Perform Input Modification Checks At the End of the Build

Sources/SwiftDriver/Utilities/VirtualPath.swift

@@ -648,7 +648,7 @@ extension TSCBasic.FileSystem {
     try resolvingVirtualPath(path, apply: exists)
   }
 
-  func lastModificationTime(for file: VirtualPath) throws -> Date {
+  public func lastModificationTime(for file: VirtualPath) throws -> Date {
     try resolvingVirtualPath(file) { path in
       #if os(macOS)
       var s = Darwin.stat()

Sources/SwiftDriverExecution/MultiJobExecutor.swift

@@ -281,6 +281,16 @@ public final class MultiJobExecutor {
 
     context.reportSkippedJobs()
 
+    // Check for any inputs that were modified during the build. Report these
+    // as errors so we don't e.g. reuse corrupted incremental build state.
+    for (input, recordedModTime) in context.recordedInputModificationDates {
+      guard try fileSystem.lastModificationTime(for: input.file) == recordedModTime else {
+        let err = Job.InputError.inputUnexpectedlyModified(input)
+        context.diagnosticsEngine.emit(err)
+        throw err
+      }
+    }
+
     // Throw the stub error the build didn't finish successfully.
     if !result.success {
       throw Diagnostics.fatalError
@@ -535,8 +545,6 @@ class ExecuteJobRule: LLBuildRule {
       let arguments: [String] = try resolver.resolveArgumentList(for: job,
                                                                  forceResponseFiles: context.forceResponseFiles)
 
-      try job.verifyInputsNotModified(since: context.recordedInputModificationDates, fileSystem: engine.fileSystem)
-
       let process = try context.processType.launchProcess(
         arguments: arguments, env: env
       )

Tests/SwiftDriverTests/JobExecutorTests.swift

@@ -323,9 +323,12 @@ final class JobExecutorTests: XCTestCase {
           $0 <<< "let bar = 3"
         }
 
-        // FIXME: It's unfortunate we diagnose this twice, once for each job which uses the input.
-        verifier.expect(.error("input file '\(other.description)' was modified during the build"))
         verifier.expect(.error("input file '\(other.description)' was modified during the build"))
+        // There's a tool-specific linker error that usually happens here from
+        // whatever job runs last - probably the linker.
+        // It's no use testing for a particular error message, let's just make
+        // sure we emit the diagnostic we need.
+        verifier.permitUnexpected(.error)
         XCTAssertThrowsError(try driver.run(jobs: jobs))
       }
     }