Memory leak when using fs.readAllAlloc multiple times

[leroycep]

Not sure if this is a bug in the LeakCountAllocator, or readAllAlloc, but the following code will leak an allocation:

test "readAllAlloc memory leak" {
    const cwd = fs.cwd();
    try cwd.writeFile("test.txt", "Hello, world!\n");
    const file = try cwd.openFile("test.txt", .{ .read = true });

    const file_contents = try file.inStream().stream.readAllAlloc(testing.allocator, 50);
    testing.allocator.free(file_contents);

    const file_contents2 = try file.inStream().stream.readAllAlloc(testing.allocator, 50);
    testing.allocator.free(file_contents2);
}

The following output is generated when using zig test:

test "readAllAlloc memory leak"...error - detected leaked allocations without matching free: 1

/home/leroycep/sources/github.com/ziglang/zig/lib/std/special/test_runner.zig:52:46: 0x243b78 in std.special.main (test)
                error.Leak => std.debug.panic("", .{}),
                                             ^
/home/leroycep/sources/github.com/ziglang/zig/lib/std/start.zig:256:37: 0x20a36b in std.start.posixCallMainAndExit (test)
            const result = root.main() catch |err| {
                                    ^
/home/leroycep/sources/github.com/ziglang/zig/lib/std/start.zig:120:5: 0x20a19f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^

Edit: I should also mention that if the second readAllAlloc is removed, no memory leak is detected.

[LemonBoy]

CC @daurnimator

This is caused by the use of a Buffer in readAllBuffer, the second call reads exactly zero bytes since the first readAllAlloc moved the seek cursor to the end and the temporary Buffer is resized to 0 + 1 bytes. This is not ok as the presence of the trailing zero is not transferred to the callee as the return is just []u8 and the underlying 1-sized slice is never freed.

Edit: There's already an open PR (#4405)