Add max-sessions parameter on pkcs11 kms

This commit adds the max-sessions parameter to limit the maximum number
of open sessions that we will keep with the HSM before reusing
a previous one. It defaults to the crypto11 default, 1024.

kms/pkcs11/pkcs11.go

@@ -59,12 +59,14 @@ type PKCS11 struct {
 //   - pkcs11:serial=1a2b3c4d5e6f?pin-source=/path/to/pin.txt
 //   - pkcs11:slot-id=5?pin-value=password
 //   - pkcs11:module-path=/path/to/module.so;token=smallstep?pin-value=password
+//   - pkcs11:token=smallstep;max-sessions=100?pin-value=password
 //
 // The scheme is "pkcs11"; "token", "serial", or "slot-id" defines the
 // cryptographic device to use. "module-path" is the path of the PKCS#11 module
 // to use. It will default to the proxy module of the p11-kit project if none is
 // specified (p11-kit-proxy.so). "pin-value" provides the user's PIN, and
-// "pin-source" defines a file that contains the PIN.
+// "pin-source" defines a file that contains the PIN. "max-sessions" defines the
+// maximum number of PKCS#11 sessions, it defaults to 1024.
 //
 // A cryptographic key or object is identified by its "id" or "object"
 // attributes. The "id" is the key identifier for the object, it's a hexadecimal
@@ -96,6 +98,13 @@ func New(_ context.Context, opts apiv1.Options) (*PKCS11, error) {
 		}
 		config.SlotNumber = &n
 	}
+	if v := u.Get("max-sessions"); v != "" {
+		n, err := strconv.Atoi(v)
+		if err != nil {
+			return nil, errors.Wrap(err, "kms uri 'max-sessions' is not valid")
+		}
+		config.MaxSessions = n
+	}
 
 	// Get module or default to use p11-kit-proxy.so.
 	//

kms/pkcs11/pkcs11_test.go

@@ -64,6 +64,10 @@ func TestNew(t *testing.T) {
 			Type: "pkcs11",
 			URI:  "pkcs11:module-path=/usr/local/lib/softhsm/libsofthsm2.so;slot-id=0?pin-value=password",
 		}}, k, false},
+		{"ok with max-sessions", args{context.Background(), apiv1.Options{
+			Type: "pkcs11",
+			URI:  "pkcs11:module-path=/usr/local/lib/softhsm/libsofthsm2.so;token=pkcs11-test;max-sessions=100?pin-value=password",
+		}}, k, false},
 		{"ok with pin", args{context.Background(), apiv1.Options{
 			Type: "pkcs11",
 			URI:  "pkcs11:module-path=/usr/local/lib/softhsm/libsofthsm2.so;token=pkcs11-test",
@@ -110,6 +114,10 @@ func TestNew(t *testing.T) {
 			Type: "pkcs11",
 			URI:  "pkcs11:module-path=/usr/local/lib/softhsm/libsofthsm2.so;slot-id=x?pin-value=password",
 		}}, nil, true},
+		{"fail max-sessions", args{context.Background(), apiv1.Options{
+			Type: "pkcs11",
+			URI:  "pkcs11:module-path=/usr/local/lib/softhsm/libsofthsm2.so;token=pkcs11-test;max-sessions=0F?pin-value=password",
+		}}, nil, true},
 		{"fail scheme", args{context.Background(), apiv1.Options{
 			Type: "pkcs11",
 			URI:  "foo:module-path=/usr/local/lib/softhsm/libsofthsm2.so;token=pkcs11-test?pin-value=password",