whisker_dev_server/hotpatch/

wrapper.rs

//! Fat-build runner + captured-args loader.
//!
//! The other half of the rustc/linker hijack started in I4g-4a.
//! `whisker-rustc-shim` writes a JSON file per rustc invocation into a
//! cache dir; this module:
//!
//! 1. Spawns the *fat build* — a normal cargo build with
//!    `RUSTC_WORKSPACE_WRAPPER=whisker-rustc-shim` set, so the cache
//!    fills up.
//! 2. Loads those JSON files back into a `HashMap<String,
//!    CapturedRustcInvocation>` keyed by crate name, picking the
//!    most recent timestamp when a crate was rebuilt mid-session.
//! 3. (Future, I4g-5) hands the captured args to a thin-rebuild
//!    driver that only recompiles the changed crate and re-links.
//!
//! `CapturedRustcInvocation` is currently *defined* here, not in
//! whisker-cli, so that the shim binary doesn't need to pull in the
//! whole dev-server dep tree (tokio / axum / notify / object). The
//! shim has its own copy of the struct shape; serde keeps the wire
//! format compatible. A future cleanup will extract a tiny
//! `whisker-hotpatch-types` crate and dedupe both sides — see TODO.

use anyhow::{Context, Result};
use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::process::Command;

use crate::Target;

/// Mirrors `whisker_cli::rustc_shim::CapturedRustcInvocation` exactly.
/// Kept duplicated (rather than imported) so the shim binary stays
/// dep-light. JSON wire format is what binds them — both sides go
/// through serde, so a field rename in one without the other will
/// trip the deserialize step at run time and emit a clear error.
#[derive(Debug, Clone, serde::Serialize, serde::Deserialize, PartialEq, Eq)]
pub struct CapturedRustcInvocation {
    pub crate_name: String,
    pub args: Vec<String>,
    pub timestamp_micros: u128,
}

/// Mirrors `whisker_cli::linker_shim::CapturedLinkerInvocation` exactly.
/// Same duplication rationale as [`CapturedRustcInvocation`].
#[derive(Debug, Clone, serde::Serialize, serde::Deserialize, PartialEq, Eq)]
pub struct CapturedLinkerInvocation {
    pub output: Option<String>,
    pub args: Vec<String>,
    pub timestamp_micros: u128,
}

/// Optional linker-shim wiring for [`run_fat_build`]. Provide all
/// three when you want the linker invocation captured (Tier 1 needs
/// it); leave this `None` for plain Tier 2 / Tier 0 fat builds.
#[derive(Debug, Clone)]
pub struct LinkerCaptureConfig<'a> {
    /// Path to `whisker-linker-shim`. The shim is the value rustc sees
    /// for `-C linker=<…>`.
    pub shim_path: &'a Path,
    /// Where the shim writes its JSON cache files.
    pub cache_dir: &'a Path,
    /// What the shim forwards to. Typically `xcrun -f clang` on
    /// macOS or PATH-resolved `cc` on Linux. Required because the
    /// shim doesn't know the real linker on its own.
    pub real_linker: &'a Path,
}

/// Spawn a `cargo` build for the given target with `RUSTC_WORKSPACE_WRAPPER`
/// pointed at `shim_path` and `WHISKER_RUSTC_CACHE_DIR` pointed at `cache_dir`.
/// Inherits stdout/stderr so cargo's progress is visible. After the build
/// completes successfully, [`load_captured_args`] can read the cache.
///
/// When `linker_capture` is `Some(_)`, the build *also* installs the
/// linker shim by setting `RUSTFLAGS=-Clinker=<shim>` plus the
/// shim's two env vars. The two captures (rustc-args and linker-args)
/// then both fill up during the same fat build, and Tier 1's
/// thin_rebuild_obj can replay them together.
///
/// `target` is currently a hint only; the cargo command we run is the
/// host build (`cargo build -p <pkg>`). I4g-5 will switch to the
/// platform-specific `whisker-build` invocations once thin rebuild
/// is wired up.
pub fn run_fat_build(
    workspace_root: &Path,
    package: &str,
    _target: Target,
    shim_path: &Path,
    cache_dir: &Path,
    linker_capture: Option<&LinkerCaptureConfig<'_>>,
) -> Result<()> {
    std::fs::create_dir_all(cache_dir)
        .with_context(|| format!("create cache dir {}", cache_dir.display()))?;
    let mut cmd = Command::new("cargo");
    cmd.args(["build", "-p", package])
        .current_dir(workspace_root)
        .env("RUSTC_WORKSPACE_WRAPPER", shim_path)
        .env("WHISKER_RUSTC_CACHE_DIR", cache_dir);

