ustream: add function ustream_read().

It can be used to fill caller-specified buffer with data already in
ustream read buffer.  Useful in the following use pattern.

	int available = ustream_pending_data(s, false);
	if (available >= sizeof(struct msghdr)) {
		struct msghdr h;
		ustream_read(s, &h, sizeof(h));
	}

Signed-off-by: Yousong Zhou <[email protected]>

ustream.c

@@ -333,6 +333,26 @@ char *ustream_get_read_buf(struct ustream *s, int *buflen)
 	return data;
 }
 
+int ustream_read(struct ustream *s, char *buf, int buflen)
+{
+	char *chunk;
+	int chunk_len;
+	int len = 0;
+
+	do {
+		chunk = ustream_get_read_buf(s, &chunk_len);
+		if (!chunk)
+			break;
+		if (chunk_len > buflen - len)
+			chunk_len = buflen - len;
+		memcpy(buf + len, chunk, chunk_len);
+		ustream_consume(s, chunk_len);
+		len += chunk_len;
+	} while (len < buflen);
+
+	return len;
+}
+
 static void ustream_write_error(struct ustream *s)
 {
 	if (!s->write_error)

ustream.h

@@ -143,6 +143,11 @@ void ustream_free(struct ustream *s);
 /* ustream_consume: remove data from the head of the read buffer */
 void ustream_consume(struct ustream *s, int len);
 
+/*
+ * ustream_read: read and consume data in read buffer into caller-specified
+ * area.  Return length of data read.
+ */
+int ustream_read(struct ustream *s, char *buf, int buflen);
 /* ustream_write: add data to the write buffer */
 int ustream_write(struct ustream *s, const char *buf, int len, bool more);
 int ustream_printf(struct ustream *s, const char *format, ...);