Fix a hang that can happen when the sender is sending an extra file-list

and no one is reading (i.e. do advantageous reading in perform_io()).

io.c

@@ -69,6 +69,7 @@ int ignore_timeout = 0;
 int batch_fd = -1;
 int msgdone_cnt = 0;
 int forward_flist_data = 0;
+BOOL flist_receiving_enabled = False;
 
 /* Ignore an EOF error if non-zero. See whine_about_eof(). */
 int kluge_around_eof = 0;
@@ -147,6 +148,7 @@ enum festatus { FES_SUCCESS, FES_REDO, FES_NO_SEND };
 
 static flist_ndx_list redo_list, hlink_list;
 
+static void read_a_msg(void);
 static void drain_multiplex_messages(void);
 static void sleep_for_bwlimit(int bytes_written);
 
@@ -487,6 +489,19 @@ void restore_iobuf_size(xbuf *out)
 	}
 }
 
+static void slide_iobuf_in(size_t needed)
+{
+	memmove(iobuf.in.buf, iobuf.in.buf + iobuf.in.pos, iobuf.in.len);
+	if (DEBUG_GTE(IO, 4)) {
+		rprintf(FINFO,
+			"[%s] moved %ld bytes from %ld to 0 in the input buffer (size=%ld, needed=%ld).\n",
+			who_am_i(), (long)iobuf.in.len, (long)iobuf.in.pos, (long)iobuf.in.size, (long)needed);
+	}
+	if (iobuf.raw_input_ends_before)
+		iobuf.raw_input_ends_before -= iobuf.in.pos;
+	iobuf.in.pos = 0;
+}
+
 /* Perform buffered input and output until specified conditions are met.  When
  * given a "needed" read requirement, we'll return without doing any I/O if the
  * iobuf.in bytes are already available.  When reading, we'll read as many
@@ -559,19 +574,8 @@ static char *perform_io(size_t needed, int flags)
 			}
 			realloc_xbuf(&iobuf.in, new_size);
 		}
-		if (iobuf.in.size - iobuf.in.pos < needed
-		 || (iobuf.in.len < needed && iobuf.in.len < 1024
-		  && iobuf.in.size - (iobuf.in.pos + iobuf.in.len) < 1024)) {
-			memmove(iobuf.in.buf, iobuf.in.buf + iobuf.in.pos, iobuf.in.len);
-			if (DEBUG_GTE(IO, 4)) {
-				rprintf(FINFO,
-					"[%s] moved %ld bytes from %ld to 0 in the input buffer (size=%ld, needed=%ld).\n",
-					who_am_i(), (long)iobuf.in.len, (long)iobuf.in.pos, (long)iobuf.in.size, (long)needed);
-			}
-			if (iobuf.raw_input_ends_before)
-				iobuf.raw_input_ends_before -= iobuf.in.pos;
-			iobuf.in.pos = 0;
-		}
+		if (iobuf.in.size - iobuf.in.pos < needed)
+			slide_iobuf_in(needed);
 		break;
 
 	case PIO_NEED_OUTROOM:
@@ -633,6 +637,9 @@ static char *perform_io(size_t needed, int flags)
 			break;
 		}
 
+		if (iobuf.in.len < 1024 && iobuf.in.size - (iobuf.in.pos + iobuf.in.len) < 1024)
+			slide_iobuf_in(flags & PIO_NEED_INPUT ? needed : 0);
+
 		max_fd = -1;
 
 		FD_ZERO(&r_fds);
@@ -837,6 +844,15 @@ static char *perform_io(size_t needed, int flags)
 			}
 		}
 
+		/* We need to help prevent deadlock by doing what reading
+		 * we can whenever we are here trying to write. */
+		if (IN_MULTIPLEXED && !(flags & PIO_NEED_INPUT)) {
+			while (!iobuf.raw_input_ends_before && iobuf.in.len > 512)
+				read_a_msg();
+			if (flist_receiving_enabled && iobuf.in.len > 512)
+				wait_for_receiver(); /* generator only */
+		}
+
 		if (ff_forward_fd >= 0 && FD_ISSET(ff_forward_fd, &r_fds)) {
 			/* This can potentially flush all output and enable
 			 * multiplexed output, so keep this last in the loop
@@ -1570,6 +1586,7 @@ void wait_for_receiver(void)
 			}
 		} else {
 			struct file_list *flist;
+			flist_receiving_enabled = False;
 			if (DEBUG_GTE(FLIST, 2)) {
 				rprintf(FINFO, "[%s] receiving flist for dir %d\n",
 					who_am_i(), ndx);
@@ -1580,6 +1597,7 @@ void wait_for_receiver(void)
 			if (preserve_hard_links)
 				match_hard_links(flist);
 #endif
+			flist_receiving_enabled = True;
 		}
 	}
 }

main.c

@@ -73,6 +73,7 @@ extern int send_msgs_to_gen;
 extern pid_t cleanup_child_pid;
 extern size_t bwlimit_writemax;
 extern unsigned int module_dirlen;
+extern BOOL flist_receiving_enabled;
 extern BOOL shutting_down;
 extern struct stats stats;
 extern char *stdout_format;
@@ -898,6 +899,7 @@ static int do_recv(int f_in, int f_out, char *local_name)
 	}
 
 	am_generator = 1;
+	flist_receiving_enabled = True;
 
 	io_end_multiplex_in(MPLX_SWITCHING);
 	if (write_batch && !am_server)