security: bound variable substitution expansion - #2923

Human

AI

Summary

• enforce a compilation-wide 65,536-unit default budget before variable substitutions or automatic copies materialize
• cover scalar and Markdown output, array and map spreads, composite aliases, boards, imports, and classes
• honor context cancellation throughout IR and graph compilation

Security

A few hundred bytes of D2 source could build a shared composite-variable DAG that materialized as hundreds of thousands of graph objects, exhausting more than a gigabyte of memory before cancellation could run. The compiler now accounts for every multiplicative occurrence and automatic copy before allocating it, while Markdown matching is linear in the input plus bounded match work.

Compatibility

Explicitly authored graph nodes do not consume the new budget unless D2 automatically copies them for boards, imports, or classes. The default is 65,536 work units; trusted Go callers can raise it with MaxVariableExpansion. Negative limits are rejected.

Testing

• full Go suite and focused race tests
• scalar, Markdown, array, map, composite, board, import, and class regression cases
• deterministic cancellation tests during IR and graph compilation
• adversarial reproductions for the prior composite, scalar, array, and class amplification paths