lib: add darr_strlen_fixup() to update len based on NUL term

Signed-off-by: Christian Hopps <chopps@labn.net>

lib/darr.h

@@ -62,6 +62,7 @@
  *  - darr_strdup
  *  - darr_strdup_cap
  *  - darr_strlen
+ *  - darr_strlen_fixup
  *  - darr_strnul
  *  - darr_sprintf, darr_vsprintf
  */
@@ -752,6 +753,22 @@ void *__darr_resize(void *a, uint count, size_t esize, struct memtype *mt);
 		__size;                                                        \
 	})
 
+/**
+ * Fixup darr_len (and thus darr_strlen) for `S` based on its strlen(S)
+ * (i.e., scan for NUL byte). The dynamic array length will be set to strlen(S) + 1.
+ *
+ * Args:
+ *	S: The dynamic array with a NUL terminated string, cannot be NULL.
+ *
+ * Return:
+ *      The calculated strlen() value.
+ */
+#define darr_strlen_fixup(S)                                                                       \
+	({                                                                                         \
+		_darr_len(S) = strlen(S) + 1;                                                      \
+		darr_strlen(S);                                                                    \
+	})
+
 /**
  * darr_vsprintf() - vsprintf into a new dynamic array.
  *

tests/lib/test_darr.c

@@ -48,6 +48,7 @@
  * [x] - darr_strdup
  * [x] - darr_strdup_cap
  * [x] - darr_strlen
+ * [x] - darr_strlen_fixup
  * [x] - darr_strnul
  * [ ] - darr_vsprintf
  */
@@ -406,6 +407,9 @@ static void test_string(void)
 	assert(!strcmp(da1, "0123456789: DEADBEEF"));
 	assert(darr_strlen(da1) == 20);
 	assert(darr_cap(da1) == 128);
+
+	da1[5] = 0;
+	assert(darr_strlen_fixup(da1) == 5);
 	darr_free(da1);
 
 	da1 = darr_sprintf("0123456789: %08x", 0xDEADBEEF);