Detach from device, if it doesn't speak IPP over USB protocol (see #52)

    Some devices (so far, only HP-rebranded Samsung devices
    known to have such a defect) offer 7/1/4 interfaces, but
    actually provide no functionality behind these interfaces
    and respond with `HTTP 404 Not found` to all the HTTP
    requests sent to USB

    ipp-usb ignores such devices to let legacy/proprietary
    drivers a chance to work with them

Author: alexpevzner

device.go

@@ -131,6 +131,21 @@ func NewDevice(desc UsbDeviceDesc) (*Device, error) {
 		}
 	}
 
+	// Skip the device, if it cannot do something useful
+	//
+	// Some devices (so far, only HP-rebranded Samsung devices
+	// known to have such a defect) offer 7/1/4 interfaces, but
+	// actually provide no functionality behind these interfaces
+	// and respond with `HTTP 404 Not found` to all the HTTP
+	// requests sent to USB
+	//
+	// ipp-usb ignores such devices to let a chance for
+	// legacy/proprietary drivers to work with them
+	if len(dnssdServices) == 0 {
+		err = ErrUnusable
+		goto ERROR
+	}
+
 	// Advertise Web service. Assume it always exists
 	dnssdServices.Add(DNSSdSvcInfo{Type: "_http._tcp", Port: dev.State.HTTPPort})
 

err.go

@@ -18,4 +18,5 @@ var (
 	ErrShutdown     = errors.New("Shutdown requested")
 	ErrBlackListed  = errors.New("Device is blacklisted")
 	ErrInitTimedOut = errors.New("Device initialization timed out")
+	ErrUnusable     = errors.New("Device doesn't implement print or scan service")
 )

pnp.go

@@ -26,7 +26,13 @@ const (
 )
 
 // pnpRetryTime returns time of next retry of failed device initialization
-func pnpRetryTime() time.Time {
+func pnpRetryTime(err error) time.Time {
+	if err == ErrBlackListed || err == ErrUnusable {
+		// These errors are unrecoverable.
+		// Forget about device for the next million hours :-)
+		return time.Now().Add(time.Hour * 1e6)
+	}
+
 	return time.Now().Add(DevInitRetryInterval)
 }
 
@@ -74,7 +80,7 @@ loop:
 					devByAddr[addr] = dev
 				} else {
 					Log.Error('!', "PNP %s: %s", addr, err)
-					retryByAddr[addr] = pnpRetryTime()
+					retryByAddr[addr] = pnpRetryTime(err)
 				}
 			}
 
@@ -103,7 +109,7 @@ loop:
 					delete(retryByAddr, addr)
 				} else {
 					Log.Error('!', "PNP %s: %s", addr, err)
-					retryByAddr[addr] = pnpRetryTime()
+					retryByAddr[addr] = pnpRetryTime(err)
 				}
 			}
 		}