[lib/cbuf] [app/tests] fix off by one in lib/cbuf. add a test.

Previously, if tail was == 0, and we wrote exactly enough bytes to
the end of the buffer, then head would end up at 0 as well. This
would make the buffer instaneously empty, as head == tail.

lib/cbuf/cbuf.c

@@ -84,8 +84,19 @@ size_t cbuf_write(cbuf_t *cbuf, const void *_buf, size_t len, bool canreschedule
 
     while (pos < len && cbuf_space_avail(cbuf) > 0) {
         if (cbuf->head >= cbuf->tail) {
-            write_len = MIN(valpow2(cbuf->len_pow2) - cbuf->head, len - pos);
+            if (cbuf->tail == 0) {
+                // Special case - if tail is at position 0, we can't write all
+                // the way to the end of the buffer. Otherwise, head ends up at
+                // 0, head == tail, and buffer is considered "empty" again.
+                write_len =
+                    MIN(valpow2(cbuf->len_pow2) - cbuf->head - 1, len - pos);
+            } else {
+                // Write to the end of the buffer.
+                write_len =
+                    MIN(valpow2(cbuf->len_pow2) - cbuf->head, len - pos);
+            }
         } else {
+            // Write from head to tail-1.
             write_len = MIN(cbuf->tail - cbuf->head - 1, len - pos);
         }
 
@@ -260,4 +271,3 @@ retry:
 
     return ret;
 }
-

lib/cbuf/include/lib/cbuf.h

@@ -47,6 +47,7 @@ typedef struct cbuf {
  *
  * @param[in] cbuf A pointer to the cbuf structure to allocate.
  * @param[in] len The minimum number of bytes for the underlying data buffer.
+ * Must be a power of two.
  */
 void cbuf_initialize(cbuf_t *cbuf, size_t len);
 
@@ -56,7 +57,8 @@ void cbuf_initialize(cbuf_t *cbuf, size_t len);
  * Initialize a cbuf structure using the supplied buffer for internal storage.
  *
  * @param[in] cbuf A pointer to the cbuf structure to allocate.
- * @param[in] len The size of the supplied buffer, in bytes.
+ * @param[in] len The size of the supplied buffer, in bytes.  Must be a power
+ * of two.
  * @param[in] buf A pointer to the memory to be used for internal storage.
  */
 void cbuf_initialize_etc(cbuf_t *cbuf, size_t len, void *buf);