Eliminated redundant checking for unknown ops.

The user must already have this checking, so we don't need it. Getting rid of
it will allow us to call beginOp before calling Convert, necessary for putting
aditional state keeping in that method.

connection.go

@@ -127,7 +127,7 @@ func (c *Connection) ReadOp() (op fuseops.Op, err error) {
 			continue
 		}
 
-		// Convert it, if possible.
+		// Convert it.
 		logForOp := func(calldepth int, format string, v ...interface{}) {
 			c.log(opID, calldepth+1, format, v...)
 		}
@@ -135,12 +135,6 @@ func (c *Connection) ReadOp() (op fuseops.Op, err error) {
 		finished := func(err error) { c.finishOp() }
 
 		op = fuseops.Convert(c.parentCtx, bfReq, logForOp, finished)
-		if op == nil {
-			c.log(opID, 1, "-> ENOSYS: %v", bfReq)
-			bfReq.RespondError(ENOSYS)
-			continue
-		}
-
 		c.beginOp()
 		return
 	}

fuseops/convert.go

@@ -25,9 +25,12 @@ import (
 // This function is an implementation detail of the fuse package, and must not
 // be called by anyone else.
 //
-// Convert the supplied bazilfuse request struct to an Op, returning nil if it
-// is unknown. finished will be called with the error supplied to o.Respond
-// when the user invokes that method.
+// Convert the supplied bazilfuse request struct to an Op. finished will be
+// called with the error supplied to o.Respond when the user invokes that
+// method.
+//
+// It is guaranteed that o != nil. If the op is unknown, a special unexported
+// type will be used.
 func Convert(
 	opCtx context.Context,
 	r bazilfuse.Request,
@@ -213,7 +216,9 @@ func Convert(
 		co = &to.commonOp
 
 	default:
-		return
+		to := &unknownOp{}
+		io = to
+		co = &to.commonOp
 	}
 
 	co.init(opCtx, io, r, logForOp, finished)

fuseops/ops.go

@@ -40,10 +40,11 @@ type Op interface {
 	Context() context.Context
 
 	// Repond to the operation with the supplied error. If there is no error, set
-	// any necessary output fields and then call Respond(nil).
+	// any necessary output fields and then call Respond(nil). The user must not
+	// call with a nil error for unrecognized ops; instead, use ENOSYS.
 	//
-	// Once this is invoked, you must exclude any further calls to any method of
-	// this op.
+	// Once this is invoked, the user must exclude any further calls to any
+	// method of this op.
 	Respond(error)
 
 	// Log information tied to this operation, with semantics equivalent to
@@ -833,3 +834,13 @@ type ReleaseFileHandleOp struct {
 func (o *ReleaseFileHandleOp) toBazilfuseResponse() (bfResp interface{}) {
 	return
 }
+
+// A sentinel used for unknown ops. The user is expected to respond with a
+// non-nil error.
+type unknownOp struct {
+	commonOp
+}
+
+func (o *unknownOp) toBazilfuseResponse() (bfResp interface{}) {
+	panic(fmt.Sprintf("Should never get here for unknown op: %s", o.ShortDesc()))
+}