std.Io.Reader: use readVec for fill functions

[andrewrk]

addresses #24695, as well as just being a better, simpler implementation for these functions now that readVec is back in the vtable.

[mrjbq7]

I believe the panic.z and fail_read.z cases both still fail, but with this error trace:

const std = @import("std");
const flate = std.compress.flate;

pub fn main() !void {
    const input_file = try std.fs.cwd().openFile("panic.z", .{});
    var input_buffer: [4096]u8 = undefined;
    var input_reader = input_file.reader(&input_buffer);

    var buffer: [flate.max_window_len]u8 = undefined;
    var decompress: flate.Decompress = .init(&input_reader.interface, .zlib, &buffer);

    // should decompress to 4160 bytes
    var out_buffer: [4160]u8 = undefined;
    var out_writer: std.Io.Writer = .fixed(&out_buffer);

    const written = try decompress.reader.streamRemaining(&out_writer);
    std.debug.print("decompressed {} bytes\n", .{written});
}

fails with

thread 17129927 panic: reached unreachable code
zig/build/stage3/lib/zig/std/debug.zig:559:14: 0x100be3e07 in assert (min)
    if (!ok) unreachable; // assertion failure
             ^
zig/build/stage3/lib/zig/std/Io/Reader.zig:1840:11: 0x100c7d90f in writableVectorPosix (min)
    assert(r.seek == r.end);
          ^
zig/build/stage3/lib/zig/std/fs/File.zig:1327:82: 0x100c7b0cf in readVec (min)
                const dest_n, const data_size = try io_reader.writableVectorPosix(&iovecs_buffer, data);
                                                                                 ^
zig/build/stage3/lib/zig/std/Io/Reader.zig:1064:56: 0x100c956c3 in fillUnbuffered (min)
    while (r.end < r.seek + n) _ = try r.vtable.readVec(r, &bufs);
                                                       ^
zig/build/stage3/lib/zig/std/Io/Reader.zig:1050:26: 0x100c953fb in fill (min)
    return fillUnbuffered(r, n);
                         ^
zig/build/stage3/lib/zig/std/Io/Reader.zig:489:15: 0x100c94ac3 in peek (min)
    try r.fill(n);
              ^
zig/build/stage3/lib/zig/std/Io/Reader.zig:570:23: 0x100c8cb37 in peekArray__anon_25952 (min)
    return (try r.peek(n))[0..n];
                      ^
zig/build/stage3/lib/zig/std/Io/Reader.zig:1117:46: 0x100c9081b in peekBits__anon_26138 (min)
    return std.mem.readInt(T, try r.peekArray(n), endian);
                                             ^
zig/build/stage3/lib/zig/std/compress/flate/Decompress.zig:610:27: 0x100c90b4f in takeBitsRuntime (min)
    const x = try peekBits(d, u16);
                          ^
zig/build/stage3/lib/zig/std/compress/flate/Decompress.zig:184:43: 0x100c90da3 in decodeLength (min)
        ml.base + try self.takeBitsRuntime(ml.extra_bits);
                                          ^
zig/build/stage3/lib/zig/std/compress/flate/Decompress.zig:423:54: 0x100c87d2b in streamInner (min)
                    const length = try d.decodeLength(sym.symbol);
                                                     ^
zig/build/stage3/lib/zig/std/compress/flate/Decompress.zig:248:23: 0x100c85157 in streamFallible (min)
    return streamInner(d, w, limit) catch |err| switch (err) {
                      ^
zig/build/stage3/lib/zig/std/compress/flate/Decompress.zig:171:23: 0x100c85afb in streamIndirectInner (min)
    _ = streamFallible(d, &writer, .limited(writer.buffer.len - writer.end)) catch |err| switch (err) {
                      ^
zig/build/stage3/lib/zig/std/compress/flate/Decompress.zig:244:31: 0x100c95947 in streamIndirect (min)
    return streamIndirectInner(d);
                              ^
zig/build/stage3/lib/zig/std/Io/Reader.zig:180:34: 0x100bf6a7b in stream (min)
    const n = try r.vtable.stream(r, w, limit);
                                 ^
zig/build/stage3/lib/zig/std/Io/Reader.zig:259:27: 0x100c754c7 in streamRemaining (min)
        offset += r.stream(w, .unlimited) catch |err| switch (err) {
                          ^
min.zig:25:58: 0x100c750df in main (min)
    const written = try decompress.reader.streamRemaining(&out_writer);
                                                         ^
zig/build/stage3/lib/zig/std/start.zig:627:37: 0x100c758ff in main (min)
            const result = root.main() catch |err| {
                                    ^
???:?:?: 0x19ac72b97 in ??? (???)
???:?:?: 0x0 in ??? (???)
[1]    56574 abort      zig/build/stage3/bin/zig run min.zig
``

[andrewrk]

@mrjbq7 I believe you ran that before I had this branch even working locally. I tried your example after latest push and it prints

decompressed 4160 bytes

[mrjbq7]

@mrjbq7 I believe you ran that before I had this branch even working locally. I tried your example after latest push and it prints

Sorry, I didn't realize it was WIP.

Can confirm this branch fixes both panic.z and fail_read.z!

[andrewrk]

Thanks for checking!