    if let Some(lc) = linker_capture {
        std::fs::create_dir_all(lc.cache_dir)
            .with_context(|| format!("create linker cache dir {}", lc.cache_dir.display()))?;
        // Prepend our -Clinker to any existing RUSTFLAGS rather than
        // clobbering them — the user may have set their own through
        // .cargo/config.toml or env.
        let mut rustflags = std::env::var("RUSTFLAGS").unwrap_or_default();
        if !rustflags.is_empty() {
            rustflags.push(' ');
        }
        rustflags.push_str(&format!("-Clinker={}", lc.shim_path.display()));
        cmd.env("RUSTFLAGS", rustflags)
            .env("WHISKER_LINKER_CACHE_DIR", lc.cache_dir)
            .env("WHISKER_REAL_LINKER", lc.real_linker);
    }

    let status = cmd.status().context("spawn cargo for fat build")?;
    if !status.success() {
        anyhow::bail!("fat build failed: cargo exited {status}");
    }
    Ok(())
}

/// Walk `cache_dir`, deserialise every `*.json` produced by
/// `whisker-rustc-shim`, and collapse duplicates per crate by keeping
/// the most-recent timestamp. Empty / unparseable files are skipped
/// with a warning rather than aborting the whole load — a partial
/// fat build shouldn't take the dev loop down.
///
/// `target_triple_filter` restricts the load to entries whose
/// captured `--target` matches. Multi-arch fat builds (e.g. iOS Simulator
/// on Apple Silicon, which xcodebuild compiles for both
/// `aarch64-apple-ios-sim` and `x86_64-apple-ios`) write one JSON per
/// (crate, triple) pair; without the filter the newest-timestamp wins
/// regardless of triple, leaving the patcher likely to pick the wrong
/// arch and emit an `.o` that fails to link into the runtime dylib
/// (`found architecture 'x86_64', required architecture 'arm64'`).
/// Pass `None` to disable filtering (host / single-arch paths).
pub fn load_captured_args(
    cache_dir: &Path,
    target_triple_filter: Option<&str>,
) -> Result<HashMap<String, CapturedRustcInvocation>> {
    let mut by_crate: HashMap<String, CapturedRustcInvocation> = HashMap::new();
    if !cache_dir.is_dir() {
        return Ok(by_crate); // empty cache is fine, just nothing to do
    }
    for entry in
        std::fs::read_dir(cache_dir).with_context(|| format!("read_dir {}", cache_dir.display()))?
    {
        let entry = entry?;
        let path = entry.path();
        if path.extension().and_then(|e| e.to_str()) != Some("json") {
            continue;
        }
        let body = match std::fs::read_to_string(&path) {
            Ok(b) => b,
            Err(e) => {
                whisker_build::ui::warn(format!("skip {}: {e}", path.display()));
                continue;
            }
        };
        let inv: CapturedRustcInvocation = match serde_json::from_str(&body) {
            Ok(i) => i,
            Err(e) => {
                whisker_build::ui::warn(format!("skip {}: malformed json: {e}", path.display()));
                continue;
            }
        };
        if let Some(want) = target_triple_filter {
            if invocation_target_triple(&inv) != Some(want) {
                continue;
            }
        }
        keep_newest(&mut by_crate, inv);
    }
    Ok(by_crate)
}

