Use Cch API more cleanly, abstract string munging into a computed variable

Author: grynspan

Sources/Testing/ExitTests/SpawnProcess.swift

@@ -297,18 +297,7 @@ func spawnExecutable(
       // unlikely to be deleted, one hopes).
       //
       // SEE: https://devblogs.microsoft.com/oldnewthing/20101109-00/?p=12323
-      var workingDirectoryPath = UnsafeMutableBufferPointer<wchar_t>.allocate(capacity: Int(MAX_PATH))
-      defer {
-        workingDirectoryPath.deallocate()
-      }
-      let systemDrive = Environment.variable(named: "SYSTEMDRIVE") ?? "C:"
-      systemDrive.withCString(encodedAs: UTF16.self) { systemDrive in
-        wcscpy_s(workingDirectoryPath.baseAddress!, workingDirectoryPath.count, systemDrive)
-        let rAddBackslash = PathCchAddBackslashEx(workingDirectoryPath.baseAddress!, workingDirectoryPath.count, nil, nil)
-        guard rAddBackslash == S_OK || rAddBackslash == S_FALSE else {
-          fatalError("Unexpected error when normalizing system drive path '\(systemDrive)': HRESULT(\(rAddBackslash))")
-        }
-      }
+      let workingDirectoryPath = rootDirectoryPath
 
       var flags = DWORD(CREATE_NO_WINDOW | CREATE_UNICODE_ENVIRONMENT | EXTENDED_STARTUPINFO_PRESENT)
 #if DEBUG
@@ -318,30 +307,32 @@ func spawnExecutable(
 
       return try commandLine.withCString(encodedAs: UTF16.self) { commandLine in
         try environ.withCString(encodedAs: UTF16.self) { environ in
-          var processInfo = PROCESS_INFORMATION()
-
-          guard CreateProcessW(
-            nil,
-            .init(mutating: commandLine),
-            nil,
-            nil,
-            true, // bInheritHandles
-            flags,
-            .init(mutating: environ),
-            workingDirectoryPath,
-            startupInfo.pointer(to: \.StartupInfo)!,
-            &processInfo
-          ) else {
-            throw Win32Error(rawValue: GetLastError())
-          }
+          try workingDirectoryPath.withCString(encodedAs: UTF16.self) { workingDirectoryPath in
+            var processInfo = PROCESS_INFORMATION()
+
+            guard CreateProcessW(
+              nil,
+              .init(mutating: commandLine),
+              nil,
+              nil,
+              true, // bInheritHandles
+              flags,
+              .init(mutating: environ),
+              workingDirectoryPath,
+              startupInfo.pointer(to: \.StartupInfo)!,
+              &processInfo
+            ) else {
+              throw Win32Error(rawValue: GetLastError())
+            }
 
 #if DEBUG
-          // Resume the process.
-          _ = ResumeThread(processInfo.hThread!)
+            // Resume the process.
+            _ = ResumeThread(processInfo.hThread!)
 #endif
 
-          _ = CloseHandle(processInfo.hThread)
-          return processInfo.hProcess!
+            _ = CloseHandle(processInfo.hThread)
+            return processInfo.hProcess!
+          }
         }
       }
     }

Sources/Testing/Support/FileHandle.swift

@@ -719,4 +719,37 @@ func setFD_CLOEXEC(_ flag: Bool, onFileDescriptor fd: CInt) throws {
   }
 }
 #endif
+
+/// The path to the root directory of the boot volume.
+///
+/// On Windows, this string is usually of the form `"C:\"`. On UNIX-like
+/// platforms, it is always equal to `"/"`.
+let rootDirectoryPath: String = {
+#if os(Windows)
+  if let systemDrive = Environment.variable(named: "SYSTEMDRIVE")?.utf16 {
+    // Copy the system drive string with room for up to "C:\" and a NUL.
+    var buffer = UnsafeMutableBufferPointer<wchar_t>.allocate(capacity: systemDrive.count + 4)
+    defer {
+      buffer.deallocate()
+    }
+    buffer.initialize(fromContentsOf: systemDrive)
+    buffer[systemDrive.count] = 0
+
+    // On the assumption that the value of %SYSTEMDRIVE% is "C:" or similar,
+    // ensure a trailing slash is added to refer to the root directory. If
+    // somebody decides to set this environment variable to something
+    // nonsensical or to a deeper path, we should accept it silently.
+    let rAddBackslash = PathCchAddBackslashEx(buffer.baseAddress!, buffer.count, nil, nil)
+    if rAddBackslash == S_OK || rAddBackslash == S_FALSE,
+       let result = String.decodeCString(buffer.baseAddress!, as: UTF16.self)?.result {
+      return result
+    }
+  }
+  // We weren't able to get a path, so fall back to "C:\" on the assumption that
+  // it's the common case and most likely correct.
+  return #"C:\"#
+#else
+  return "/"
+#endif
+}()
 #endif

Tests/TestingTests/Support/FileHandleTests.swift

@@ -201,6 +201,26 @@ struct FileHandleTests {
 #endif
   }
 #endif
+
+  @Test("Root directory path is correct")
+  func rootDirectoryPathIsCorrect() async {
+#if os(Windows)
+#if !SWT_NO_EXIT_TESTS
+    await #expect(processExitsWith: .success) {
+      #expect(Environment.setVariable(nil, named: "SYSTEMDRIVE"))
+      #expect(rootDirectoryPath == #"C:\"#)
+
+      #expect(Environment.setVariable("Q:", named: "SYSTEMDRIVE"))
+      #expect(rootDirectoryPath == #"Q:\"#)
+
+      #expect(Environment.setVariable("Q:\abc123", named: "SYSTEMDRIVE"))
+      #expect(rootDirectoryPath == #"Q:\abc123\"#)
+    }
+#endif
+#else
+    #expect(rootDirectoryPath == "/")
+#endif
+  }
 }
 
 // MARK: - Fixtures