[windows] add a check for a matching python architecture

Sources/SwiftDriver/CMakeLists.txt

@@ -116,6 +116,7 @@ add_library(SwiftDriver
   Utilities/FileMetadata.swift
   Utilities/FileType.swift
   Utilities/PredictableRandomNumberGenerator.swift
+  Utilities/PythonArchitecture.swift
   Utilities/RelativePathAdditions.swift
   Utilities/Sanitizer.swift
   Utilities/StringAdditions.swift

Sources/SwiftDriver/Driver/Driver.swift

@@ -864,6 +864,14 @@ public struct Driver {
     if case .subcommand = try Self.invocationRunMode(forArgs: args).mode {
       throw Error.subcommandPassedToDriver
     }
+
+    #if os(Windows)
+      if args[1] == "-repl" {  // a `-` gets automatically added to `swift repl`.
+        try checkIfMatchingPythonArch(
+          cwd: ProcessEnv.cwd, env: env, diagnosticsEngine: diagnosticsEngine)
+      }
+    #endif
+
     var args = args
     if let additional = env["ADDITIONAL_SWIFT_DRIVER_FLAGS"] {
       args.append(contentsOf: additional.components(separatedBy: " "))
@@ -955,7 +963,7 @@ public struct Driver {
                                                       negative: .disableIncrementalFileHashing,
                                                       default: false)
     self.recordedInputMetadata = .init(uniqueKeysWithValues:
-      Set(inputFiles).compactMap { inputFile -> (TypedVirtualPath, FileMetadata)? in 
+      Set(inputFiles).compactMap { inputFile -> (TypedVirtualPath, FileMetadata)? in
         guard let modTime = try? fileSystem.lastModificationTime(for: inputFile.file) else { return nil }
         if incrementalFileHashes {
             guard let data = try? fileSystem.readFileContents(inputFile.file)  else { return nil }

Sources/SwiftDriver/Utilities/PythonArchitecture.swift

@@ -0,0 +1,122 @@
+import Foundation
+
+import struct TSCBasic.AbsolutePath
+import struct TSCBasic.ByteString
+import struct TSCBasic.Diagnostic
+import protocol TSCBasic.DiagnosticData
+import class TSCBasic.DiagnosticsEngine
+import protocol TSCBasic.FileSystem
+import protocol TSCBasic.OutputByteStream
+import func TSCBasic.getEnvSearchPaths
+import func TSCBasic.lookupExecutablePath
+
+/// Check that the architecture of the toolchain matches the architecture
+/// of the Python installation.
+///
+/// When installing the x86 toolchain on ARM64 Windows, if the user does not
+/// install an x86 version of Python, they will get acryptic error message
+/// when running lldb (`0xC000007B`). Calling this function before invoking
+/// lldb gives them a warning to help troublshoot the issue.
+///
+/// - Parameters:
+///   - cwd: The current working directory.
+///   - env: A dictionary of the environment variables and their values. Usually of the parent shell.
+///   - diagnosticsEngine: DiagnosticsEngine instance to use for printing the warning.
+public func checkIfMatchingPythonArch(
+  cwd: AbsolutePath?, env: [String: String], diagnosticsEngine: DiagnosticsEngine
+) throws {
+  #if arch(arm64)
+    let toolchainArchitecture = COFFBinaryExecutableArchitecture.arm64
+  #elseif arch(x86_64)
+    let toolchainArchitecture = COFFBinaryExecutableArchitecture.x64
+  #elseif arch(x86)
+    let toolchainArchitecture = COFFBinaryExecutableArchitecture.x86
+  #else
+    return
+  #endif
+  let pythonArchitecture = COFFBinaryExecutableArchitecture.readWindowsExecutableArchitecture(
+    cwd: cwd, env: env, filename: "python.exe")
+
+  if toolchainArchitecture != pythonArchitecture {
+    diagnosticsEngine.emit(
+      .warning(
+        """
+        There is an architecture mismatch between the installed toolchain and the resolved Python's architecture:
+        Toolchain: \(toolchainArchitecture)
+        Python: \(pythonArchitecture)
+        """))
+  }
+}
+
+/// Some of the architectures that can be stored in a COFF header.
+enum COFFBinaryExecutableArchitecture: String {
+  case x86 = "X86"
+  case x64 = "X64"
+  case arm64 = "ARM64"
+  case unknown = "Unknown"
+
+  static func fromPEMachineByte(machine: UInt16) -> Self {
+    // https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#machine-types
+    switch machine {
+    case 0x014c: return .x86
+    case 0x8664: return .x64
+    case 0xAA64: return .arm64
+    default: return .unknown
+    }
+  }
+
+  /// Resolves the filename from the `Path` environment variable and read its COFF header to determine the architecture
+  /// of the binary.
+  ///
+  /// - Parameters:
+  ///   - cwd: The current working directory.
+  ///   - env: A dictionary of the environment variables and their values. Usually of the parent shell.
+  ///   - filename: The name of the file we are resolving the architecture of.
+  /// - Returns: The architecture of the file which was found in the `Path`.
+  static func readWindowsExecutableArchitecture(
+    cwd: AbsolutePath?, env: [String: String], filename: String
+  ) -> Self {
+    let searchPaths = getEnvSearchPaths(pathString: env["Path"], currentWorkingDirectory: cwd)
+    guard let filePath = lookupExecutablePath(
+        filename: filename, currentWorkingDirectory: cwd, searchPaths: searchPaths)
+    else {
+      return .unknown
+    }
+    guard let fileHandle = FileHandle(forReadingAtPath: filePath.pathString) else {
+      return .unknown
+    }
+
+    defer { fileHandle.closeFile() }
+
+    // Infering the architecture of a Windows executable from its COFF header involves the following:
+    // 1. Get the COFF header offset from the pointer located at the 0x3C offset (4 bytes long).
+    // 2. Jump to that offset and read the next 6 bytes.
+    // 3. The first 4 are the signature which should be equal to 0x50450000.
+    // 4. The last 2 are the machine architecture which can be infered from the value we get.
+    //
+    // The link below provides a visualization of the COFF header and the process to get to it.
+    // https://upload.wikimedia.org/wikipedia/commons/1/1b/Portable_Executable_32_bit_Structure_in_SVG_fixed.svg
+    fileHandle.seek(toFileOffset: 0x3C)
+    guard let offsetPointer = try? fileHandle.read(upToCount: 4),
+      offsetPointer.count == 4
+    else {
+      return .unknown
+    }
+
+    let peHeaderOffset = offsetPointer.withUnsafeBytes { $0.load(as: UInt32.self) }
+
+    fileHandle.seek(toFileOffset: UInt64(peHeaderOffset))
+    guard let coffHeader = try? fileHandle.read(upToCount: 6), coffHeader.count == 6 else {
+      return .unknown
+    }
+
+    let signature = coffHeader.prefix(4)
+    let machineBytes = coffHeader.suffix(2)
+
+    guard signature == Data([0x50, 0x45, 0x00, 0x00]) else {
+      return .unknown
+    }
+
+    return .fromPEMachineByte(machine: machineBytes.withUnsafeBytes { $0.load(as: UInt16.self) })
+  }
+}