Add a mechanism to clear the global module cache at the end of a build for testing

Sources/SWBBuildSystem/BuildOperation.swift

@@ -1555,6 +1555,7 @@ internal final class OperationSystemAdaptor: SWBLLBuild.BuildSystemDelegate, Act
     func waitForCompletion(buildSucceeded: Bool) async {
         let completionToken = await dynamicOperationContext.waitForCompletion()
         cleanupCompilationCache()
+        cleanupGlobalModuleCache()
 
         await queue.sync {
             self.isCompleted = true
@@ -1601,6 +1602,33 @@ internal final class OperationSystemAdaptor: SWBLLBuild.BuildSystemDelegate, Act
         }
     }
 
+    func cleanupGlobalModuleCache() {
+        let settings = operation.requestContext.getCachedSettings(operation.request.parameters)
+        if settings.globalScope.evaluate(BuiltinMacros.KEEP_GLOBAL_MODULE_CACHE_DIRECTORY) {
+            return // Keep the cache directory.
+        }
+
+        let cachePath = settings.globalScope.evaluate(BuiltinMacros.MODULE_CACHE_DIR)
+        guard !cachePath.isEmpty, operation.fs.exists(cachePath) else {
+            return
+        }
+
+        let signatureCtx = InsecureHashContext()
+        signatureCtx.add(string: "CleanupGlobalModuleCache")
+        signatureCtx.add(string: cachePath.str)
+        let signature = signatureCtx.signature
+
+        withActivity(ruleInfo: "CleanupGlobalModuleCache \(cachePath.str)", executionDescription: "Cleanup global module cache at \(cachePath)", signature: signature, target: nil, parentActivity: nil) { activity in
+            do {
+                try operation.fs.removeDirectory(cachePath)
+            } catch {
+                // Log error but do not fail the build.
+                emit(diagnostic: Diagnostic.init(behavior: .warning, location: .unknown, data: DiagnosticData("Failed to remove \(cachePath): \(error.localizedDescription)")), for: activity, signature: signature)
+            }
+            return .succeeded
+        }
+    }
+
     /// Get the active output delegate for an executing command.
     ///
     /// - returns: The active delegate, or nil if not found.

Sources/SWBCore/Settings/BuiltinMacros.swift

@@ -547,6 +547,7 @@ public final class BuiltinMacros {
     public static let COMPILATION_CACHE_ENABLE_PLUGIN = BuiltinMacros.declareBooleanMacro("COMPILATION_CACHE_ENABLE_PLUGIN")
     public static let COMPILATION_CACHE_ENABLE_STRICT_CAS_ERRORS = BuiltinMacros.declareBooleanMacro("COMPILATION_CACHE_ENABLE_STRICT_CAS_ERRORS")
     public static let COMPILATION_CACHE_KEEP_CAS_DIRECTORY = BuiltinMacros.declareBooleanMacro("COMPILATION_CACHE_KEEP_CAS_DIRECTORY")
+    public static let KEEP_GLOBAL_MODULE_CACHE_DIRECTORY = BuiltinMacros.declareBooleanMacro("SCANNING_PCM_KEEP_CACHE_DIRECTORY")
     public static let COMPILATION_CACHE_CAS_PATH = BuiltinMacros.declareStringMacro("COMPILATION_CACHE_CAS_PATH")
     public static let COMPILATION_CACHE_LIMIT_PERCENT = BuiltinMacros.declareStringMacro("COMPILATION_CACHE_LIMIT_PERCENT")
     public static let COMPILATION_CACHE_LIMIT_SIZE = BuiltinMacros.declareStringMacro("COMPILATION_CACHE_LIMIT_SIZE")
@@ -1551,6 +1552,7 @@ public final class BuiltinMacros {
         COMPILATION_CACHE_ENABLE_PLUGIN,
         COMPILATION_CACHE_ENABLE_STRICT_CAS_ERRORS,
         COMPILATION_CACHE_KEEP_CAS_DIRECTORY,
+        KEEP_GLOBAL_MODULE_CACHE_DIRECTORY,
         COMPILATION_CACHE_LIMIT_PERCENT,
         COMPILATION_CACHE_LIMIT_SIZE,
         COMPILATION_CACHE_CAS_PATH,

Sources/SWBCore/Settings/Settings.swift

@@ -1983,6 +1983,11 @@ private class SettingsBuilder {
         exportedMacroNames.formUnion(info.exportedMacros)
         errors.append(contentsOf:info.errors)
 
+        // Default to preserving the module cache directory.
+        pushTable(.exported) {
+            $0.push(BuiltinMacros.KEEP_GLOBAL_MODULE_CACHE_DIRECTORY, literal: true)
+        }
+
         if self.parameters.action == .indexBuild {
             pushTable(.exported) {
                 $0.push(BuiltinMacros.INDEX_ENABLE_BUILD_ARENA, literal: true)