/// Extract `--target <triple>` from a captured rustc args list, if
/// present. Returns `None` for host-build invocations (which omit
/// `--target` and let cargo's default kick in).
fn invocation_target_triple(inv: &CapturedRustcInvocation) -> Option<&str> {
    let mut iter = inv.args.iter();
    while let Some(a) = iter.next() {
        if a == "--target" {
            return iter.next().map(String::as_str);
        }
        if let Some(rest) = a.strip_prefix("--target=") {
            return Some(rest);
        }
    }
    None
}

/// Pure helper for the load loop's "keep most-recent per crate"
/// decision. Pulled out so unit tests don't have to write JSON to
/// disk to exercise the merge.
pub fn keep_newest(
    map: &mut HashMap<String, CapturedRustcInvocation>,
    inv: CapturedRustcInvocation,
) {
    match map.get(&inv.crate_name) {
        Some(prev) if prev.timestamp_micros >= inv.timestamp_micros => {
            // already have a newer or equal-timestamp entry; ignore.
        }
        _ => {
            map.insert(inv.crate_name.clone(), inv);
        }
    }
}

/// Walk a `whisker-linker-shim` cache dir and collapse duplicates per
/// output filename, keeping the most-recent timestamp. Same shape
/// as [`load_captured_args`] but for the linker side. Empty /
/// unparseable files get a warning, not an abort.
///
/// Keying by the **basename of the output path** (`libdemo.dylib` →
/// `libdemo.dylib`) lets the patcher look up the linker call that
/// produced a specific crate's library without having to re-derive
/// the cargo's hash-suffixed path. Output-less captures are keyed
/// under `_unknown`.
pub fn load_captured_linker_args(
    cache_dir: &Path,
) -> Result<HashMap<String, CapturedLinkerInvocation>> {
    let mut by_output: HashMap<String, CapturedLinkerInvocation> = HashMap::new();
    if !cache_dir.is_dir() {
        return Ok(by_output);
    }
    for entry in
        std::fs::read_dir(cache_dir).with_context(|| format!("read_dir {}", cache_dir.display()))?
    {
        let entry = entry?;
        let path = entry.path();
        if path.extension().and_then(|e| e.to_str()) != Some("json") {
            continue;
        }
        let body = match std::fs::read_to_string(&path) {
            Ok(b) => b,
            Err(e) => {
                whisker_build::ui::warn(format!("skip {}: {e}", path.display()));
                continue;
            }
        };
        let inv: CapturedLinkerInvocation = match serde_json::from_str(&body) {
            Ok(i) => i,
            Err(e) => {
                whisker_build::ui::warn(format!("skip {}: malformed json: {e}", path.display()));
                continue;
            }
        };
        keep_newest_linker(&mut by_output, inv);
    }
    Ok(by_output)
}

/// Pure helper for [`load_captured_linker_args`] — same "keep
/// most-recent per key" pattern as [`keep_newest`], but the key is
/// the output filename rather than a crate name.
pub fn keep_newest_linker(
    map: &mut HashMap<String, CapturedLinkerInvocation>,
    inv: CapturedLinkerInvocation,
) {
    let key = inv
        .output
        .as_deref()
        .and_then(|s| Path::new(s).file_name())
        .and_then(|n| n.to_str())
        .unwrap_or("_unknown")
        .to_string();
    match map.get(&key) {
        Some(prev) if prev.timestamp_micros >= inv.timestamp_micros => {}
        _ => {
            map.insert(key, inv);
        }
    }
}

/// Convenience: best-effort default cache dir under the workspace's
/// `target/.whisker/rustc-args/`. Created on demand.
pub fn default_cache_dir(workspace_root: &Path) -> PathBuf {
    workspace_root.join("target/.whisker/rustc-args")
}

/// Counterpart of [`default_cache_dir`] for the linker side:
/// `target/.whisker/linker-args/`. Created on demand.
pub fn default_linker_cache_dir(workspace_root: &Path) -> PathBuf {
    workspace_root.join("target/.whisker/linker-args")
}

