[5.5] Use stat Instead of lstat

Sources/SwiftDriver/Utilities/VirtualPath.swift

@@ -715,18 +715,33 @@ extension TSCBasic.FileSystem {
     try resolvingVirtualPath(path, apply: exists)
   }
 
+  /// Retrieves the last modification time of the file referenced at the given path.
+  ///
+  /// If the given file path references a symbolic link, the modification time for the *linked file*
+  /// - not the symlink itself - is returned.
+  ///
+  /// - Parameter file: The path to a file.
+  /// - Throws: `SystemError` if the underlying `stat` operation fails.
+  /// - Returns: A `Date` value containing the last modification time.
   public func lastModificationTime(for file: VirtualPath) throws -> Date {
     try resolvingVirtualPath(file) { path in
       #if os(macOS)
       var s = Darwin.stat()
-      let err = lstat(path.pathString, &s)
+      let err = stat(path.pathString, &s)
       guard err == 0 else {
         throw SystemError.stat(errno, path.pathString)
       }
       let ti = (TimeInterval(s.st_mtimespec.tv_sec) - kCFAbsoluteTimeIntervalSince1970) + (1.0e-9 * TimeInterval(s.st_mtimespec.tv_nsec))
       return Date(timeIntervalSinceReferenceDate: ti)
       #else
-      return try localFileSystem.getFileInfo(file).modTime
+      // `getFileInfo` is going to ask Foundation to stat this path, and
+      // Foundation is always going to use `lstat` to do so. This is going to
+      // do the wrong thing for symbolic links, for which we always want to
+      // retrieve the mod time of the underlying file. This makes build systems
+      // that regenerate lots of symlinks but do not otherwise alter the
+      // contents of files - like Bazel - quite happy.
+      let path = resolveSymlinks(path)
+      return try localFileSystem.getFileInfo(path).modTime
       #endif
     }
   }

Tests/SwiftDriverTests/IncrementalCompilationTests.swift

@@ -287,6 +287,22 @@ extension IncrementalCompilationTests {
       tryNoChange(checkDiagnostics: true)
     }
   }
+
+  func testSymlinkModification() throws {
+    // Remap
+    // main.swift -> links/main.swift
+    // other.swift -> links/other.swift
+    for (file, _) in self.inputPathsAndContents {
+      try localFileSystem.createDirectory(tempDir.appending(component: "links"))
+      let linkTarget = tempDir.appending(component: "links").appending(component: file.basename)
+      try localFileSystem.move(from: file, to: linkTarget)
+      try localFileSystem.removeFileTree(file)
+      try localFileSystem.createSymbolicLink(file, pointingAt: linkTarget, relative: false)
+    }
+    try tryInitial()
+    try checkReactionToTouchingSymlinks(checkDiagnostics: true)
+    try checkReactionToTouchingSymlinkTargets(checkDiagnostics: true)
+  }
 }
 
 // MARK: - Incremental test stages
@@ -535,6 +551,68 @@ extension IncrementalCompilationTests {
       ],
       whenAutolinking: autolinkLifecycleExpectations)
   }
+
+  private func checkReactionToTouchingSymlinks(
+    checkDiagnostics: Bool = false,
+    extraArguments: [String] = []
+  ) throws {
+    for (file, _) in self.inputPathsAndContents {
+      try localFileSystem.removeFileTree(file)
+      let linkTarget = tempDir.appending(component: "links").appending(component: file.basename)
+      try localFileSystem.createSymbolicLink(file, pointingAt: linkTarget, relative: false)
+    }
+    try doABuild(
+      "touch both symlinks; non-propagating",
+      checkDiagnostics: checkDiagnostics,
+      extraArguments: extraArguments,
+      expectingRemarks: [
+        "Enabling incremental cross-module building",
+        "Incremental compilation: Read dependency graph",
+        "Incremental compilation: May skip current input:  {compile: main.o <= main.swift}",
+        "Incremental compilation: May skip current input:  {compile: other.o <= other.swift}",
+        "Incremental compilation: Skipping input:  {compile: main.o <= main.swift}",
+        "Incremental compilation: Skipping input:  {compile: other.o <= other.swift}",
+        "Incremental compilation: Skipping job: Linking theModule",
+        "Skipped Compiling main.swift",
+        "Skipped Compiling other.swift",
+      ],
+      whenAutolinking: autolinkLifecycleExpectations)
+  }
+
+  private func checkReactionToTouchingSymlinkTargets(
+    checkDiagnostics: Bool = false,
+    extraArguments: [String] = []
+  ) throws {
+    for (file, contents) in self.inputPathsAndContents {
+      let linkTarget = tempDir.appending(component: "links").appending(component: file.basename)
+      try! localFileSystem.writeFileContents(linkTarget) { $0 <<< contents }
+    }
+    try doABuild(
+      "touch both symlink targets; non-propagating",
+      checkDiagnostics: checkDiagnostics,
+      extraArguments: extraArguments,
+      expectingRemarks: [
+        "Enabling incremental cross-module building",
+        "Incremental compilation: Read dependency graph",
+        "Incremental compilation: Scheduing changed input  {compile: main.o <= main.swift}",
+        "Incremental compilation: Scheduing changed input  {compile: other.o <= other.swift}",
+        "Incremental compilation: Queuing (initial):  {compile: main.o <= main.swift}",
+        "Incremental compilation: Queuing (initial):  {compile: other.o <= other.swift}",
+        "Incremental compilation: not scheduling dependents of main.swift; unknown changes",
+        "Incremental compilation: not scheduling dependents of other.swift; unknown changes",
+        "Found 2 batchable jobs",
+        "Forming into 1 batch",
+        "Adding {compile: main.swift} to batch 0",
+        "Adding {compile: other.swift} to batch 0",
+        "Forming batch job from 2 constituents: main.swift, other.swift",
+        "Starting Compiling main.swift, other.swift",
+        "Finished Compiling main.swift, other.swift",
+        "Incremental compilation: Scheduling all post-compile jobs because something was compiled",
+        "Starting Linking theModule",
+        "Finished Linking theModule",
+      ],
+      whenAutolinking: autolinkLifecycleExpectations)
+  }
 }
 
 // MARK: - Incremental test perturbation helpers