string-list: introduce string_list_setlen()

It is sometimes useful to reduce the size of a `string_list`'s list of
items without having to re-allocate them. For example, doing the
following:

    struct strbuf buf = STRBUF_INIT;
    struct string_list parts = STRING_LIST_INIT_NO_DUP;
    while (strbuf_getline(&buf, stdin) != EOF) {
      parts.nr = 0;
      string_list_split_in_place(&parts, buf.buf, ":", -1);
      /* ... */
    }
    string_list_clear(&parts, 0);

is preferable over calling `string_list_clear()` on every iteration of
the loop. This is because `string_list_clear()` causes us free our
existing `items` array. This means that every time we call
`string_list_split_in_place()`, the string-list internals re-allocate
the same size array.

Since in the above example we do not care about the individual parts
after processing each line, it is much more efficient to pretend that
there aren't any elements in the `string_list` by setting `list->nr` to
0 while leaving the list of elements allocated as-is.

This allows `string_list_split_in_place()` to overwrite any existing
entries without needing to free and re-allocate them.

However, setting `list->nr` manually is not safe in all instances. There
are a couple of cases worth worrying about:

  - If the `string_list` is initialized with `strdup_strings`,
    truncating the list can lead to overwriting strings which are
    allocated elsewhere. If there aren't any other pointers to those
    strings other than the ones inside of the `items` array, they will
    become unreachable and leak.

    (We could ourselves free the truncated items between
    string_list->items[nr] and `list->nr`, but no present or future
    callers would benefit from this additional complexity).

  - If the given `nr` is larger than the current value of `list->nr`,
    we'll trick the `string_list` into a state where it thinks there are
    more items allocated than there actually are, which can lead to
    undefined behavior if we try to read or write those entries.

Guard against both of these by introducing a helper function which
guards assignment of `list->nr` against each of the above conditions.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

string-list.c

@@ -203,6 +203,15 @@ void string_list_clear_func(struct string_list *list, string_list_clear_func_t c
 	list->nr = list->alloc = 0;
 }
 
+void string_list_setlen(struct string_list *list, size_t nr)
+{
+	if (list->strdup_strings)
+		BUG("cannot setlen a string_list which owns its entries");
+	if (nr > list->nr)
+		BUG("cannot grow a string_list with setlen");
+	list->nr = nr;
+}
+
 struct string_list_item *string_list_append_nodup(struct string_list *list,
 						  char *string)
 {

string-list.h

@@ -134,6 +134,16 @@ typedef void (*string_list_clear_func_t)(void *p, const char *str);
 /** Call a custom clear function on each util pointer */
 void string_list_clear_func(struct string_list *list, string_list_clear_func_t clearfunc);
 
+/*
+ * Set the length of a string_list to `nr`, provided that (a) `list`
+ * does not own its own storage, and (b) that `nr` is no larger than
+ * `list->nr`.
+ *
+ * Useful when "shrinking" `list` to write over existing entries that
+ * are no longer used without reallocating.
+ */
+void string_list_setlen(struct string_list *list, size_t nr);
+
 /**
  * Apply `func` to each item. If `func` returns nonzero, the
  * iteration aborts and the return value is propagated.