Proposal: Fix brotli decompressor buffer typo, replace framing/header magic numbers

Summary & Motivation

This proposal tracks eliminating unexplained magic numeric literals across the actix-web workspace, fixing buffer alignment anomalies, and replacing hardcoded RFC framing constants with self-documenting compile-time constants (const).

Audit Telemetry: This scan and static analysis audit was performed across all 318 .rs files (86,266 LOC) using GEMINI-3.7-FLASH.

Implementation Plan & Note for Maintainers

If maintainers approve of this cleanup, I will implement these changes iteratively across small, focused PRs (e.g. splitting the Brotli buffer fix, RFC 7231 Date header cleanup, and WebSocket constants) so reviewing remains straightforward and modular.

• Part 1: Brotli Decompressor Buffer Capacity Fix — #4228
• Part 2: RFC 7231 Date Header Scratchpad Buffer Constants
• Part 3: RFC 6455 WebSocket Framing & Protocol Constants
• Part 4: HTTP/1.1 Framing & Encoder Pre-allocation Cleanup

Additionally, minor magic numbers (e.g. trivial 1-byte offsets, string split delimiters, or obvious test assertions) will be intentionally left as-is to avoid unnecessary abstraction churn. The focus is solely on allocator-impacting buffer sizes, protocol framing thresholds, and timeouts.

Major Issues Identified

1. The Brotli Decompressor Buffer Typo (actix-http/src/encoding/decoder.rs) — (Resolved in #4228)

// Current code in actix-http/src/encoding/decoder.rs:
#[cfg(feature = "compress-brotli")]
ContentEncoding::Brotli => Some(ContentDecoder::Brotli(Box::new(
    brotli::DecompressorWriter::new(Writer::new(), 8_096), // <-- Typo
))),

• The Problem: $8,096$ is neither a power of two nor an integer multiple of the standard $4\text{ KiB}$ memory page ($8,192 - 96$). CPU cache lines and memory allocators cry themselves to sleep whenever an unaligned $8,096$-byte chunk is requested. The rest of the encoding module uses $8\text{ KiB}$ or $32\text{ KiB}$ buffers.
• The Fix: Define const DEFAULT_DECODER_CAPACITY: usize = 8 * 1024; ($8\text{ KiB}$) to restore clean page alignment (Implemented in #4228).

2. The Date Header Scratchpad Buffer (actix-http/src/config.rs)

Unit tests were already importing use crate::date::DATE_VALUE_LENGTH; (29), but the production date writer is doing manual index math:

// Current code in actix-http/src/config.rs:
let mut buf: [u8; 37] = [0; 37];
buf[..6].copy_from_slice(if camel_case { b"Date: " } else { b"date: " });
self.0.date_service.with_date(|date| buf[6..35].copy_from_slice(&date.bytes));
buf[35..].copy_from_slice(b"\r\n");

• The Problem: If the date format ever shifted by even 1 byte, this function would silently slice corrupted HTTP headers or panic at runtime on an index out of bounds.
• The Fix:
  rust复制

  const PREFIX_LEN: usize = 6; // b"Date: ".len()
  const CRLF_LEN: usize = 2;   // b"\r\n".len()
  const DATE_HEADER_LEN: usize = PREFIX_LEN + DATE_VALUE_LENGTH + CRLF_LEN; // 37

  Bound all slice offsets to compiler-verified const arithmetic.

3. RFC 6455 WebSocket Framing Literals (actix-http/src/ws/frame.rs)

Replace raw binary bitmasks and protocol thresholds with standardized named constants:

• Masks: FIN_MASK = 0x80, MASK_BIT = 0x80, OPCODE_MASK = 0x0F, PAYLOAD_LEN_MASK = 0x7F.
• Length thresholds: EXT_LEN_U16 = 126, EXT_LEN_U64 = 127, MAX_CONTROL_FRAME_PAYLOAD = 125, U16_PAYLOAD_LIMIT = 65_535.
• RFC 6455 Section 7.4.1 Close status codes mapped cleanly rather than using naked numeric literals.

Detailed Audit: Magic Numbers by Category

1. Buffer Capacities, Pre-allocations & Limits

2. WebSocket Framing & Protocol Constants

3. HTTP Protocol Framing, Slicing & Parsing

4. Client Configuration & Pool Lifetimes

5. Compression Parameters & Codecs