Replace global MCP_MODE with struct-based config propagation

[momokun7]

Source: Code quality review 2026-03-31 (Score: 52/100)
Category: Rust Code Quality (10/20)
Priority: 4/5

Problem

// mcp.rs:8
static MCP_MODE: AtomicBool = AtomicBool::new(false);

is_mcp_mode() is referenced across search.rs, lib.rs, candidates.rs, and mcp_tools.rs to suppress stderr output. Global mutable state hurts testability and blocks future multi-instance support (searching multiple repos in one process).

Proposed Solution

Add a config field to Xgrep struct:

pub struct Xgrep {
    root: PathBuf,
    index_path: PathBuf,
    config: Config,
}

pub struct Config {
    pub quiet: bool,  // suppresses stderr warnings
}

Replace all is_mcp_mode() calls with self.config.quiet checks. MCP server sets quiet: true when constructing Xgrep.

Why Now

Small refactor now prevents entrenching the global state pattern as more features are added.

Estimated effort: 1-2 hours

[momokun7]

Resolved in v0.3.0.

Implementation:

• lib.rs: Added Config struct with quiet: bool field and with_config() builder method on Xgrep
• search.rs: All public search functions (search, search_files, search_regex, search_files_regex) and internal functions (scan_indexed, scan_direct) accept quiet: bool parameter
• candidates.rs: resolve_regex_candidates() accepts quiet: bool for warning suppression
• mcp.rs: Removed QUIET_MODE: AtomicBool, set_quiet(), is_quiet(), is_mcp_mode() entirely
• index/reader.rs: madvise warning wrapped in #[cfg(debug_assertions)]
• main.rs: MCP serve mode uses with_config(Config { quiet: true })

Verification: cargo test — 291 tests pass. xg "AtomicBool" src/ returns zero matches confirming complete removal.