fix positional reader discarding (#31593)

closes #30151
closes #30126

Reviewed-on: https://codeberg.org/ziglang/zig/pulls/31593
Reviewed-by: Ryan Liptak <squeek502@noreply.codeberg.org>
Co-authored-by: glowsquid <sachabarsayuracko@gmail.com>
Co-committed-by: glowsquid <sachabarsayuracko@gmail.com>

lib/std/Io/File/Reader.zig

@@ -309,7 +309,9 @@ fn discard(io_reader: *Io.Reader, limit: Io.Limit) Io.Reader.Error!usize {
                 return 0;
             };
             const logical_pos = logicalPos(r);
-            const delta = @min(@intFromEnum(limit), size - logical_pos);
+            const bytes_remaining = size - logical_pos;
+            if (bytes_remaining == 0) return error.EndOfStream;
+            const delta = @min(@intFromEnum(limit), bytes_remaining);
             setLogicalPos(r, logical_pos + delta);
             return delta;
         },

lib/std/Io/test.zig

@@ -85,6 +85,28 @@ test "File.Writer.seekTo" {
     try expect(fw.logicalPos() == 1234);
 }
 
+test "file discard" {
+    var tmp = tmpDir(.{});
+    defer tmp.cleanup();
+
+    const io = testing.io;
+
+    const tmp_file_name = "temp_test_file.txt";
+    var file = try tmp.dir.createFile(io, tmp_file_name, .{ .read = true });
+    defer file.close(io);
+
+    var fw = file.writerStreaming(io, &.{});
+
+    try fw.interface.writeAll("test");
+
+    var fr = file.reader(io, &.{});
+    const r = &fr.interface;
+
+    try std.testing.expectEqual(error.EndOfStream, r.discardAll(1024));
+    try fr.seekTo(0);
+    try std.testing.expectEqual(4, fr.interface.discardRemaining());
+}
+
 test "File.setLength" {
     const io = testing.io;