Preallocate binary joins and preserve buffer lifetimes - #8880

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Walkthrough

Join handling for bytes, bytearray, and str now uses updated iterable processing. Bytes and bytearray share BytesJoin, which validates buffer items and assembles output while handling mutable inputs. Tests cover iterator errors, buffer behavior, separator mutation, and single-item bytes identity.

Changes

Join processing

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant PyBytesJoin
  participant BytesJoin
  participant Iterable
  participant BufferProtocol
  Caller->>PyBytesJoin: Call join with iterable
  PyBytesJoin->>BytesJoin: Create join state
  BytesJoin->>Iterable: Materialize items when needed
  BytesJoin->>BufferProtocol: Acquire contiguous item buffers
  BufferProtocol-->>BytesJoin: Return item buffers
  BytesJoin->>BytesJoin: Read separator and assemble output
  BytesJoin-->>Caller: Return joined bytes

Security Blast Radius

• inferred — The demonstrated reach is a caller invoking VM join with a general iterable. Potential resource consumption is confined to that execution context on the available evidence; no network, tenant, credential, or deployment path was established.

Security Findings and Attack Paths

• inferred — An iterable yielding an invalid first item followed by extensive further output can require that output to be consumed before TypeError. This is a conditional resource-amplification path, not an established production exploit.

Trust Boundaries and Controls

• observed — Join can invoke user-defined iteration and buffer acquisition. Binary join requests a simple buffer, rejects noncontiguous buffers, retains acquired exports through copying, and prevents bytearray-separator resizing while its join is active.

Resilience and Maintainability Implications

• inferred — The retained buffer handles and deferred separator-export release cover ordinary completion and error returns. The focused evidence does not establish behavior for every concurrent or nonconforming exporter interaction.

Hardening Proposals

• proposed — Applications that pass untrusted general iterables to join should enforce input-size or execution budgets at their own boundary while retaining the intended join semantics.

❌ Failed checks (1 warning)

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