commit bd17c2a4e237ca1f38544db65053ecfea6054009
Author: Tomas Korbar <tkorbar@redhat.com>
Date:   Thu Sep 24 13:17:45 2020 +0200

    Skip files for transfer that has been truncated during negotiation

diff --git a/rsync.1 b/rsync.1
index 6cabd44..855dd47 100644
--- a/rsync.1
+++ b/rsync.1
@@ -1004,8 +1004,10 @@ This causes rsync to update a file by appending data onto
 the end of the file, which presumes that the data that already exists on
 the receiving side is identical with the start of the file on the sending
 side.  If a file needs to be transferred and its size on the receiver is
-the same or longer than the size on the sender, the file is skipped.  This
-does not interfere with the updating of a file\(cq\&s non\-content attributes
+the same or longer than the size on the sender, the file is skipped. It
+also skips any files whose size on the sending side gets shorter during
+the send negotiations (rsync warns about a "diminished" file when this
+happens). This does not interfere with the updating of a file\(cq\&s non\-content attributes
 (e.g. permissions, ownership, etc.) when the file does not need to be
 transferred, nor does it affect the updating of any non\-regular files.
 Implies \fB\-\-inplace\fP.
diff --git a/sender.c b/sender.c
index 1cc28a1..e22eadd 100644
--- a/sender.c
+++ b/sender.c
@@ -379,6 +379,16 @@ void send_files(int f_in, int f_out)
 			}
 		}
 
+		if (append_mode > 0 && st.st_size < F_LENGTH(file)) {
+			rprintf(FWARNING, "skipped diminished file: %s\n",
+				full_fname(fname));
+			free_sums(s);
+			close(fd);
+			if (protocol_version >= 30)
+				send_msg_int(MSG_NO_SEND, ndx);
+			continue;
+		}
+
 		if (st.st_size) {
 			int32 read_size = MAX(s->blength * 3, MAX_MAP_SIZE);
 			mbuf = map_file(fd, st.st_size, read_size, s->blength);