Relevant files: src/rs232_port_owner.{h,c}, src/rs232_hw_test.c, src/serial_service.c, src/session_broker.c
Resource IDs are generation-safe
Decision: Broker clients, SSH/WebSocket slots, queued admin work, and user principals carry generations or random stable IDs to reject stale references and slot reuse.
Rationale/evidence: Broker IDs encode slot generation; transports track slot generations; admin tokens include session/slot generation; user principal currentness includes account ID and authentication generation.
Consequence for future changes: Preserve transport-slot generations and account-authentication generations as distinct concepts. Validate tokens immediately before side effects and discard late work after disconnect/reuse/revocation.
Relevant files: src/session_broker.{h,c}, src/ssh_transport.c, src/web_serial_transport.c, src/admin_ssh_console.c, src/user_database.{h,c}
UART0 is the physical recovery authority
Decision: UART0 remains independent of UART1 and networking. Initial administrator bootstrap and explicit unavailable-user-database recovery are restricted to UART0.
Rationale/evidence: main.c configures UART0 separately; command policy and user handlers deny these operations remotely. README/roadmap identify UART0 as the trusted recovery console.
Consequence for future changes: Network failures or credential corruption must not remove UART0 recovery. Do not expose bootstrap/recovery through web or admin SSH without an explicit security redesign.
Relevant files: src/main.c, src/admin_ssh_console.c, src/user_console.c, docs/roadmap.md
Admin SSH and user SSH are different routes
Decision: A role-user SSH session becomes a broker serial client. A role-admin session enters the administration console and never obtains a broker client/writer lease.
Rationale/evidence: Role routing is explicit after SSH authentication. The administrative shell is intended for command execution, not multiplexed serial data.
Consequence for future changes: Do not silently give administrators both streams or infer that higher privilege means UART1 ownership. A route-switch feature would require explicit protocol, lifecycle, and authorization design.
Relevant files: src/ssh_transport.c, src/admin_ssh_console.{h,c}, src/session_broker.c
One dispatcher executes the canonical command registry
Decision: UART0 and admin SSH submit complete lines to one fixed queue; one task is the sole caller of esp_console_run().
Rationale/evidence: The implementation treats ESP-IDF console execution as non-reentrant and removes the need for separate remote command implementations.
Consequence for future changes: Register one canonical handler rather than creating a second SSH dispatcher. Long commands/prompts block all administration, so keep handlers bounded or explicitly asynchronous. Preserve output routing and remote principal checks.
Relevant files: src/admin_ssh_console.c, src/main.c, src/console_input.c, all src/*_console.c
Selected self-affecting admin SSH actions use bounded deferred control
Decision: Remote reboot, SSH stop/disconnect, and host-key rotate/reset are deferred until command state and administration/transport application buffers appear drained, with a ten-second limit and short final delay.
Rationale/evidence: admin_ssh_console has a separate bounded control task and pending-action state. The check is a best-effort application-buffer heuristic, not peer-delivery confirmation. User account mutations and their immediate revocation calls do not use this path.
Consequence for future changes: Actions that would invalidate their own SSH transport should integrate with deferred control when acknowledgement preservation matters. Prevent new input while an action is pending, keep the wait bounded, and do not describe it as guaranteed delivery.
Relevant files: src/admin_ssh_console.c, src/system_console.c, src/ssh_console.c, src/ssh_transport.c
Authentication uses copied principals and fail-safe currentness checks
Decision: Network sessions retain secret-free copied principals. Account mutations invalidate generations/IDs; after commit, the command layer requests best-effort targeted transport revocation, while ongoing currentness checks are authoritative.
Rationale/evidence: user_database issues principals without secrets; web/SSH check currentness during admission and active sessions. Mutating console paths call transport revocation hooks.
Consequence for future changes: Do not retain pointers to database records or treat login as permanently authoritative. New authenticated sessions/transports must revalidate at admission, before sensitive input, and periodically or on relevant events. Database mutation APIs alone do not perform transport notification, and notification failure must not roll back an already committed mutation.
Relevant files: src/user_database.{h,c}, src/user_console.c, src/web_server.c, src/web_serial_transport.c, src/ssh_transport.c
Security material and configuration use bounded, versioned NVS records
Decision: Application settings, users, and identities use separate fixed/versioned NVS blobs. Serial, Wi-Fi, and local-UI working edits are RAM-only until explicitly saved. User mutations and HTTPS/SSH identity changes commit directly as part of the operation. Invalid ordinary configuration generally selects RAM defaults without erasing storage; malformed security material fails closed and needs explicit reset.
Rationale/evidence: Serial, Wi-Fi, local UI, web security, users, and SSH security each validate schema/size and own their namespace. User/security mutations build and validate candidate state before committing it; security modules avoid silently replacing an established identity.