/// Resolve the system linker driver we want to forward to from the
/// shim. Same logic the integration test uses, lifted into one
/// place so production and tests agree:
///
///   - `CC` env var wins.
///   - macOS: `xcrun -f clang` (active toolchain).
///   - Otherwise: PATH-resolved `cc`.
///
/// Returns the resolved path. Caller should `.canonicalize()` if
/// they want absolute, but the shim only needs an executable name
/// the OS can find.
pub fn resolve_host_linker() -> PathBuf {
    if let Some(cc) = std::env::var_os("CC") {
        return PathBuf::from(cc);
    }
    if cfg!(target_os = "macos") {
        if let Ok(out) = Command::new("xcrun").args(["-f", "clang"]).output() {
            if out.status.success() {
                let path = String::from_utf8_lossy(&out.stdout).trim().to_string();
                if !path.is_empty() {
                    return PathBuf::from(path);
                }
            }
        }
        return PathBuf::from("clang");
    }
    PathBuf::from("cc")
}

// ============================================================================
// Tests
// ============================================================================

#[cfg(test)]
mod tests {
    use super::*;
    use std::sync::atomic::{AtomicU64, Ordering};

    fn s(v: &[&str]) -> Vec<String> {
        v.iter().map(|s| s.to_string()).collect()
    }

    fn unique_tempdir() -> PathBuf {
        static SEQ: AtomicU64 = AtomicU64::new(0);
        let n = SEQ.fetch_add(1, Ordering::Relaxed);
        let pid = std::process::id();
        let p = std::env::temp_dir().join(format!("whisker-wrapper-test-{pid}-{n}"));
        let _ = std::fs::remove_dir_all(&p);
        std::fs::create_dir_all(&p).unwrap();
        p
    }

    fn write_invocation(dir: &Path, inv: &CapturedRustcInvocation) {
        let name = format!(
            "{}-{}.json",
            inv.crate_name.replace(['-', '/'], "_"),
            inv.timestamp_micros,
        );
        let body = serde_json::to_string_pretty(inv).unwrap();
        std::fs::write(dir.join(name), body).unwrap();
    }

    // ----- load_captured_args ------------------------------------------

    #[test]
    fn load_captured_args_returns_empty_for_missing_cache_dir() {
        let map = load_captured_args(Path::new("/nope/does/not/exist"), None).unwrap();
        assert!(map.is_empty());
    }

    #[test]
    fn load_captured_args_filters_by_target_triple_when_specified() {
        // Multi-arch fat build (iOS Simulator on Apple Silicon): one
        // invocation per triple lands in the cache, both with the same
        // crate name. Without filtering, the newest-timestamp wins
        // regardless of triple — which is wrong when the dev-server's
        // runtime triple is the older one, and the patcher's resulting
        // `.o` fails to link into the runtime dylib (`found architecture
        // 'x86_64', required architecture 'arm64'`). Verify the filter
        // picks the matching triple even when its timestamp is older.
        let dir = unique_tempdir();
        write_invocation(
            &dir,
            &CapturedRustcInvocation {
                crate_name: "podcast".into(),
                args: s(&[
                    "--crate-name",
                    "podcast",
                    "--target",
                    "aarch64-apple-ios-sim",
                ]),
                timestamp_micros: 100,
            },
        );
        write_invocation(
            &dir,
            &CapturedRustcInvocation {
                crate_name: "podcast".into(),
                args: s(&["--crate-name", "podcast", "--target", "x86_64-apple-ios"]),
                timestamp_micros: 200,
            },
        );

        let map = load_captured_args(&dir, Some("aarch64-apple-ios-sim")).unwrap();
        assert_eq!(map.len(), 1);
        assert_eq!(map["podcast"].timestamp_micros, 100);

        let _ = std::fs::remove_dir_all(&dir);
    }

    #[test]
    fn load_captured_args_returns_one_entry_per_crate_for_distinct_crates() {
        let dir = unique_tempdir();
        write_invocation(
            &dir,
            &CapturedRustcInvocation {
                crate_name: "foo".into(),
                args: s(&["--crate-name", "foo", "src/lib.rs"]),
                timestamp_micros: 100,
            },
        );
        write_invocation(
            &dir,
            &CapturedRustcInvocation {
                crate_name: "bar".into(),
                args: s(&["--crate-name", "bar", "src/lib.rs"]),
                timestamp_micros: 200,
            },
        );

