Fix bug uncovered by changing alloca to auto buffer.

* coding.c (growable_destination): New function.
(produce_chars): Use it for sanity checks.  Do not fiddle with
dst_end if the source and destination are both nil, as it's
the caller's responsibility to avoid overlap.
* keyboard.c (read_decoded_event_from_main_queue):
The destination must be MAX_MULTIBYTE_LENGTH times the max source
length, not 4 times, to prevent decode_coding_c_string from trying
to reallocate a destination.  This removes the need for the FIXME.

Fixes: debbugs:18410

3 files changed, 31 insertions, 12 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 88ab1eedca8..2654abd32d4 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,15 @@
 2014-09-07  Paul Eggert  <eggert@cs.ucla.edu>
 
+        Fix bug uncovered by changing alloca to auto buffer (Bug#18410).
+        * coding.c (growable_destination): New function.
+        (produce_chars): Use it for sanity checks.  Do not fiddle with
+        dst_end if the source and destination are both nil, as it's
+        the caller's responsibility to avoid overlap.
+        * keyboard.c (read_decoded_event_from_main_queue):
+        The destination must be MAX_MULTIBYTE_LENGTH times the max source
+        length, not 4 times, to prevent decode_coding_c_string from trying
+        to reallocate a destination.  This removes the need for the FIXME.
+
         * callproc.c (exec_failed) [DOS_NT]: Define a dummy.
         All callers simplified.  Add a comment about exec_failed, vfork,
         and alloca.
diff --git a/src/coding.c b/src/coding.c
index 84b774b4355..ed107a297ba 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -690,6 +690,14 @@ CHECK_NATNUM_CDR (Lisp_Object x)
   XSETCDR (x, tmp);
 }
 
+/* True if CODING's destination can be grown.  */
+
+static bool
+growable_destination (struct coding_system *coding)
+{
+  return STRINGP (coding->dst_object) || BUFFERP (coding->dst_object);
+}
+
 
 /* Safely get one byte from the source text pointed by SRC which ends
    at SRC_END, and set C to that byte.  If there are not enough bytes
@@ -7019,8 +7027,10 @@ produce_chars (struct coding_system *coding, Lisp_Object translation_table,
       int *buf = coding->charbuf;
       int *buf_end = buf + coding->charbuf_used;
 
-      if (EQ (coding->src_object, coding->dst_object))
+      if (EQ (coding->src_object, coding->dst_object)
+          && ! NILP (coding->dst_object))
         {
+          eassert (growable_destination (coding));
           coding_set_source (coding);
           dst_end = ((unsigned char *) coding->source) + coding->consumed;
         }
@@ -7059,6 +7069,7 @@ produce_chars (struct coding_system *coding, Lisp_Object translation_table,
 
               if ((dst_end - dst) / MAX_MULTIBYTE_LENGTH < to_nchars)
                 {
+                  eassert (growable_destination (coding));
                   if (((min (PTRDIFF_MAX, SIZE_MAX) - (buf_end - buf))
                        / MAX_MULTIBYTE_LENGTH)
                       < to_nchars)
@@ -7103,7 +7114,10 @@ produce_chars (struct coding_system *coding, Lisp_Object translation_table,
       const unsigned char *src_end = src + coding->consumed;
 
       if (EQ (coding->dst_object, coding->src_object))
-        dst_end = (unsigned char *) src;
+        {
+          eassert (growable_destination (coding));
+          dst_end = (unsigned char *) src;
+        }
       if (coding->src_multibyte != coding->dst_multibyte)
         {
           if (coding->src_multibyte)
@@ -7119,6 +7133,7 @@ produce_chars (struct coding_system *coding, Lisp_Object translation_table,
                   ONE_MORE_BYTE (c);
                   if (dst == dst_end)
                     {
+                      eassert (growable_destination (coding));
                       if (EQ (coding->src_object, coding->dst_object))
                         dst_end = (unsigned char *) src;
                       if (dst == dst_end)
@@ -7149,6 +7164,7 @@ produce_chars (struct coding_system *coding, Lisp_Object translation_table,
 
                 if (dst >= dst_end - 1)
                   {
+                    eassert (growable_destination (coding));
                     if (EQ (coding->src_object, coding->dst_object))
                       dst_end = (unsigned char *) src;
                     if (dst >= dst_end - 1)
diff --git a/src/keyboard.c b/src/keyboard.c
index eef0770684e..c435ba74faa 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -2355,22 +2355,15 @@ read_decoded_event_from_main_queue (struct timespec *end_time,
               struct coding_system *coding
                 = TERMINAL_KEYBOARD_CODING (terminal);
               unsigned char src[MAX_ENCODED_BYTES];
-              unsigned char dest[4 * sizeof src];
+              unsigned char dest[MAX_ENCODED_BYTES * MAX_MULTIBYTE_LENGTH];
               int i;
               for (i = 0; i < n; i++)
                 src[i] = XINT (events[i]);
               if (meta_key != 2)
                 for (i = 0; i < n; i++)
                   src[i] &= ~0x80;
-
-              /* FIXME: For some reason decode_coding_c_string requires a
-                 fresh output buffer each time, and reusing the old buffer can
-                 make Emacs dump core.  Avoid triggering the problem for now
-                 by allocating a new buffer each time through the loop.  */
-              bool please_fixme = true;
-              coding->destination = please_fixme ? alloca (n * 4) : dest;
-
-              coding->dst_bytes = n * 4;
+              coding->destination = dest;
+              coding->dst_bytes = sizeof dest;
               decode_coding_c_string (coding, src, n, Qnil);
               eassert (coding->produced_char <= n);
               if (coding->produced_char == 0)