membuf: Add a function to set up a static membuf

Add a way to set up a membuf with some pre-loaded data, so it is
possible to read it out using membuf_readline(), etc.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/membuf.h

@@ -222,7 +222,9 @@ int membuf_readline(struct membuf *mb, char *str, int maxlen, int minch,
 int membuf_extend_by(struct membuf *mb, int by, int max);
 
 /**
- * membuf_init() - set up a new membuff using an existing membuff
+ * membuf_init() - set up a new membuff using an existing buffer
+ *
+ * The buffer is initially empty
  *
  * @mb: membuff to set up
  * @buff: Address of buffer
@@ -230,6 +232,19 @@ int membuf_extend_by(struct membuf *mb, int by, int max);
  */
 void membuf_init(struct membuf *mb, char *buff, int size);
 
+/**
+ * membuf_init_with_data() - set up a new membuff using existing data
+ *
+ * The buffer is set up to contain the provided data. with its size set to
+ * @size + 1 (less MEMBUF_FULL), so that there is enough space for the head/tail
+ * differece.
+ *
+ * @mb: membuff to set up
+ * @buff: Address of buffer
+ * @size: Number of bytes to put into the membuf
+ */
+void membuf_init_with_data(struct membuf *mb, char *buff, int size);
+
 /**
  * membuf_uninit() - clear a membuff so it can no longer be used
  *

lib/membuf.c

@@ -405,6 +405,14 @@ void membuf_init(struct membuf *mb, char *buff, int size)
 	membuf_purge(mb);
 }
 
+void membuf_init_with_data(struct membuf *mb, char *buff, int size)
+{
+	char *data;
+
+	membuf_init(mb, buff, size + !MEMBUF_FULL);
+	membuf_putraw(mb, size, true, &data);
+}
+
 int membuf_new(struct membuf *mb, int size)
 {
 	mb->start = malloc(size);

test/lib/membuf.c

@@ -242,3 +242,22 @@ static int lib_test_membuf_readline(struct unit_test_state *uts)
 	return 0;
 }
 LIB_TEST(lib_test_membuf_readline, 0);
+
+/* test membuf_readline() with generated data */
+static int lib_test_membuf_init(struct unit_test_state *uts)
+{
+	struct membuf mb;
+	char buf[10], out[10];
+	int len;
+
+	strcpy(buf, "hello");
+	len = strlen(buf);
+	membuf_init_with_data(&mb, buf, len);
+	ut_asserteq_ptr(buf, mb.start);
+	ut_asserteq(len, membuf_avail(&mb));
+	ut_asserteq(len, membuf_readline(&mb, out, sizeof(out), 0, false));
+	ut_asserteq_str("hello", out);
+
+	return 0;
+}
+LIB_TEST(lib_test_membuf_init, 0);