1 files changed, 31 insertions, 5 deletions

diff --git a/src/fileio.c b/src/fileio.c
index cbdfeacf8e4..5d383a7b44d 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4289,12 +4289,38 @@ by calling `format-decode', which see.  */)
                 {
                   off_t tailoff = emacs_fd_lseek (fd, - 3 * 1024, SEEK_END);
                   if (tailoff < 0)
-                    seekable = false;
-                  else if (1024 < tailoff)
                     {
-                      ptrdiff_t ntail = emacs_fd_read (fd, read_buf + 1024,
-                                                       3 * 1024);
-                      nread = ntail < 0 ? ntail : 1024 + ntail;
+                      seekable = false;
+                      tailoff = nread;
+                    }
+
+                  /* When appending the last 3 KiB, read until EOF
+                     without trusting tailoff, as the file may be in
+                     /proc or be mutating.  */
+                  nread = 1024;
+                  for (;;)
+                    {
+                      ptrdiff_t r = emacs_fd_read (fd, read_buf + nread,
+                                                   sizeof read_buf - nread);
+                      if (r <= 0)
+                        {
+                          if (r < 0)
+                            nread = r;
+                          else
+                            file_size_hint = tailoff;
+                          break;
+                        }
+                      tailoff += r;
+                      nread += r;
+                      bool eof = nread < sizeof read_buf;
+                      if (4 * 1024 < nread)
+                        {
+                          memmove (read_buf + 1024,
+                                   read_buf + nread - 3 * 1024, 3 * 1024);
+                          nread = 4 * 1024;
+                        }
+                      if (eof)
+                        break;
                     }
                 }