Modify seal function to be callable from C (zcash#99)

* modify seal function to be callable from cgo

- can't pass arrays by value in C; need to pass pointer to first cell
- seal accepts pointer to result-array so that callers manage memory instead of Rust

* line reorg

* rename symbol

* apply cargo fmt

* add comments for seal parameters

* create lib crate-type for Rust tests

* dylib seems to work with cgo

* kick off ci

Cargo.toml

@@ -3,6 +3,9 @@ name = "proofs"
 version = "0.1.0"
 authors = ["dignifiedquire <[email protected]>"]
 
+[lib]
+crate-type = ["rlib", "dylib"]
+
 [dependencies]
 sapling-crypto = { git = "https://github.com/zcash-hackworks/sapling-crypto", branch = "master" }
 rand = "0.4"

src/api/mod.rs

@@ -1,14 +1,24 @@
 use libc;
 use std::ffi::{CStr, CString};
+use std::slice;
 
 type CstrT = *const libc::c_char;
 
 type SectorID = i32;
 type Commitment = u32;
 type ProverID = [u8; 31];
+type ChallengeSeed = [u32; 8];
+type RandomSeed = [u32; 8];
+
+// arrays cannot be passed by value in C; callers instead pass a pointer to the
+// head and Rust makes runtime assertions of length while marshaling
+type ProverIDPtr = *const u8;
+type ChallengeSeedPtr = *const u32;
+type RandomSeedPtr = *const u32;
+type ResultPtr = *const Commitment;
 
 type SectorAccess = CstrT;
-type SectorAccessor = extern "C" fn(i: SectorID) -> CstrT;
+type SectorAccessor = extern "C" fn(i: SectorID) -> SectorAccess;
 
 fn from_cstr(c_str: CstrT) -> String {
     unsafe {
@@ -19,21 +29,99 @@ fn from_cstr(c_str: CstrT) -> String {
     }
 }
 
+fn u32ptr_to_array8(x: *const u32) -> [u32; 8] {
+    let s = unsafe { slice::from_raw_parts(x, 8).to_owned() };
+
+    assert_eq!(
+        s.len(),
+        8,
+        "actual len(s) = {}, expected len(s) = {}",
+        s.len(),
+        8
+    );
+
+    let mut out: [u32; 8] = Default::default();
+    out.copy_from_slice(&s[0..8]);
+    out
+}
+
+fn u8ptr_to_array31(x: *const u8) -> [u8; 31] {
+    let s = unsafe { slice::from_raw_parts(x, 31).to_owned() };
+
+    assert_eq!(
+        s.len(),
+        31,
+        "actual len(s) = {}, expected len(s) = {}",
+        s.len(),
+        31
+    );
+
+    let mut out: [u8; 31] = Default::default();
+    out.copy_from_slice(&s[0..31]);
+    out
+}
+
 fn to_cstring(s: &str) -> CString {
     CString::new(s).unwrap()
 }
 
 const DUMMY_COMM_R: Commitment = 12345;
 const DUMMY_COMM_D: Commitment = 54321;
 
+/// Seals a sector.
+///
+/// # Arguments
+///
+/// * `sector_id`           - identity of the unsealed sector
+/// * `unsealed`            - function pointer used to get access to unsealed
+///                           sector
+/// * `sealed`              - function pointer used to get access to sealed
+///                           sector
+/// * `_prover_id_ptr`      - pointer to first cell in a 31-length array of u8
+/// * `_challenge_seed_ptr` - pointer to first cell in a 8-length array of u32
+/// * `_random_seed_ptr`    - pointer to first cell in a 8-length array of u32
+/// * `result_ptr`          - pointer to first cell in a 2-length array of u32,
+///                           mutated by seal in order to pass commitments back
+///                           to caller
+/// ```
 #[no_mangle]
 pub extern "C" fn seal(
+    sector_id: SectorID,
+    unsealed: SectorAccessor,
+    sealed: SectorAccessor,
+    _prover_id_ptr: ProverIDPtr,
+    _challenge_seed_ptr: ChallengeSeedPtr,
+    _random_seed_ptr: RandomSeedPtr,
+    result_ptr: ResultPtr,
+) -> () {
+    let prover_id = u8ptr_to_array31(_prover_id_ptr);
+    let challenge_seed = u32ptr_to_array8(_challenge_seed_ptr);
+    let random_seed = u32ptr_to_array8(_random_seed_ptr);
+
+    let comms = seal_internal(
+        sector_id,
+        unsealed,
+        sealed,
+        prover_id,
+        challenge_seed,
+        random_seed,
+    );
+
+    // let caller manage this memory, preventing the need for calling back into
+    // Rust code later to deallocate
+    unsafe {
+        *(result_ptr.offset(0 as isize) as *mut u32) = comms[0];
+        *(result_ptr.offset(1 as isize) as *mut u32) = comms[1];
+    };
+}
+
+fn seal_internal(
     sector_id: SectorID,
     unsealed: SectorAccessor,
     sealed: SectorAccessor,
     _prover_id: ProverID,
-    _challenge_seed: [u32; 8],
-    _random_seed: [u32; 8],
+    _challenge_seed: ChallengeSeed,
+    _random_seed: RandomSeed,
 ) -> [Commitment; 2] {
     let _in_path = from_cstr(unsealed(sector_id));
     let _out_path = from_cstr(sealed(sector_id));
@@ -73,7 +161,29 @@ mod tests {
 
     #[test]
     fn seal_verify() {
-        let comms = seal(
+        let result: [u32; 2] = [0; 2];
+        let prover_id: [u8; 31] = [1; 31];
+        let challenge_seed: [u32; 8] = [2; 8];
+        let random_seed: [u32; 8] = [3; 8];
+
+        seal(
+            123,
+            sector_accessor,
+            sector_accessor,
+            &prover_id[0],
+            &challenge_seed[0],
+            &random_seed[0],
+            &result[0],
+        );
+
+        assert_ne!(result[0], result[1]);
+        assert!(verifySeal(result[0], result[1]));
+        assert!(!verifySeal(result[1], result[0]));
+    }
+
+    #[test]
+    fn seal_internal_verify() {
+        let comms = seal_internal(
             123,
             sector_accessor,
             sector_accessor,