Add solverrpc.StartSolver function

This allows callers to start the solver background process prior to the first
call to Roots, thereby detecting any issues earlier (e.g. csppsolver process not
found) before they have already begun a mixing session.

solverrpc/rpc.go

@@ -70,6 +70,13 @@ func startSolver() {
 	})
 }
 
+// StartSolver starts the solver's background process.  This can be used to
+// detect errors starting the solver process before the first call to Roots.
+func StartSolver() error {
+	once.Do(startSolver)
+	return onceErr
+}
+
 type repeatedRoot big.Int
 
 func (r *repeatedRoot) Error() string          { return "repeated roots" }
@@ -78,9 +85,8 @@ func (r *repeatedRoot) RepeatedRoot() *big.Int { return (*big.Int)(r) }
 // Roots solves for len(a)-1 roots of the polynomial with coefficients a (mod F).
 // Repeated roots are considered an error for the purposes of unique slot assignment.
 func Roots(a []*big.Int, F *big.Int) ([]*big.Int, error) {
-	once.Do(startSolver)
-	if onceErr != nil {
-		return nil, onceErr
+	if err := StartSolver(); err != nil {
+		return nil, err
 	}
 
 	var args struct {