Use a copy of the cache to avoid issues

src/index.ts

@@ -23,6 +23,7 @@ import projectExprTmpl from "./yarn-project.nix.in";
 const generate = async (project: Project, report: Report) => {
   const { configuration, cwd } = project;
   const yarnPathAbs = configuration.get(`yarnPath`);
+  const cacheFolderAbs = configuration.get(`cacheFolder`);
   const lockfileFilename = configuration.get(`lockfileFilename`);
 
   // TODO: Should try to remove this. Our binary wrappers currently do
@@ -40,7 +41,16 @@ const generate = async (project: Project, report: Report) => {
     yarnPath = yarnPathAbs;
     report.reportWarning(
       0,
-      `The Yarn path ${yarnPathAbs} is outside the project directory - it cannot be reached by the Nix build`
+      `The Yarn path ${yarnPathAbs} is outside the project - it may not be reachable by the Nix build`
+    );
+  }
+
+  let cacheFolder = ppath.relative(cwd, cacheFolderAbs);
+  if (cacheFolder.startsWith(`../`)) {
+    cacheFolder = cacheFolderAbs;
+    report.reportWarning(
+      0,
+      `The cache folder ${cacheFolderAbs} is outside the project - it may not be reachable by the Nix build`
     );
   }
 
@@ -74,7 +84,7 @@ const generate = async (project: Project, report: Report) => {
       if (inputPath !== yarnPath) {
         report.reportWarning(
           0,
-          `The path ${inputPath} is outside the project directory and was ignored - it may not be reachable in the Nix build`
+          `The path ${inputPath} is outside the project and was ignored - it may not be reachable in the Nix build`
         );
       }
       continue;
@@ -91,7 +101,6 @@ const generate = async (project: Project, report: Report) => {
 
   // Build the Nix output-hash by hashing the Yarn cache folder. The
   // derivation should build the exact same.
-  const cacheFolder = configuration.get(`cacheFolder`);
   let cacheHash = ``;
   try {
     const hasherResult = await execUtils.execvp(
@@ -116,7 +125,8 @@ const generate = async (project: Project, report: Report) => {
   const projectName = ident ? structUtils.stringifyIdent(ident) : `workspace`;
   const projectExpr = projectExprTmpl
     .replace(`@@PROJECT_NAME@@`, JSON.stringify(projectName))
-    .replace(`@@OFFLINE_CACHE_HASH@@`, JSON.stringify(cacheHash))
+    .replace(`@@CACHE_FOLDER@@`, JSON.stringify(cacheFolder))
+    .replace(`@@CACHE_HASH@@`, JSON.stringify(cacheHash))
     .replace(`@@YARN_PATH@@`, JSON.stringify(yarnPath))
     .replace(
       `@@YARN_CLOSURE_ENTRIES@@`,

src/yarn-project.nix.in

@@ -9,7 +9,8 @@ let
 
   # Variables provided by the generator.
   project-name = @@PROJECT_NAME@@;
-  offline-cache-hash = @@OFFLINE_CACHE_HASH@@;
+  cache-folder = @@CACHE_FOLDER@@;
+  cache-hash = @@CACHE_HASH@@;
   yarn-path = @@YARN_PATH@@;
   yarn-closure-entries = @@YARN_CLOSURE_ENTRIES@@;
 
@@ -30,13 +31,13 @@ let
       elem "${type}:${srcRel path}" yarn-closure-entries;
   };
 
-  # Build just the offline cache for the project.
+  # Build just the cache for the project.
   offline-cache = stdenv.mkDerivation {
     name = "${project-name}-offline-cache";
     buildInputs = [ nodejs ];
     builder = writeText "builder.sh" ''
       source $stdenv/setup
-      cd ${yarn-closure}
+      cd '${yarn-closure}'
 
       # Yarn may need a writable home directory for the global cache mirror.
       # TODO: Can't disable the mirror, because it changes cache filenames.
@@ -51,7 +52,7 @@ let
     '';
     outputHashMode = "recursive";
     outputHashAlgo = "sha256";
-    outputHash = offline-cache-hash;
+    outputHash = cache-hash;
   };
 
 in stdenv.mkDerivation {
@@ -72,18 +73,15 @@ in stdenv.mkDerivation {
   configurePhase = ''
     runHook preConfigure
 
-    # Move the entire project to the output directory.
-    # TODO: Would rather do this in 'installPhase',
-    # but '.pnp.js' is generated with relative paths.
-    mkdir -p $out/libexec $out/bin
-    mv $PWD "$out/libexec/$sourceRoot"
-    cd "$out/libexec/$sourceRoot"
+    # Copy over the Yarn cache.
+    # TODO: Can we do without the copy somehow?
+    rm -fr '${cache-folder}'
+    mkdir -p "$(dirname '${cache-folder}')"
+    cp -r '${offline-cache}' '${cache-folder}'
+    chmod -R u+w '${cache-folder}'
 
     # Store the absolute path to Yarn for the 'yarn' alias.
-    export NIX_YARN_PATH="$(readlink -f ${yarn-path})"
-
-    # Point Yarn to the offline cache built separately.
-    yarn config set cacheFolder '${offline-cache}'
+    export NIX_YARN_PATH="$(readlink -f '${yarn-path}')"
 
     # Run normal Yarn install to complete dependency installation.
     yarn install --immutable --immutable-cache
@@ -99,6 +97,15 @@ in stdenv.mkDerivation {
   installPhase = ''
     runHook preInstall
 
+    mkdir -p $out/libexec $out/bin
+
+    # Move the entire project to the output directory.
+    mv $PWD "$out/libexec/$sourceRoot"
+    cd "$out/libexec/$sourceRoot"
+
+    # Update the path to Yarn.
+    export NIX_YARN_PATH="$(readlink -f '${yarn-path}')"
+
     # Invoke a plugin internal command to setup binaries.
     yarn nixify install-bin $out/bin