[hermes] Resolve Cargo and Hermes target dirs

gherrit-pr-id: G48b27aa33306f1b4bae6f7996a68c1a1869d0a9a

Author: joshlf

tools/hermes/src/main.rs

@@ -58,5 +58,5 @@ fn main() {
     }
 
     // TODO: Create shadow skeleton.
-    // shadow::create_shadow_skeleton(&roots.workspace, todo!(), todo!(), todo!()).unwrap();
+    // shadow::create_shadow_skeleton(&roots.workspace, &roots.shadow_root, &roots.cargo_target_dir, todo!()).unwrap();
 }

tools/hermes/src/resolve.rs

@@ -1,9 +1,11 @@
-use std::{env, path::PathBuf};
+use std::{
+    env,
+    hash::{Hash as _, Hasher as _},
+    path::PathBuf,
+};
 
 use anyhow::{anyhow, Context, Result};
-use cargo_metadata::{
-    Metadata, MetadataCommand, Package, PackageName, Target, TargetKind,
-};
+use cargo_metadata::{Metadata, MetadataCommand, Package, PackageName, Target, TargetKind};
 use clap::Parser;
 
 #[derive(Parser, Debug)]
@@ -98,6 +100,8 @@ impl TryFrom<&TargetKind> for HermesTargetKind {
 
 pub struct Roots {
     pub workspace: PathBuf,
+    pub cargo_target_dir: PathBuf,
+    pub shadow_root: PathBuf,
     pub roots: Vec<(PackageName, HermesTargetKind, PathBuf)>,
 }
 
@@ -120,8 +124,12 @@ pub fn resolve_roots(args: &Args) -> Result<Roots> {
 
     let selected_packages = resolve_packages(&metadata, &args.workspace)?;
 
-    let mut roots =
-        Roots { workspace: metadata.workspace_root.as_std_path().to_owned(), roots: Vec::new() };
+    let mut roots = Roots {
+        workspace: metadata.workspace_root.as_std_path().to_owned(),
+        cargo_target_dir: metadata.target_directory.as_std_path().to_owned(),
+        shadow_root: resolve_shadow_path(&metadata),
+        roots: Vec::new(),
+    };
 
     for package in selected_packages {
         log::trace!("Scanning package: {}", package.name);
@@ -145,6 +153,22 @@ pub fn resolve_roots(args: &Args) -> Result<Roots> {
     Ok(roots)
 }
 
+fn resolve_shadow_path(metadata: &Metadata) -> PathBuf {
+    // NOTE: Automatically handles `CARGO_TARGET_DIR` env var.
+    let target_dir = metadata.target_directory.as_std_path();
+
+    // Hash the path to the workspace root to avoid collisions between different
+    // workspaces using the same target directory.
+    let workspace_root_hash = {
+        let mut hasher = std::hash::DefaultHasher::new();
+        hasher.write(b"hermes_shadow_salt");
+        metadata.workspace_root.hash(&mut hasher);
+        hasher.finish()
+    };
+
+    target_dir.join(format!("hermes_shadow_{workspace_root_hash}"))
+}
+
 /// Resolves which packages to process based on workspace flags and CWD.
 fn resolve_packages<'a>(
     metadata: &'a Metadata,

tools/hermes/src/shadow.rs

@@ -17,6 +17,11 @@ pub fn create_shadow_skeleton(
     target_dir: &Path,
     skip_paths: &HashSet<PathBuf>,
 ) -> Result<()> {
+    if dest_root.exists() {
+        std::fs::remove_dir_all(&dest_root)?;
+    }
+    std::fs::create_dir_all(&dest_root)?;
+
     let walker = WalkDir::new(source_root)
         .follow_links(false) // Security: don't follow symlinks out of the root.
         .into_iter();