digest: panic on unavailable hash algorithm

After running into a few nil pointer errors during development, it is clear
that having this function return nil when a hash is not available is the wrong
approach. Nearly every time, this lack of availability was due to a missing
import statement for the hash. This is always a programming error.

To avoid future confusion, we now appropriately panic when the hash function is
not imported by the application. More dynamic uses of the package should call
Algorithm.Available() before calling Algorithm.Hash() to avoid this panic.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

digest/digester.go

@@ -2,6 +2,7 @@ package digest
 
 import (
 	"crypto"
+	"fmt"
 	"hash"
 	"io"
 )
@@ -84,11 +85,18 @@ func (a Algorithm) New() Digester {
 	}
 }
 
-// Hash returns a new hash as used by the algorithm. If not available, nil is
-// returned. Make sure to check Available before calling.
+// Hash returns a new hash as used by the algorithm. If not available, the
+// method will panic. Check Algorithm.Available() before calling.
 func (a Algorithm) Hash() hash.Hash {
 	if !a.Available() {
-		return nil
+		// NOTE(stevvooe): A missing hash is usually a programming error that
+		// must be resolved at compile time. We don't import in the digest
+		// package to allow users to choose their hash implementation (such as
+		// when using stevvooe/resumable or a hardware accelerated package).
+		//
+		// Applications that may want to resolve the hash at runtime should
+		// call Algorithm.Available before call Algorithm.Hash().
+		panic(fmt.Sprintf("%v not available (make sure it is imported)", a))
 	}
 
 	return algorithms[a].New()