Re-land [Planning] Avoid batching compile job twice

Sources/SwiftDriver/Jobs/Planning.swift

@@ -90,16 +90,18 @@ extension Driver {
       incrementalCompilationState = nil
     }
 
-    return try (
-      // For compatibility with swiftpm, the driver produces batched jobs
-      // for every job, even when run in incremental mode, so that all jobs
-      // can be returned from `planBuild`.
-      // But in that case, don't emit lifecycle messages.
-      formBatchedJobs(jobsInPhases.allJobs,
-                      showJobLifecycle: showJobLifecycle && incrementalCompilationState == nil,
-                      jobCreatingPch: jobsInPhases.allJobs.first(where: {$0.kind == .generatePCH})),
-      incrementalCompilationState
-    )
+    let batchedJobs: [Job]
+    // If the jobs are batched during the incremental build, reuse the computation rather than computing the batches again.
+    if let incrementalState = incrementalCompilationState {
+      // For compatibility reasons, all the jobs planned will be returned, even the incremental state suggests the job is not mandatory.
+      batchedJobs = incrementalState.skippedJobs + incrementalState.mandatoryJobsInOrder + incrementalState.jobsAfterCompiles
+    } else {
+      batchedJobs = try formBatchedJobs(jobsInPhases.allJobs,
+                                        showJobLifecycle: showJobLifecycle,
+                                        jobCreatingPch: jobsInPhases.allJobs.first(where: {$0.kind == .generatePCH}))
+    }
+
+    return (batchedJobs, incrementalCompilationState)
   }
 
   /// If performing an explicit module build, compute an inter-module dependency graph.

Tests/SwiftDriverTests/IncrementalCompilationTests.swift

@@ -217,6 +217,25 @@ extension IncrementalCompilationTests {
     let expected = try AbsolutePath(validating: "\(module).autolink", relativeTo: derivedDataPath)
     XCTAssertEqual(outputs.first!.file.absolutePath, expected)
   }
+
+  // Null planning should not return an empty compile job for compatibility reason.
+  // `swift-build` wraps the jobs returned by swift-driver in `Executor` so returning an empty list of compile job will break build system.
+  func testNullPlanningCompatility() throws {
+    guard let sdkArgumentsForTesting = try Driver.sdkArgumentsForTesting() else {
+      throw XCTSkip("Cannot perform this test on this host")
+    }
+    var driver = try Driver(args: commonArgs + sdkArgumentsForTesting)
+    let initialJobs = try driver.planBuild()
+    try driver.run(jobs: initialJobs)
+
+    // Plan the build again without touching any file. This should be a null build but for compatibility reason,
+    // planBuild() should return all the jobs and supported build system will query incremental state for the actual
+    // jobs need to be executed.
+    let replanJobs = try driver.planBuild()
+    XCTAssertFalse(
+      replanJobs.filter { $0.kind == .compile }.isEmpty,
+      "more than one compile job needs to be planned")
+  }
 }
 
 // MARK: - Dot file tests