        let map = load_captured_args(&dir, None).unwrap();
        assert_eq!(map.len(), 2);
        assert_eq!(map["foo"].args, s(&["--crate-name", "foo", "src/lib.rs"]));
        assert_eq!(map["bar"].args, s(&["--crate-name", "bar", "src/lib.rs"]));

        let _ = std::fs::remove_dir_all(&dir);
    }

    #[test]
    fn load_captured_args_keeps_the_most_recent_invocation_per_crate() {
        let dir = unique_tempdir();
        // Older invocation: shorter args.
        write_invocation(
            &dir,
            &CapturedRustcInvocation {
                crate_name: "foo".into(),
                args: s(&["--old-args"]),
                timestamp_micros: 100,
            },
        );
        // Newer invocation: longer args.
        write_invocation(
            &dir,
            &CapturedRustcInvocation {
                crate_name: "foo".into(),
                args: s(&["--newer-args", "--more"]),
                timestamp_micros: 200,
            },
        );

        let map = load_captured_args(&dir, None).unwrap();
        assert_eq!(map.len(), 1);
        assert_eq!(map["foo"].timestamp_micros, 200);
        assert_eq!(map["foo"].args, s(&["--newer-args", "--more"]));

        let _ = std::fs::remove_dir_all(&dir);
    }

    #[test]
    fn load_captured_args_skips_non_json_files() {
        let dir = unique_tempdir();
        std::fs::write(dir.join("README.md"), "not json").unwrap();
        write_invocation(
            &dir,
            &CapturedRustcInvocation {
                crate_name: "foo".into(),
                args: vec![],
                timestamp_micros: 1,
            },
        );

        let map = load_captured_args(&dir, None).unwrap();
        assert_eq!(map.len(), 1);
        assert!(map.contains_key("foo"));

        let _ = std::fs::remove_dir_all(&dir);
    }

    #[test]
    fn load_captured_args_skips_malformed_json_with_a_warning() {
        let dir = unique_tempdir();
        std::fs::write(dir.join("garbage.json"), "{ not valid json").unwrap();
        write_invocation(
            &dir,
            &CapturedRustcInvocation {
                crate_name: "good".into(),
                args: vec![],
                timestamp_micros: 1,
            },
        );

        let map = load_captured_args(&dir, None).unwrap();
        assert_eq!(map.len(), 1);
        assert!(map.contains_key("good"));

        let _ = std::fs::remove_dir_all(&dir);
    }

    // ----- keep_newest --------------------------------------------------

    #[test]
    fn keep_newest_inserts_into_empty_map() {
        let mut m = HashMap::new();
        keep_newest(
            &mut m,
            CapturedRustcInvocation {
                crate_name: "x".into(),
                args: vec![],
                timestamp_micros: 1,
            },
        );
        assert_eq!(m.len(), 1);
    }

    #[test]
    fn keep_newest_replaces_when_timestamp_strictly_newer() {
        let mut m = HashMap::new();
        m.insert(
            "x".into(),
            CapturedRustcInvocation {
                crate_name: "x".into(),
                args: s(&["old"]),
                timestamp_micros: 5,
            },
        );
        keep_newest(
            &mut m,
            CapturedRustcInvocation {
                crate_name: "x".into(),
                args: s(&["new"]),
                timestamp_micros: 10,
            },
        );
        assert_eq!(m["x"].args, s(&["new"]));
    }

    #[test]
    fn keep_newest_does_not_replace_with_equal_or_older_timestamp() {
        let mut m = HashMap::new();
        m.insert(
            "x".into(),
            CapturedRustcInvocation {
                crate_name: "x".into(),
                args: s(&["incumbent"]),
                timestamp_micros: 10,
            },
        );
        keep_newest(
            &mut m,
            CapturedRustcInvocation {
                crate_name: "x".into(),
                args: s(&["equal"]),
                timestamp_micros: 10,
            },
        );
        keep_newest(
            &mut m,
            CapturedRustcInvocation {
                crate_name: "x".into(),
                args: s(&["older"]),
                timestamp_micros: 1,
            },
        );
        assert_eq!(m["x"].args, s(&["incumbent"]));
    }

