Add two-step write feature to bytebuf

If there is exactly one producer, then it can assume that the remaining
space in the buffer will only increase until it write something.

This assumption may allow the producer to write to the buffer (up to a
known safe size) without any synchronization mechanism, thus allowing
to read and write different parts of the buffer in parallel.

The producer can then commit the write with lock held, and update its
knowledge of the safe empty remaining space.

app/src/util/bytebuf.c

@@ -86,3 +86,31 @@ sc_bytebuf_write(struct sc_bytebuf *buf, const uint8_t *from, size_t len) {
     }
     buf->head = (buf->head + len) % buf->alloc_size;
 }
+
+void
+sc_bytebuf_prepare_write(struct sc_bytebuf *buf, const uint8_t *from,
+                         size_t len) {
+    // *This function MUST NOT access buf->tail (even in assert()).*
+    // The purpose of this function is to allow a reader and a writer to access
+    // different parts of the buffer in parallel simultaneously. It is intended
+    // to be called without lock (only sc_bytebuf_commit_write() is intended to
+    // be called with lock held).
+
+    assert(len < buf->alloc_size - 1);
+
+    size_t right_len = buf->alloc_size - buf->head;
+    if (len < right_len) {
+        right_len = len;
+    }
+
+    memcpy(buf->data + buf->head, from, right_len);
+    if (len > right_len) {
+        memcpy(buf->data, from + right_len, len - right_len);
+    }
+}
+
+void
+sc_bytebuf_commit_write(struct sc_bytebuf *buf, size_t len) {
+    assert(len <= sc_bytebuf_write_remaining(buf));
+    buf->head = (buf->head + len) % buf->alloc_size;
+}