Do not use `fcopyfile` if appending to non-empty file [Bug #18388]

`fcopyfile` appends `src` to `to` and then truncates `to` to it's
original size.

1 files changed, 7 insertions, 0 deletions

diff --git a/io.c b/io.c
index b2ca5a69e3..bc64938648 100644
--- a/io.c
+++ b/io.c
@@ -11483,6 +11483,13 @@ nogvl_fcopyfile(struct copy_stream_struct *stp)
         return 0;
     if (lseek(stp->dst_fptr->fd, 0, SEEK_CUR) > (off_t)0) /* if dst IO was already written */
         return 0;
+    if (fcntl(stp->dst_fptr->fd, F_GETFL) & O_APPEND) {
+        /* fcopyfile(3) appends src IO to dst IO and then truncates
+         * dst IO to src IO's original size. */
+        off_t end = lseek(stp->dst_fptr->fd, 0, SEEK_END);
+        lseek(stp->dst_fptr->fd, 0, SEEK_SET);
+        if (end > (off_t)0) return 0;
+    }
 
     if (src_offset > (off_t)0) {
         off_t r;