insert-file-contents file end EOF fixes

* src/fileio.c (Finsert_file_contents):
When counting bytes at file end, don’t trust lseek to seek
to the end of the file.  Instead, read a bit after lseeking,
to make sure we get to the end, give up if we don’t get to EOF.
This works around problems on /proc filesystems
where lseek pretends the file is empty.
Update file_size_hint when reaching EOF.

1 files changed, 33 insertions, 5 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 3c371dea98d..b7b4320d73e 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4488,13 +4488,39 @@ by calling `format-decode', which see.  */)
       /* Count how many chars at the end of the file
          match the text at the end of the buffer.  But, if we have
          already found that decoding is necessary, don't waste time.  */
+
+      off_t endpos;
+      if (!giveup_match_end)
+        {
+          endpos = end_offset;
+          if (endpos == TYPE_MAXIMUM (off_t))
+            {
+              endpos = emacs_fd_lseek (fd, 0, SEEK_END);
+              giveup_match_end = endpos < 0;
+              if (giveup_match_end)
+                seekable = false;
+              else
+                {
+                  /* Check that read reports EOF soon, to catch platforms
+                     where SEEK_END can report wildly small offsets.  */
+                  ptrdiff_t n = emacs_fd_read (fd, read_buf, sizeof read_buf);
+                  if (n < 0)
+                    report_file_error ("Read error", orig_filename);
+                  endpos += n;
+                  giveup_match_end = 0 < n;
+                  if (!giveup_match_end)
+                    file_size_hint = endpos;
+                }
+            }
+        }
+
       while (!giveup_match_end)
         {
           int total_read, nread, bufpos, trial;
           off_t curpos;
 
           /* At what file position are we now scanning?  */
-          curpos = end_offset - (ZV_BYTE - same_at_end);
+          curpos = endpos - (ZV_BYTE - same_at_end);
           /* If the entire file matches the buffer tail, stop the scan.  */
           if (curpos == 0)
             break;
@@ -4511,7 +4537,10 @@ by calling `format-decode', which see.  */)
               if (nread < 0)
                 report_file_error ("Read error", orig_filename);
               else if (nread == 0)
-                break;
+                {
+                  file_size_hint = curpos - trial + total_read;
+                  break;
+                }
               total_read += nread;
             }
 
@@ -4567,15 +4596,14 @@ by calling `format-decode', which see.  */)
           /* Don't try to reuse the same piece of text twice.  */
           overlap = (same_at_start - BEGV_BYTE
                      - (same_at_end - ZV_BYTE
-                        + (end_offset < TYPE_MAXIMUM (off_t)
-                           ? end_offset : file_size_hint)));
+                        + endpos));
           if (overlap > 0)
             same_at_end += overlap;
           same_at_end_charpos = BYTE_TO_CHAR (same_at_end);
 
           /* Arrange to read only the nonmatching middle part of the file.  */
           beg_offset += same_at_start - BEGV_BYTE;
-          end_offset -= ZV_BYTE - same_at_end;
+          end_offset = endpos - (ZV_BYTE - same_at_end);
 
           if (!NILP (visit) && BEG == BEGV && Z == ZV)
             /* This binding is to avoid ask-user-about-supersession-threat