add features for auto-release of hardware and warning on sw fallback

These features should help the crate work more seamlessly with the
existing API, at perhaps some performance penalty that is still
to be determined.

curve25519-dalek/src/backend/serial/u32e/field.rs

@@ -200,10 +200,14 @@ pub(crate) fn engine(a: &[u8; 32], b: &[u8; 32], op: EngineOp) -> Engine25519 {
                 }
             }
 
+            #[cfg(feature="auto-release")]
+            free_engine();
+
             Engine25519 { 0: result }
         }
         _ => {
             // fallback to fiat crypto field arithmetic...
+            #[cfg(feature="warn-fallback")]
             log::warn!("Hardware acceleration unavailable, falling back to software");
             let fiat_a = bytes_to_fiat(a);
             let fiat_b = bytes_to_fiat(b);

curve25519-dalek/src/montgomery.rs

@@ -471,9 +471,13 @@ impl ProjectivePoint {
                 copy_to_rf(self.U.as_bytes(), 29, rf_hw, 0);
                 copy_to_rf(self.W.as_bytes(), 30, rf_hw, 0);
 
-                MontgomeryPoint(run_job(&mut ucode_hw, &rf_hw, &mcode, 0))
+                let r = MontgomeryPoint(run_job(&mut ucode_hw, &rf_hw, &mcode, 0));
+                #[cfg(feature="auto-release")]
+                free_engine();
+                r
             }
             _ => {
+                #[cfg(feature="warn-fallback")]
                 log::warn!("Hardware acceleration unavailable, falling back to software");
                 let u = &self.U * &self.W.invert();
                 MontgomeryPoint(u.as_bytes())
@@ -651,8 +655,11 @@ pub(crate) fn differential_add_and_double(
             P.W = FieldElement::from_bytes(&copy_from_rf(21, &rf_hw, 0));
             Q.U = FieldElement::from_bytes(&copy_from_rf(22, &rf_hw, 0));
             Q.W = FieldElement::from_bytes(&copy_from_rf(23, &rf_hw, 0));
+            #[cfg(feature="auto-release")]
+            free_engine();
         }
         _ => {
+            #[cfg(feature="warn-fallback")]
             log::warn!("Hardware acceleration unavailable, falling back to software");
             let t0 = &P.U + &P.W;
             let t1 = &P.U - &P.W;
@@ -1012,9 +1019,13 @@ impl Mul<&Scalar> for &MontgomeryPoint {
                     window,
                 ); // 254 as loop counter
 
-                MontgomeryPoint(run_job(&mut ucode_hw, &rf_hw, &mcode, window))
+                let r = MontgomeryPoint(run_job(&mut ucode_hw, &rf_hw, &mcode, window));
+                #[cfg(feature="auto-release")]
+                free_engine();
+                r
             }
             _ => {
+                #[cfg(feature="warn-fallback")]
                 log::warn!("Hardware acceleration unavailable, falling back to software");
                 // We multiply by the integer representation of the given Scalar. By scalar invariant #1,
                 // the MSB is 0, so we can skip it.