refactor(@ngtools/webpack): reduce stored resource data between rebuilds

This change makes several changes to attempt to reduce retained memory within the resource loader. The first is the preemptive clearing of the resource loader's parent compilation after all modules are built. The second removes the cached sourcemaps for each resource which are not yet used by the system. And finally, the child compilations are no longer stored on the parent compilation and instead the dependencies, errors, and warnings are propagated to the parent compilation.

(cherry picked from commit d92805e)

Author: clydin

packages/ngtools/webpack/src/ivy/plugin.ts

@@ -265,6 +265,9 @@ export class AngularWebpackPlugin {
         // Rebuild any remaining AOT required modules
         await this.rebuildRequiredFiles(modules, compilation, fileEmitter);
 
+        // Clear out the Webpack compilation to avoid an extra retaining reference
+        resourceLoader.clearParentCompilation();
+
         // Analyze program for unused files
         if (compilation.errors.length > 0) {
           return;

packages/ngtools/webpack/src/resource_loader.ts

@@ -44,6 +44,10 @@ export class WebpackResourceLoader {
     }
   }
 
+  clearParentCompilation() {
+    this._parentCompilation = undefined;
+  }
+
   getModifiedResourceFiles() {
     return this.modifiedResources;
   }
@@ -172,6 +176,22 @@ export class WebpackResourceLoader {
           return;
         }
 
+        // Workaround to attempt to reduce memory usage of child compilations.
+        // This removes the child compilation from the main compilation and manually propagates
+        // all dependencies, warnings, and errors.
+        const parent = childCompiler.parentCompilation;
+        if (parent) {
+          parent.children = parent.children.filter((child) => child !== childCompilation);
+
+          parent.fileDependencies.addAll(childCompilation.fileDependencies);
+          parent.contextDependencies.addAll(childCompilation.contextDependencies);
+          parent.missingDependencies.addAll(childCompilation.missingDependencies);
+          parent.buildDependencies.addAll(childCompilation.buildDependencies);
+
+          parent.warnings.push(...childCompilation.warnings);
+          parent.errors.push(...childCompilation.errors);
+        }
+
         // Save the dependencies for this resource.
         if (filePath) {
           this._fileDependencies.set(filePath, new Set(childCompilation.fileDependencies));
@@ -188,7 +208,6 @@ export class WebpackResourceLoader {
 
         resolve({
           content: finalContent ?? '',
-          map: finalMap,
           success: childCompilation.errors?.length === 0,
         });
       });