buf: bug: empty fill pattern.

* buf.c (prepare_pattern): Do not return a zero length pattern
for an empty buffer. Callers don't deal with this properly.
Return a one byte pattern that is zero.

* test/012/buf.tl: Test case added.

* txr.1: Documented.

3 files changed, 15 insertions, 2 deletions

diff --git a/buf.c b/buf.c
index 4a35bc8b..3f7b7738 100644
--- a/buf.c
+++ b/buf.c
@@ -133,8 +133,15 @@ static void prepare_pattern(mem_t **ppat, mem_t **pfree, mem_t pbuf[SIZEOF_PTR],
   case BUF:
     {
       struct buf *pb = buf_handle(pat, self);
-      *ppat = pb->data;
-      *plen = c_unum(pb->len, self);
+      ucnum l = c_unum(pb->len, self);
+
+      if (l > 0) {
+        *ppat = pb->data;
+        *plen = l;
+      } else {
+        *ppat = pbuf;
+        *plen = 1;
+      }
     }
     break;
   default:
diff --git a/tests/012/buf.tl b/tests/012/buf.tl
index 082e0d9f..22640eac 100644
--- a/tests/012/buf.tl
+++ b/tests/012/buf.tl
@@ -611,3 +611,5 @@
     (buf-set-length b 3 #xdeadbeef) 2 b #b'ffefef'
     (buf-set-length b 4 #x-deadbeef) 3 b #b'ffefefef'
     (buf-set-length b 7 #x-deadbeef) 4 b #b'ffefefefdeadbe'))
+
+(test (make-buf 16 #b'') #b'00000000000000000000000000000000')
diff --git a/txr.1 b/txr.1
index 10d77642..daa390c2 100644
--- a/txr.1
+++ b/txr.1
@@ -29015,6 +29015,10 @@ which doesn't fit into the buffer length is truncated
 on the opposite side of the alignment mode: truncated
 on the left under right alignment and vice versa.
 
+An empty buffer
+.meta pat
+specifies one byte pattern of zero.
+
 .coNP Function @ make-buf
 .synb
 .mets (make-buf < len >> [ init-pat <> [ alloc-size ]])