Audit and fix tsort: add cycle detection, odd token validation, and stdin support

[Copilot]

The tsort implementation was missing critical POSIX-required error detection and stdin handling. It would silently fail on cycles, ignore incomplete pairs, and reject - as stdin.

Changes

Cycle detection

• Track all items during parsing vs successfully sorted items
• Items remaining after topological sort indicate cycles
• Report cycle members to stderr with exit code 1

Odd token validation

• Detect incomplete pairs (odd number of whitespace-separated tokens)
• Report error to stderr with exit code 1

Stdin handling

• Changed from input_stream_opt to input_stream(path, dashed_stdin=true)
• Now correctly treats - argument as stdin

Test coverage

• Added 18 integration tests covering basic functionality, edge cases, and error conditions
• Tests include chains, cycles, empty input, self-loops, and various input formats

Example

Before (silently fails):

$ echo "a b b a" | tsort
# no output, exit 0

After (detects cycle):

$ echo "a b b a" | tsort
stdin: input contains a loop:
stdin: a
stdin: b
a
b
# exit 1

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• pubs.opengroup.org
  • Triggering command: /usr/bin/curl curl -s REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Audit tsort utility</issue_title>
<issue_description>Study the attached POSIX spec for tsort:

tsort.html

then audit the tsort implementation for correctness, completeness and integration test completeness. identify any gaps, and plan and implement changes to address gaps. DO NOT add tests that require special test environments (i.e. other that non-root user in GH CI/CD).
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Audit tsort utility #486

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