Initial TigerBeetle specifics strip. All 7 tests passing.

src/io.zig

@@ -0,0 +1,30 @@
+const std = @import("std");
+const builtin = @import("builtin");
+const assert = std.debug.assert;
+const os = std.os;
+
+const FIFO = @import("fifo.zig").FIFO;
+const IO_Linux = @import("io/linux.zig").IO;
+const IO_Darwin = @import("io/darwin.zig").IO;
+const IO_Windows = @import("io/windows.zig").IO;
+
+pub const IO = switch (builtin.target.os.tag) {
+    .linux => IO_Linux,
+    .windows => IO_Windows,
+    .macos, .tvos, .watchos, .ios => IO_Darwin,
+    else => @compileError("IO is not supported for platform"),
+};
+
+pub fn bufferLimit(buffer_len: usize) usize {
+    // Linux limits how much may be written in a `pwrite()/pread()` call, which is `0x7ffff000` on
+    // both 64-bit and 32-bit systems, due to using a signed C int as the return value, as well as
+    // stuffing the errno codes into the last `4096` values.
+    // Darwin limits writes to `0x7fffffff` bytes, more than that returns `EINVAL`.
+    // The corresponding POSIX limit is `std.math.maxInt(isize)`.
+    const limit = switch (builtin.target.os.tag) {
+        .linux => 0x7ffff000,
+        .macos, .ios, .watchos, .tvos => std.math.maxInt(i32),
+        else => std.math.maxInt(isize),
+    };
+    return @min(limit, buffer_len);
+}