    // ----- default_cache_dir -------------------------------------------

    #[test]
    fn default_cache_dir_lives_under_target_dot_whisker() {
        let p = default_cache_dir(Path::new("/tmp/ws"));
        assert!(p.ends_with("target/.whisker/rustc-args"));
    }

    // ----- run_fat_build (integration: runs a real cargo) ---------------
    //
    // Smoke-test only: spawn `cargo --version` instead of a real
    // build to keep the test fast. The real round-trip
    // (build → JSON files appear → load_captured_args returns them)
    // is exercised in I4g-5's integration test against a fixture
    // crate.

    #[test]
    fn run_fat_build_creates_the_cache_dir_even_if_build_fails() {
        // Point the wrapper at /bin/true so cargo doesn't actually
        // compile anything; we just want to know `run_fat_build`
        // creates the cache dir and surfaces a non-zero exit as Err.
        let dir = unique_tempdir();
        let cache = dir.join("nested/cache");
        // Bogus workspace_root means cargo build will fail; that's
        // the path we want to assert on.
        let bad_workspace = unique_tempdir();
        let res = run_fat_build(
            &bad_workspace,
            "no-such-package",
            Target::Android,
            Path::new("/bin/true"),
            &cache,
            None,
        );
        assert!(res.is_err(), "build of nonexistent pkg should error");
        assert!(cache.is_dir(), "cache dir should be created up front");

        let _ = std::fs::remove_dir_all(&dir);
        let _ = std::fs::remove_dir_all(&bad_workspace);
    }

    #[test]
    fn run_fat_build_creates_linker_cache_dir_when_capture_requested() {
        // Same shape as above but with linker_capture set: both
        // dirs should be created up front, even though the cargo
        // call ultimately fails.
        let dir = unique_tempdir();
        let rustc_cache = dir.join("rustc");
        let linker_cache = dir.join("linker");
        let bad_workspace = unique_tempdir();
        let lc = LinkerCaptureConfig {
            shim_path: Path::new("/bin/true"),
            cache_dir: &linker_cache,
            real_linker: Path::new("/usr/bin/true"),
        };
        let res = run_fat_build(
            &bad_workspace,
            "no-such-package",
            Target::Android,
            Path::new("/bin/true"),
            &rustc_cache,
            Some(&lc),
        );
        assert!(res.is_err());
        assert!(rustc_cache.is_dir());
        assert!(linker_cache.is_dir());

        let _ = std::fs::remove_dir_all(&dir);
        let _ = std::fs::remove_dir_all(&bad_workspace);
    }

    // ----- load_captured_linker_args -----------------------------------

    fn write_linker_inv(dir: &Path, inv: &CapturedLinkerInvocation) {
        let stem = inv
            .output
            .as_deref()
            .and_then(|s| Path::new(s).file_name())
            .and_then(|n| n.to_str())
            .unwrap_or("_unknown")
            .replace(['/', '\\'], "_");
        let name = format!("{stem}-{}.json", inv.timestamp_micros);
        let body = serde_json::to_string_pretty(inv).unwrap();
        std::fs::write(dir.join(name), body).unwrap();
    }

    #[test]
    fn load_captured_linker_args_returns_empty_for_missing_dir() {
        let map = load_captured_linker_args(Path::new("/nope/does/not/exist")).unwrap();
        assert!(map.is_empty());
    }

