[Build] BuildOperation: Make createBuildSystem stateless

Author: xedin

Sources/Build/BuildOperation.swift

@@ -55,11 +55,9 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
     /// The path to scratch space (.build) directory.
     let scratchDirectory: AbsolutePath
 
-    /// The llbuild build delegate reference.
-    private var progressTracker: LLBuildProgressTracker?
-
-    /// The llbuild build system reference.
-    private var buildSystem: SPMLLBuild.BuildSystem?
+    /// The llbuild build system reference previously created
+    /// via \c createBuildSystem call.
+    private var current: (buildSystem: SPMLLBuild.BuildSystem, tracker: LLBuildProgressTracker)?
 
     /// If build manifest caching should be enabled.
     public let cacheBuildManifest: Bool
@@ -194,7 +192,7 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
 
     /// Cancel the active build operation.
     public func cancel(deadline: DispatchTime) throws {
-        buildSystem?.cancel()
+        current?.buildSystem.cancel()
     }
 
     // Emit a warning if a target imports another target in this build
@@ -355,8 +353,10 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
         try verifyTargetImports(in: buildDescription)
 
         // Create the build system.
-        let buildSystem = try self.createBuildSystem(buildDescription: buildDescription)
-        self.buildSystem = buildSystem
+        let (buildSystem, progressTracker) = try self.createBuildSystem(
+            buildDescription: buildDescription
+        )
+        self.current = (buildSystem, progressTracker)
 
         // If any plugins are part of the build set, compile them now to surface
         // any errors up-front. Returns true if we should proceed with the build
@@ -366,7 +366,7 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
         }
 
         // delegate is only available after createBuildSystem is called
-        self.progressTracker?.buildStart(configuration: self.productsBuildParameters.configuration)
+        progressTracker.buildStart(configuration: self.productsBuildParameters.configuration)
 
         // Perform the build.
         let llbuildTarget = try computeLLBuildTargetName(for: subset)
@@ -386,7 +386,7 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
             subsetDescriptor = nil
         }
 
-        self.progressTracker?.buildComplete(
+        progressTracker.buildComplete(
             success: success,
             duration: duration,
             subsetDescriptor: subsetDescriptor
@@ -496,7 +496,10 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
                 self.progressTracker?.preparationStepFinished(preparationStepName, result: (cachedResult.succeeded ? .succeeded : .failed))
             }
         }
-        let delegate = Delegate(preparationStepName: "Compiling plugin \(plugin.targetName)", progressTracker: self.progressTracker)
+        let delegate = Delegate(
+            preparationStepName: "Compiling plugin \(plugin.targetName)",
+            progressTracker: self.current?.tracker
+        )
         let result = try temp_await {
             pluginConfiguration.scriptRunner.compilePluginScript(
                 sourceFiles: plugin.sources.paths,
@@ -745,8 +748,10 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
 
     /// Build the package structure target.
     private func buildPackageStructure() throws -> Bool {
-        let buildSystem = try self.createBuildSystem(buildDescription: .none)
-        self.buildSystem = buildSystem
+        let (buildSystem, tracker) = try self.createBuildSystem(
+            buildDescription: .none
+        )
+        self.current = (buildSystem, tracker)
 
         // Build the package structure target which will re-generate the llbuild manifest, if necessary.
         return buildSystem.build(target: "PackageStructure")
@@ -756,7 +761,9 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
     ///
     /// The build description should only be omitted when creating the build system for
     /// building the package structure target.
-    private func createBuildSystem(buildDescription: BuildDescription?) throws -> SPMLLBuild.BuildSystem {
+    private func createBuildSystem(
+        buildDescription: BuildDescription?
+    ) throws -> (buildSystem: SPMLLBuild.BuildSystem, tracker: LLBuildProgressTracker) {
         // Figure out which progress bar we have to use during the build.
         let progressAnimation = ProgressAnimation.ninja(
             stream: self.outputStream,
@@ -782,16 +789,17 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
             observabilityScope: self.observabilityScope,
             delegate: self.delegate
         )
-        self.progressTracker = progressTracker
 
         let databasePath = self.scratchDirectory.appending("build.db").pathString
 
-        return SPMLLBuild.BuildSystem(
+        let llbuildSystem = SPMLLBuild.BuildSystem(
             buildFile: self.productsBuildParameters.llbuildManifest.pathString,
             databaseFile: databasePath,
             delegate: progressTracker,
             schedulerLanes: self.productsBuildParameters.workers
         )
+
+        return (buildSystem: llbuildSystem, tracker: progressTracker)
     }
 
     /// Runs any prebuild commands associated with the given list of plugin invocation results, in order, and returns the