Fix file seek errors

Src/IronPython/Modules/_fileio.cs

@@ -133,7 +133,7 @@ public FileIO(CodeContext/*!*/ context, string name, string mode = "r", bool clo
                         // according to [documentation](https://learn.microsoft.com/en-us/dotnet/api/system.io.filestream.safefilehandle?view=net-9.0#remarks)
                         // accessing SafeFileHandle sets the current stream position to 0
                         // in practice it doesn't seem to be the case, but better to be sure
-                        if (this.mode == "ab+") {
+                        if (this.mode.StartsWith("ab", StringComparison.InvariantCulture)) {
                             _streams.WriteStream.Seek(0L, SeekOrigin.End);
                         }
                         if (!_streams.IsSingleStream) {
@@ -372,26 +372,40 @@ public override BigInteger readinto(CodeContext/*!*/ context, object buf) {
                 return readinto(bufferProtocol);
             }
 
-            [Documentation("seek(offset: int[, whence: int]) -> None.  Move to new file position.\n\n"
-                + "Argument offset is a byte count.  Optional argument whence defaults to\n"
-                + "0 (offset from start of file, offset should be >= 0); other values are 1\n"
-                + "(move relative to current position, positive or negative), and 2 (move\n"
-                + "relative to end of file, usually negative, although many platforms allow\n"
-                + "seeking beyond the end of a file).\n"
-                + "Note that not all file objects are seekable."
-                )]
+
+            [Documentation("""
+                seek(offset: int[, whence: int]) -> int.  Change stream position.
+
+                Argument offset is a byte count.  Optional argument whence defaults to
+                0 or `os.SEEK_SET` (offset from start of file, offset should be >= 0);
+                other values are 1 or `os.SEEK_CUR` (move relative to current position,
+                positive or negative), and 2 or `os.SEEK_END` (move relative to end of
+                file, usually negative, although many platforms allow seeking beyond
+                the end of a file, by adding zeros to enlarge the file).
+
+                Return the new absolute position.
+
+                Note that not all file objects are seekable.
+                """)]
             public override BigInteger seek(CodeContext/*!*/ context, BigInteger offset, [Optional] object whence) {
                 _checkClosed();
 
-                return _streams.ReadStream.Seek((long)offset, (SeekOrigin)GetInt(whence));
-            }
+                var origin = (SeekOrigin)GetInt(whence);
+                if (origin < SeekOrigin.Begin || origin > SeekOrigin.End)
+                    throw PythonOps.OSError(PythonFileManager.EINVAL, "Invalid argument");
 
-            public BigInteger seek(double offset, [Optional] object whence) {
-                _checkClosed();
+                long ofs = checked((long)offset);
+                if (ofs < 0 && ClrModule.IsMono && origin == SeekOrigin.Current) {
+                    // Mono does not support negative offsets with SeekOrigin.Current
+                    // so we need to calculate the absolute offset
+                    ofs += _streams.ReadStream.Position;
+                    origin = SeekOrigin.Begin;
+                }
 
-                throw PythonOps.TypeError("an integer is required");
+                return _streams.ReadStream.Seek(ofs, origin);
             }
 
+
             [Documentation("seekable() -> bool.  True if file supports random-access.")]
             public override bool seekable(CodeContext/*!*/ context) {
                 _checkClosed();

Src/IronPython/Runtime/PythonFileManager.cs

@@ -222,6 +222,7 @@ internal class PythonFileManager {
         internal const int ENOENT = 2;
         internal const int EBADF = 9;
         internal const int EACCES = 13;
+        internal const int EINVAL = 22;
         internal const int EMFILE = 24;
 
         // *** END GENERATED CODE ***

Src/Scripts/generate_os_codes.py

@@ -102,7 +102,7 @@ def darwin_code_expr(codes, fmt):
 def linux_code_expr(codes, fmt):
     return fmt(codes[linux_idx])
 
-common_errno_codes = ['ENOENT', 'EBADF', 'EACCES', 'EMFILE']
+common_errno_codes = ['ENOENT', 'EBADF', 'EACCES', 'EINVAL', 'EMFILE']
 
 def generate_common_errno_codes(cw):
     for name in common_errno_codes:

Tests/test_file.py

@@ -783,6 +783,51 @@ def test_opener_uncallable(self):
 
         self.assertRaises(TypeError, open, "", "r", opener=uncallable_opener)
 
+
+    def test_seek(self):
+        with open(self.temp_file, "w") as f:
+            f.write("abc")
+            self.assertRaises(TypeError, f.buffer.seek, 1.5)
+            if is_cli:
+                self.assertRaises(TypeError, f.seek, 1.5)  # surprisingly, this doesn't raise an error in CPython
+
+        with open(self.temp_file, "rb") as f:
+            self.assertEqual(f.tell(), 0)
+            self.assertEqual(f.read(1), b"a")
+            self.assertEqual(f.tell(), 1)
+
+            f.seek(2)
+            self.assertEqual(f.tell(), 2)
+            self.assertEqual(f.read(1), b"c")
+            self.assertEqual(f.tell(), 3)
+
+            f.seek(0, os.SEEK_SET)
+            self.assertEqual(f.tell(), 0)
+            self.assertEqual(f.read(1), b"a")
+            self.assertEqual(f.tell(), 1)
+
+            f.seek(0, os.SEEK_CUR)
+            self.assertEqual(f.tell(), 1)
+            self.assertEqual(f.read(1), b"b")
+            self.assertEqual(f.tell(), 2)
+
+            f.raw.seek(2, os.SEEK_SET)
+            f.raw.seek(-2, os.SEEK_CUR)
+            self.assertEqual(f.raw.tell(), 0)
+            self.assertEqual(f.raw.read(1), b"a")
+            self.assertEqual(f.raw.tell(), 1)
+
+            f.raw.seek(-1, os.SEEK_END)
+            self.assertEqual(f.raw.tell(), 2)
+            self.assertEqual(f.raw.read(1), b"c")
+            self.assertEqual(f.raw.tell(), 3)
+
+            self.assertRaises(TypeError, f.seek, 1.5)
+            self.assertRaises(TypeError, f.raw.seek, 1.5)
+            self.assertRaises(OSError, f.raw.seek, -1)
+            self.assertRaises(OSError, f.raw.seek, 0, -1)
+
+
     def test_open_wbplus(self):
         with open(self.temp_file, "wb+") as f:
             f.write(b"abc")