Don't log out player when update aborts a command with pthreads

pthread.c's empth_select() returned -1 when empth_wakeup() interrupted
select().  The failure then got propagated all the way up, and the
player got logged out.  Fix by returning 0 in that case.  While there,
retry on EINTR, to match LWP.  Also clarify comments.

include/empthread.h

@@ -160,8 +160,8 @@ void empth_yield(void);
  * If FLAGS & EMPTH_FD_WRITE, wake up if FD is ready for output.
  * At most one thread may sleep on the same file descriptor.
  * TIMEOUT, if non-null, limits the sleep time.
- * Return one when the FD is ready, zero on timeout, -1 on error with
+ * Return one when the FD is ready, zero on timeout or early wakeup by
- * errno set.
+ * empth_wakeup(), -1 on error with errno set.
  * Note: Currently, Empire sleeps only on network I/O, i.e. FD is a
  * socket.  Implementations should not rely on that.
  */
@@ -169,7 +169,7 @@ int empth_select(int fd, int flags, struct timeval *timeout);
 
 /*
  * Awaken THREAD if it is sleeping in empth_select() or empth_sleep().
- * Note: This must not awaken threads sleeping in other functions.
+ * This does not awaken threads sleeping in other functions.
  * Does not yield the processor.
  */
 void empth_wakeup(empth_t *thread);

src/lib/empthread/io.c

@@ -110,11 +110,11 @@ io_close(struct iop *iop)
 }
 
 /*
- * Return number of bytes read on success, zero on timeout or EOF, -1
+ * Return number of bytes read on success, zero on timeout, early
- * on error, with errno set appropriately.  In particular, return -1
+ * wakeup or EOF, -1 on error, with errno set appropriately.  In
- * with errno set to EAGAIN or EWOULDBLOCK when no data is available
+ * particular, return -1 with errno set to EAGAIN or EWOULDBLOCK when
- * for non-blocking input (WAITFORINPUT false).  Use io_eof() to
+ * no data is available for non-blocking input (WAITFORINPUT false).
- * distinguish timeout from EOF.
+ * Use io_eof() to distinguish timeout and early wakeup from EOF.
  */
 int
 io_input(struct iop *iop, int waitforinput)

src/lib/empthread/pthread.c

@@ -280,11 +280,13 @@ empth_select(int fd, int flags, struct timeval *timeout)
     fd_set writemask;
     struct timeval tv;
     int n;
+    empth_t *ctx;
     int res = 0;
 
     pthread_mutex_unlock(&mtx_ctxsw);
     empth_status("select on %d for %d", fd, flags);
 
+again:
     FD_ZERO(&readmask);
     FD_ZERO(&writemask);
     if (flags & EMPTH_FD_READ)
@@ -292,18 +294,21 @@ empth_select(int fd, int flags, struct timeval *timeout)
     if (flags & EMPTH_FD_WRITE)
 	FD_SET(fd, &writemask);
 
-    if (timeout) {
+    if (timeout)
 	tv = *timeout;
-	timeout = &tv;
+    n = select(fd + 1, &readmask, &writemask, NULL, timeout ? &tv : NULL);
-    }
-    n = select(fd + 1, &readmask, &writemask, NULL, timeout);
-
     if (n < 0) {
-	if (errno == EINTR) /* go handle the signal */
+	ctx = pthread_getspecific(ctx_key);
+	if (ctx->wakeup) {
+	    empth_status("select woken up");
+	    res = 0;
+	} else if (errno == EINTR) {
 	    empth_status("select broken by signal");
-	 else
+	    goto again;
+	} else {
 	    empth_status("select failed (%s)", strerror(errno));
-	res = -1;
+	    res = -1;
+	}
     } else if (n == 0) {
 	empth_status("select timed out");
 	res = 0;