Perform Input Modification Checks At the End of the Build

Also deduplicate them while we're at it. We don't need to ask each job for its input set, we just need to check the same set of files we recorded mod times for in the first place. This saves a quadratic amount of disk I/O in the incremental and non-batch builds, which can be a huge wall time savings for large compiles.

Author: CodaFi

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))
       }
     }