preproc: fix infinite loop in paste_tokens with leading/consecutive %+

[agourakis82]

Fixes #294.

Summary

In asm/preproc.c, paste_tokens() tracks prev_nonspace as a Token ** pointing to the preceding non-whitespace, non-paste token in the stream, which serves as the left token for explicit token pasting (%+).

Two bugs in pointer tracking caused infinite loops on malformed inputs such as %+s%+t:

1. When a leading %+ is dropped (t == NULL), *prev_next = tok = next advances *head to the token following %+ (e.g. s). However, prev_nonspace remained NULL, failing to record that head now points to a non-space token.
2. In the unpasted token advancement path, prev_nonspace was updated by checking whether next was non-space/non-paste, rather than whether tok was:

if (next && next->type != TOKEN_WHITESPACE && next->type != TOKEN_PASTE)
    prev_nonspace = prev_next;

Because prev_next was already advanced to &tok->next and next was the second %+, this check evaluated to false, leaving prev_nonspace as NULL.

Consequently, upon reaching the second %+, !prev_nonspace was evaluated as true again. The second %+ was mistakenly treated as another leading %+, setting nextp = head and rewinding next back to *head (s), trapping the while loop in an endless cycle.

Fix

• When dropping a leading %+ (!t), update prev_nonspace to prev_next if the new head token is not whitespace/paste.
• In the token advancement path, update prev_nonspace = prev_next whenever tok is not whitespace/paste before advancing prev_next = &tok->next.

Verified assembling %+s%+t and %+ts %+ts completes cleanly without hanging.

[Copilot]

Copilot review overview

🟡 Changes recommended

Add regression fixtures covering leading/consecutive %+ inputs.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 1

Open (1)

• Add regression fixtures for preprocessor infinite-loop inputs · New

What changed in this PR

Fixes an infinite loop in preprocessor token pasting involving leading or consecutive %+ tokens.

Changes:

• Corrects prev_nonspace tracking after dropping %+.
• Fixes token advancement bookkeeping in paste_tokens().

File	Summary
asm/​preproc.c	Fixes %+ token traversal; add regression fixtures for the reported hangs.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[agourakis82]

Follow-up regression fixture is now included in a76c3ff16: it covers leading %+, consecutive %+, and the combined leading+consecutive case that previously hung in paste_tokens(). Could you please re-review when convenient?

[agourakis82]

The regression coverage update is published in b105d3a. NASM CI/CD on that exact head is still action_required. Could a maintainer approve the workflow run? The same approval hold affects my #320 and #322; their heads were not changed in this pass.