    #[test]
    fn load_captured_linker_args_keys_by_output_basename() {
        let dir = unique_tempdir();
        write_linker_inv(
            &dir,
            &CapturedLinkerInvocation {
                output: Some("/cargo/target/debug/deps/libfoo.dylib".into()),
                args: s(&["-shared", "-o", "/cargo/target/debug/deps/libfoo.dylib"]),
                timestamp_micros: 100,
            },
        );
        write_linker_inv(
            &dir,
            &CapturedLinkerInvocation {
                output: Some("/cargo/target/debug/deps/libbar.dylib".into()),
                args: s(&["-shared", "-o", "/cargo/target/debug/deps/libbar.dylib"]),
                timestamp_micros: 200,
            },
        );

        let map = load_captured_linker_args(&dir).unwrap();
        assert_eq!(map.len(), 2);
        assert!(map.contains_key("libfoo.dylib"));
        assert!(map.contains_key("libbar.dylib"));

        let _ = std::fs::remove_dir_all(&dir);
    }

    #[test]
    fn load_captured_linker_args_keeps_most_recent_per_output() {
        let dir = unique_tempdir();
        write_linker_inv(
            &dir,
            &CapturedLinkerInvocation {
                output: Some("/path/libfoo.dylib".into()),
                args: s(&["old"]),
                timestamp_micros: 100,
            },
        );
        write_linker_inv(
            &dir,
            &CapturedLinkerInvocation {
                output: Some("/path/libfoo.dylib".into()),
                args: s(&["new"]),
                timestamp_micros: 200,
            },
        );

        let map = load_captured_linker_args(&dir).unwrap();
        assert_eq!(map.len(), 1);
        assert_eq!(map["libfoo.dylib"].timestamp_micros, 200);
        assert_eq!(map["libfoo.dylib"].args, s(&["new"]));

        let _ = std::fs::remove_dir_all(&dir);
    }

    #[test]
    fn load_captured_linker_args_skips_malformed_json() {
        let dir = unique_tempdir();
        std::fs::write(dir.join("garbage.json"), "{ not json").unwrap();
        write_linker_inv(
            &dir,
            &CapturedLinkerInvocation {
                output: Some("/path/lib.dylib".into()),
                args: vec![],
                timestamp_micros: 1,
            },
        );

        let map = load_captured_linker_args(&dir).unwrap();
        assert_eq!(map.len(), 1);
        assert!(map.contains_key("lib.dylib"));

        let _ = std::fs::remove_dir_all(&dir);
    }

    // ----- keep_newest_linker -------------------------------------------

    #[test]
    fn keep_newest_linker_inserts_into_empty() {
        let mut m = HashMap::new();
        keep_newest_linker(
            &mut m,
            CapturedLinkerInvocation {
                output: Some("/path/lib.so".into()),
                args: vec![],
                timestamp_micros: 1,
            },
        );
        assert_eq!(m.len(), 1);
        assert!(m.contains_key("lib.so"));
    }

    #[test]
    fn keep_newest_linker_does_not_replace_with_older() {
        let mut m = HashMap::new();
        keep_newest_linker(
            &mut m,
            CapturedLinkerInvocation {
                output: Some("/path/lib.so".into()),
                args: s(&["incumbent"]),
                timestamp_micros: 10,
            },
        );
        keep_newest_linker(
            &mut m,
            CapturedLinkerInvocation {
                output: Some("/path/lib.so".into()),
                args: s(&["older"]),
                timestamp_micros: 5,
            },
        );
        assert_eq!(m["lib.so"].args, s(&["incumbent"]));
    }

    #[test]
    fn keep_newest_linker_keys_anonymous_invocations_under_unknown() {
        let mut m = HashMap::new();
        keep_newest_linker(
            &mut m,
            CapturedLinkerInvocation {
                output: None,
                args: vec![],
                timestamp_micros: 1,
            },
        );
        assert!(m.contains_key("_unknown"));
    }

    // ----- default_linker_cache_dir + resolve_host_linker --------------

    #[test]
    fn default_linker_cache_dir_lives_under_target_dot_whisker() {
        let p = default_linker_cache_dir(Path::new("/tmp/ws"));
        assert!(p.ends_with("target/.whisker/linker-args"));
    }

    #[test]
    fn resolve_host_linker_returns_something_executable_or_a_path() {
        let p = resolve_host_linker();
        // We don't `which` here — just sanity that it isn't empty.
        assert!(!p.as_os_str().is_empty());
    }
}