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GOP 2 – Login Expression Detection in Execution Loop for Console Streamers

  • Author: The GOP Editors
  • Status: Accepted
  • Type: Standards Track
  • Topic: Drivers/Transports
  • Created: 2026-03-15
  • Last-Modified: 2026-03-15

Abstract

When using a console streamer, executing a logout command (exit, quit) does not cause an EOF as it does with SSH or Telnet. Instead the device drops back to the login screen, sending a login prompt (e.g., Username:). This GOP introduces an optional check for the login expression inside the GenericExecute command execution loop. When the streamer reports the LoginInsteadEOF feature, a matched login expression during command execution is treated as an EOF, allowing graceful session termination.

Motivation

SSH and Telnet transports close the underlying connection when the user logs out, producing an EOF that gnetcli handles naturally. Console connections (serial/RS-232) are persistent physical links — there is no connection to close. After exit or quit, the device simply returns to its login prompt:

write "exit\n"
read  "exit\r\n"          ← echo
read  "Username: "        ← login prompt instead of EOF

Without this change, GenericExecute returns read error.

Goals

  • Detect logout on console connections by recognizing the login prompt during command execution.
  • Translate a login prompt match into an EOFException, keeping the error handling consistent with SSH/Telnet EOF behavior.
  • Keep this behavior opt-in so that SSH and Telnet streamers are not affected.

Non-Goals

  • Automatic re-login after logout on console. This is a higher-level concern.
  • Changes to the login procedure itself.

Proposal

Streamer Feature Flag

A new feature constant LoginInsteadEOF is added to the streamer package.

Console streamers return true for HasFeature(LoginInsteadEOF). SSH and Telnet streamers continue to return false.

Execution Loop Change

When GenericExecute builds the expression list for the read loop, the login expression is included only if the streamer reports LoginInsteadEOF support and the login expression is configured for the device. If the login expression fires during command execution, it is treated as a session termination and an EOFException is returned — the same behavior that SSH and Telnet produce when the connection is closed.

Compatibility

  • SSH/Telnet streamers: unaffected — HasFeature(LoginInsteadEOF) returns false, login expression is never added to the execution loop.
  • Console streamer: gains the ability to properly detect logout via login prompt.
  • Existing device drivers: no changes required unless they use console transport and need logout detection. In that case they must already have login expressions configured via WithLoginExprs().

Rationale

Using the HasFeature mechanism keeps the change scoped to streamers that need it. The login expression is already available in GenericCLI (set via WithLoginExprs), so no additional configuration is required — only the streamer must declare the feature.

Translating the match into EOFException (rather than a new error type) ensures that upstream code handling EOF from SSH/Telnet works identically for console logout.

Alternatives

  1. Do nothing — console logout commands would time out, requiring workarounds in user code.
  2. Always check login expression in the execution loop — risks false positives on SSH/Telnet if the output contains text matching the login pattern.
  3. New dedicated expression type for logout detection — adds complexity without clear benefit, since the login expression already serves this purpose on console.

Testing Plan

Tests are located in pkg/device/genericcli/gop2_login_instead_eof_test.go:

  • TestExitCmdLoginPromptOnConsole — verifies that executing exit on a LoginInsteadEOF connector returns an EOFException when the device responds with a login prompt.
  • TestExitCmdLoginPromptOnSSH — verifies that on a standard SSH connector (no LoginInsteadEOF), the login expression is not checked in the execution loop.
  • TestCmdThenExitOnConsole — verifies that a regular command executes normally, followed by exit producing an EOFException.

References

  • pkg/streamer/streamer.go — LoginInsteadEOF constant.
  • pkg/streamer/console/console.go — HasFeature returns true for LoginInsteadEOF.
  • pkg/device/genericcli/genericcli.go — GenericExecute changes.