wasm: move data buffer to library crate (DRY)

Signed-off-by: Xe Iaso <me@xeiaso.net>

wasm/anubis/src/lib.rs

@@ -1,5 +1,10 @@
+use std::boxed::Box;
+use std::sync::{LazyLock, Mutex};
+
 #[cfg(target_arch = "wasm32")]
 mod hostimport {
+    use crate::{DATA_BUFFER, DATA_LENGTH};
+
     #[link(wasm_import_module = "anubis")]
     unsafe extern "C" {
         /// The runtime expects this function to be defined. It is called whenever the Anubis check
@@ -13,6 +18,18 @@ mod hostimport {
             anubis_update_nonce(nonce);
         }
     }
+
+    #[unsafe(no_mangle)]
+    pub extern "C" fn data_ptr() -> *const u8 {
+        let challenge = &DATA_BUFFER;
+        challenge.as_ptr()
+    }
+
+    #[unsafe(no_mangle)]
+    pub extern "C" fn set_data_length(len: u32) {
+        let mut data_length = DATA_LENGTH.lock().unwrap();
+        *data_length = len as usize;
+    }
 }
 
 #[cfg(not(target_arch = "wasm32"))]
@@ -22,4 +39,18 @@ mod hostimport {
     }
 }
 
+/// The data buffer is a bit weird in that it doesn't have an explicit length as it can
+/// and will change depending on the challenge input that was sent by the server.
+/// However, it can only fit 4096 bytes of data (one amd64 machine page). This is
+/// slightly overkill for the purposes of an Anubis check, but it's fine to assume
+/// that the browser can afford this much ram usage.
+///
+/// Callers should fetch the base data pointer, write up to 4096 bytes, and then
+/// `set_data_length` the number of bytes they have written
+///
+/// This is also functionally a write-only buffer, so it doesn't really matter that
+/// the length of this buffer isn't exposed.
+pub static DATA_BUFFER: LazyLock<[u8; 4096]> = LazyLock::new(|| [0; 4096]);
+pub static DATA_LENGTH: LazyLock<Mutex<usize>> = LazyLock::new(|| Mutex::new(0));
+
 pub use hostimport::update_nonce;