Consequence for future changes: Add schema versions and transactional candidate validation. Do not overwrite unknown records automatically; provide explicit migration/reset behavior. Preserve the distinct persistence contracts: explicit save/load/default/reset for working configuration and per-blob commit-before-live-install for user and identity mutation. Pre-bootstrap legacy credential rotation spans web_sec/material and user_db/database, is not cross-namespace atomic, and relies on boot reconciliation after interruption.
Relevant files: src/serial_config.c, src/wifi_config.c, src/local_ui_config.c, src/web_security.c, src/user_database.c, src/ssh_security.c
NVS is persistence, not a physical security boundary
Decision: The current firmware stores Wi-Fi credentials, recovery credentials, and TLS/SSH private keys in unencrypted application NVS. The reserved NVS-key partition does not enable encryption.
Rationale/evidence: partitions.csv, README security notes, and current code show no NVS-encryption setup. Original rationale for deferring encryption is outside the implementation; the observable limitation is explicit.
Consequence for future changes: Do not claim resistance to flash extraction. Avoid increasing stored secret exposure. Enabling encryption requires migration/recovery planning, not just changing the partition table.
Relevant files: partitions.csv, README.md, src/web_security.c, src/ssh_security.c, src/wifi_config.c
Wi-Fi callbacks enqueue; the manager owns policy
Decision: ESP event callbacks copy bounded event data into the Wi-Fi manager queue. A permanent manager task performs driver operations, profile/AP policy, deadlines, and reconciliation.
Rationale/evidence: Callback paths avoid blocking, NVS, and policy work. Manager deadlines consult authoritative driver/netif state so dropped events are recoverable.
Consequence for future changes: Keep callbacks short and nonblocking. Add state transitions to the manager rather than directly invoking Wi-Fi policy from consoles, UI, or callbacks. Preserve queue-drop observability.
Relevant files: src/wifi_manager.{h,c}, src/wifi_config.{h,c}
Optional local UI cannot become a core dependency
Decision: The OLED/display may fail without stopping serial, UART0, USB, or networking. The UI consumes copied snapshots and calls public APIs; it never parses CLI output or joins the broker.
Rationale/evidence: main.c logs display failures and continues. local_status_ui collects snapshots before display frames and exposes limited confirmed controls.
Consequence for future changes: Keep OLED/I2C work bounded and outside service locks. Do not put credentials or core ownership into UI state. A missing display must remain nonfatal.
Relevant files: src/main.c, src/local_display.{h,c}, src/local_status_ui.c, src/local_ui_config.c
Hardware and library access has designated owners
Decision: The serial task owns UART1 while active, local_display owns I2C/framebuffer access, the SSH owner task owns runtime wolfSSH contexts/calls after caller-side library initialization, and the console dispatcher alone runs registered commands.
Rationale/evidence: These constraints are enforced by module structure, mutex/task assertions, and transport indirection. Original rationale varies; the observable effect is serialized library/hardware access.
Consequence for future changes: Cross-task requests should use existing queues/public APIs. Do not make post-initialization wolfSSH calls, mutate display frames, or run console handlers from arbitrary tasks.
Relevant files: src/serial_service.c, src/local_display.c, src/ssh_transport.c, src/admin_ssh_console.c
Software cryptography settings are a validated concurrency workaround
Decision: wolfSSL ESP32 AES/SHA acceleration is disabled, and HTTPS uses software AES for PSRAM-backed TLS records. Internal task stacks are retained where cache-disable safety matters.
Rationale/evidence: Root CMakeLists.txt disables wolfSSL hardware crypto. The roadmap reports a reproduced watchdog stall involving mbedTLS external-RAM hardware-AES DMA, uncoordinated mbedTLS/wolfSSL hardware locks, and a successful software-crypto concurrency retest; no standalone execution record is checked in.
Consequence for future changes: Do not remove these definitions as a performance cleanup. Any re-enablement needs target-hardware concurrency testing with simultaneous USB, WebSocket, SSH, and serial traffic plus watchdog/stack telemetry.
Relevant files: CMakeLists.txt, src/CMakeLists.txt, docs/roadmap.md, relevant sdkconfig.defaults crypto settings
Embedded web assets are checked-in generated artifacts
Decision: Vendored xterm assets are compressed and embedded ahead of the normal firmware build; src/web_assets_data.c is compiled directly.
Rationale/evidence: src/CMakeLists.txt lists generated data as a source, and web_assets/SOURCES.md documents pinned versions, hashes, and deterministic gzip inputs.
Consequence for future changes: Edit authored web UI separately. Changes to its inline bootstrap loader must update the hard-coded CSP hash atomically and preserve the response security policy. When dependency assets change, follow the documented provenance/generation process and review generated diffs; do not hand-edit arrays or regenerate assets during unrelated work.
Relevant files: web_assets/SOURCES.md, web_assets/generate_embedded_assets.py, src/web_assets_data.{h,c}, src/web_ui.c