fold: don't truncate multibyte characters at the end of the buffer

* src/fold.c (fold_file): Replace invalid characters with the original
byte read. Copy multibyte sequences that may not yet be read to the
start of the buffer before reading more bytes.
* tests/fold/fold-characters.sh: Add a test case.

src/fold.c

@@ -139,6 +139,7 @@ fold_file (char const *filename, size_t width)
   idx_t offset_out = 0;		/* Index in 'line_out' for next char. */
   static char line_out[IO_BUFSIZE];
   static char line_in[IO_BUFSIZE];
+  static size_t offset_in = 0;
   static size_t length_in = 0;
   int saved_errno;
 
@@ -158,14 +159,30 @@ fold_file (char const *filename, size_t width)
 
   fadvise (istream, FADVISE_SEQUENTIAL);
 
-  while (0 < (length_in = fread (line_in, 1, sizeof line_in, istream)))
+  while (0 < (length_in = fread (line_in + offset_in, 1,
+                                 sizeof line_in - offset_in, istream)))
     {
       char *p = line_in;
-      char *lim = p + length_in;
+      char *lim = p + length_in + offset_in;
       mcel_t g;
       for (; p < lim; p += g.len)
         {
           g = mcel_scan (p, lim);
+          if (g.err)
+            {
+              /* Replace the character with the byte if it cannot be a
+                 truncated multibyte sequence.  */
+              if (!(lim - p <= MCEL_LEN_MAX))
+                g.ch = p[0];
+              else
+                {
+                  /* It may be a truncated multibyte sequence.  Move it to the
+                     front of the input buffer.  */
+                  memmove (line_in, p, lim - p);
+                  offset_in = lim - p;
+                  goto next_line;
+                }
+            }
           if (g.ch == '\n')
             {
               memcpy (line_out + offset_out, p, g.len);
@@ -241,6 +258,9 @@ fold_file (char const *filename, size_t width)
         }
       if (feof (istream))
         break;
+      /* We read a full buffer of complete characters.  */
+      offset_in = 0;
+    next_line:
     }
 
   saved_errno = errno;

tests/fold/fold-characters.sh

@@ -58,6 +58,25 @@ compare column-exp2 column-out2 || fail=1
 fold --characters -w 10 input2 > character-out2 || fail=1
 compare character-exp2 character-out2 || fail=1
 
+# Test a Unicode character on the edge of the input buffer.
+# Keep in sync with IO_BUFSIZE - 1.
+yes a | head -n 262143 | tr -d '\n' > input3 || framework_failure_
+env printf '\uB250' >> input3 || framework_failure_
+yes a | head -n 100 | tr -d '\n' >> input3 || framework_failure_
+env printf '\n' >> input3 || framework_failure_
+
+yes a | head -n 80 | tr -d '\n' > exp3 || framework_failure_
+env printf '\n' >> exp3 || framework_failure_
+yes a | head -n 63 | tr -d '\n' >> exp3 || framework_failure_
+env printf '\uB250' >> exp3 || framework_failure_
+yes a | head -n 16 | tr -d '\n' >> exp3 || framework_failure_
+env printf '\n' >> exp3 || framework_failure_
+yes a | head -n 80 | tr -d '\n' >> exp3 || framework_failure_
+env printf '\naaaa\n' >> exp3 || framework_failure_
+
+fold --characters input3 | tail -n 4 > out3 || fail=1
+compare exp3 out3 || fail=1
+
 # Ensure bounded memory operation
 vm=$(get_min_ulimit_v_ fold /dev/null) && {
   yes | tr -d '\n' | (ulimit -v $(($vm+8000)) && fold 2>err) | head || fail=1