Fix cross-platform plugin test prerequisites and repository paths

Canonicalize the repository root before relative path calculation on Windows. Treat Wasmtime disk caching as optional when the cache directory is unavailable. Install the WASI target for coverage integration tests. AI-assisted by Codex.

Author: ketan0

.github/workflows/coverage.yml

@@ -10,11 +10,17 @@ jobs:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
 
       - name: Install Rust
-        run: rustup toolchain install stable --component llvm-tools-preview
+        run: rustup toolchain install stable --component llvm-tools-preview --target wasm32-wasip2
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@ff869c26e4a7acc3087d9d6c1163a80d3957bca7 # cargo-llvm-cov
 
+      # WASI guests cannot use the host LLVM coverage runtime.
+      - name: Build WASM plugins
+        run: cargo +stable xtask build-plugins
+
       - name: Generate code coverage
+        env:
+          DIFFR_PLUGINS_BUILT: "1"
         run: cargo +stable llvm-cov --all-features --workspace --lcov --output-path lcov.info
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@75cd11691c0faa626561e295848008c8a7dddffe # v5

src/cli.rs

@@ -223,7 +223,9 @@ fn select(
         }
     }
     paths.extend(explicit_paths);
-    let prefix = location.strip_prefix(repo.workdir().unwrap_or(repo.path()))?;
+    // Match canonical path forms, including Windows verbatim path prefixes.
+    let root = std::fs::canonicalize(repo.workdir().unwrap_or(repo.path()))?;
+    let prefix = location.strip_prefix(&root)?;
     let paths = paths
         .into_iter()
         .map(|path| normalize_path(prefix, &path))

src/plugin/wasm.rs

@@ -47,9 +47,10 @@ use bindings::diffr::plugin::{host, types};
 pub(crate) fn engine() -> anyhow::Result<Engine> {
     let mut config = Config::new();
     config.wasm_component_model(true);
-    config.cache(Some(
-        Cache::new(CacheConfig::new()).context("wasmtime's compilation cache")?,
-    ));
+    // The disk cache is an optimization; read-only homes must still run plugins.
+    if let Ok(cache) = Cache::new(CacheConfig::new()) {
+        config.cache(Some(cache));
+    }
     Engine::new(&config)
 }