std.fs.File.Reader: fix wrong calculation when reading

lib/std/fs/File.zig

@@ -1083,7 +1083,7 @@ pub const Reader = struct {
                     return error.EndOfStream;
                 }
                 r.pos += n;
-                return w.advanceVector(n);
+                return n;
             },
             .streaming_reading => {
                 if (is_windows) {
@@ -1106,7 +1106,7 @@ pub const Reader = struct {
                     return error.EndOfStream;
                 }
                 r.pos += n;
-                return w.advanceVector(n);
+                return n;
             },
             .failure => return error.ReadFailed,
         }

lib/std/http.zig

@@ -701,9 +701,13 @@ pub const Reader = struct {
         var hp: HeadParser = .{ .state = .seen_rn };
         var trailers_len: usize = 2;
         while (true) {
-            if (trailers_len >= in.buffer.len) return error.HttpHeadersOversize;
-            try in.fill(trailers_len + 1);
-            trailers_len += hp.feed(in.buffered()[trailers_len..]);
+            if (in.buffer.len - trailers_len == 0) return error.HttpHeadersOversize;
+            const remaining = in.buffered()[trailers_len..];
+            if (remaining.len == 0) {
+                try in.fillMore();
+                continue;
+            }
+            trailers_len += hp.feed(remaining);
             if (hp.state == .finished) {
                 reader.state = .ready;
                 reader.trailers = in.buffered()[0..trailers_len];

lib/std/io/Reader.zig

@@ -942,7 +942,7 @@ pub fn fill(r: *Reader, n: usize) Error!void {
         writer.end = r.end;
         r.end += r.vtable.stream(r, &writer, .limited(r.buffer.len - r.end)) catch |err| switch (err) {
             error.WriteFailed => unreachable,
-            else => |e| return e,
+            error.ReadFailed, error.EndOfStream => |e| return e,
         };
     }
 }
@@ -1168,9 +1168,9 @@ fn takeMultipleOf7Leb128(r: *Reader, comptime Result: type) TakeLeb128Error!Resu
 
 /// Left-aligns data such that `r.seek` becomes zero.
 pub fn rebase(r: *Reader) void {
+    if (r.seek == 0) return;
     const data = r.buffer[r.seek..r.end];
-    const dest = r.buffer[0..data.len];
-    std.mem.copyForwards(u8, dest, data);
+    @memmove(r.buffer[0..data.len], data);
     r.seek = 0;
     r.end = data.len;
 }

lib/std/net.zig

@@ -1943,7 +1943,7 @@ pub const Stream = struct {
                     else => |err| return windows.unexpectedWSAError(err),
                 };
                 if (n == 0) return error.EndOfStream;
-                return io_w.advanceVector(n);
+                return n;
             }
         },
         else => struct {