py/qstr: Add qstr_from_strn_static() helper function.

Allows an interned string to reference static/ROM data, instead of
allocating it on the GC heap.

Signed-off-by: Damien George <damien@micropython.org>

py/qstr.c

@@ -309,7 +309,7 @@ qstr qstr_from_str(const char *str) {
     return qstr_from_strn(str, strlen(str));
 }
 
-qstr qstr_from_strn(const char *str, size_t len) {
+static qstr qstr_from_strn_helper(const char *str, size_t len, bool data_is_static) {
     QSTR_ENTER();
     qstr q = qstr_find_strn(str, len);
     if (q == 0) {
@@ -321,6 +321,12 @@ qstr qstr_from_strn(const char *str, size_t len) {
             mp_raise_msg(&mp_type_RuntimeError, MP_ERROR_TEXT("name too long"));
         }
 
+        if (data_is_static) {
+            // Given string data will be forever available so use it directly.
+            assert(str[len] == '\0');
+            goto add;
+        }
+
         // compute number of bytes needed to intern this string
         size_t n_bytes = len + 1;
 
@@ -364,12 +370,26 @@ qstr qstr_from_strn(const char *str, size_t len) {
         // store the interned strings' data
         memcpy(q_ptr, str, len);
         q_ptr[len] = '\0';
-        q = qstr_add(len, q_ptr);
+        str = q_ptr;
+
+    add:
+        q = qstr_add(len, str);
     }
     QSTR_EXIT();
     return q;
 }
 
+qstr qstr_from_strn(const char *str, size_t len) {
+    return qstr_from_strn_helper(str, len, false);
+}
+
+#if MICROPY_VFS_ROM
+// Create a new qstr that can forever reference the given string data.
+qstr qstr_from_strn_static(const char *str, size_t len) {
+    return qstr_from_strn_helper(str, len, true);
+}
+#endif
+
 mp_uint_t qstr_hash(qstr q) {
     const qstr_pool_t *pool = find_qstr(&q);
     #if MICROPY_QSTR_BYTES_IN_HASH

py/qstr.h

@@ -101,6 +101,9 @@ qstr qstr_find_strn(const char *str, size_t str_len); // returns MP_QSTRnull if
 
 qstr qstr_from_str(const char *str);
 qstr qstr_from_strn(const char *str, size_t len);
+#if MICROPY_VFS_ROM
+qstr qstr_from_strn_static(const char *str, size_t len);
+#endif
 
 mp_uint_t qstr_hash(qstr q);
 const char *qstr_str(qstr q);