FileHandle.truncateFile should reposition on truncate

- Accepting truncating to any position, not just 0.
- Reposition to the truncate offset.
- Darwin throws exceptions on error so we use fatalError() instead of silently failing.

(cherry picked from commit fb1e71b)

Author: nevil

Foundation/FileHandle.swift

@@ -143,14 +143,13 @@ open class FileHandle : NSObject, NSSecureCoding {
         precondition(_fd >= 0, "Bad file descriptor")
         lseek(_fd, off_t(offset), SEEK_SET)
     }
-    
+
     open func truncateFile(atOffset offset: UInt64) {
         precondition(_fd >= 0, "Bad file descriptor")
-        if lseek(_fd, off_t(offset), SEEK_SET) == 0 {
-            ftruncate(_fd, off_t(offset))
-        }
+        if lseek(_fd, off_t(offset), SEEK_SET) < 0 { fatalError("lseek() failed.") }
+        if ftruncate(_fd, off_t(offset)) < 0 { fatalError("ftruncate() failed.") }
     }
-    
+
     open func synchronizeFile() {
         precondition(_fd >= 0, "Bad file descriptor")
         fsync(_fd)

TestFoundation/TestFileHandle.swift

@@ -12,6 +12,7 @@ class TestFileHandle : XCTestCase {
         return [
             ("test_constants", test_constants),
             ("test_nullDevice", test_nullDevice),
+            ("test_truncate", test_truncate),
         ]
     }
 
@@ -38,4 +39,29 @@ class TestFileHandle : XCTestCase {
         fh.seek(toFileOffset: 0)
         XCTAssertEqual(fh.readDataToEndOfFile().count, 0)
     }
+
+    func test_truncate() throws {
+        let fm = FileManager.default
+        let tempDir = fm.temporaryDirectory
+        let filePath = tempDir.appendingPathComponent("temp_file")
+        guard fm.createFile(atPath: filePath.path, contents: nil, attributes: nil) else {
+            XCTAssertTrue(false, "Unable to create temporary file");
+            return
+        }
+        guard let fh = FileHandle(forWritingAtPath: filePath.path) else {
+            XCTAssertTrue(false, "Unable to open temporary file")
+            return
+        }
+        defer { try? fm.removeItem(atPath: filePath.path) }
+
+        for newSize: UInt64 in [0, 100, 5, 1] {
+            fh.truncateFile(atOffset: newSize)
+            guard let size = (try fm.attributesOfItem(atPath: filePath.path))[.size] as? NSNumber else {
+                XCTFail("Cant get size")
+                continue
+            }
+            XCTAssertEqual(newSize, size.uint64Value)
+            XCTAssertEqual(newSize, fh.offsetInFile)
+        }
+    }
 }