tools/blktap2: Handle read/write interrupts in blktap2 control plane.

The following patch:

tools: Retry blktap2 tapdisk message on interrupt.

Addressed a long standing regression with the blktap2 control
plane.  An interruption of the select system call would
prematurely terminate the message sequence needed to properly
shutdown a blktap2 tapdisk instance.

Ian Jackson correctly noted that the read and write systems calls
responsible for receiving and sending the control messages could
also return EINTR resulting in similar effects.  While this
regression was not noted in field testing this patch adds support
to re-start the calls to provide a technically complete
implementation of control plane management in the presence of
signals.

Signed-off-by: Dr. Greg Wettstein <xen@wind.enjellic.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/blktap2/control/tap-ctl-ipc.c

@@ -71,8 +71,11 @@ tap_ctl_read_message(int fd, tapdisk_message_t *message, int timeout)
 		}
 		else if (FD_ISSET(fd, &readfds)) {
 			ret = read(fd, message + offset, len - offset);
-			if (ret <= 0)
+			if (ret <= 0) {
+				if (errno == EINTR)
+					continue;
 				break;
+			}
 			offset += ret;
 		} else
 			break;
@@ -124,8 +127,11 @@ tap_ctl_write_message(int fd, tapdisk_message_t *message, int timeout)
 		}
 		else if (FD_ISSET(fd, &writefds)) {
 			ret = write(fd, message + offset, len - offset);
-			if (ret <= 0)
+			if (ret <= 0) {
+				if (errno == EINTR)
+					continue;
 				break;
+			}
 			offset += ret;
 		} else
 			break;