basic/strv: add optimizable version of strv_push/consume/extend

This will be helpful in cases where we are repeatedly adding entries
to a long strv and want to skip the iteration over old entries leading
to quadratic behaviour.

Note that we don't want to calculate the length if not necessary, so
the calculation is delayed until after we've checked that value is not
NULL.

src/basic/strv.c

@@ -403,27 +403,33 @@ char* strv_join_full(char * const *l, const char *separator, const char *prefix,
         return r;
 }
 
-int strv_push(char ***l, char *value) {
-        char **c;
-        size_t n;
+int strv_push_with_size(char ***l, size_t *n, char *value) {
+        /* n is a pointer to a variable to store the size of l.
+         * If not given (i.e. n is NULL or *n is SIZE_MAX), size will be calculated using strv_length().
+         * If n is not NULL, the size after the push will be returned.
+         * If value is empty, no action is taken and *n is not set. */
 
         if (!value)
                 return 0;
 
-        n = strv_length(*l);
+        size_t size = n ? *n : SIZE_MAX;
+        if (size == SIZE_MAX)
+                size = strv_length(*l);
 
         /* Check for overflow */
-        if (n > SIZE_MAX-2)
+        if (size > SIZE_MAX-2)
                 return -ENOMEM;
 
-        c = reallocarray(*l, GREEDY_ALLOC_ROUND_UP(n + 2), sizeof(char*));
+        char **c = reallocarray(*l, GREEDY_ALLOC_ROUND_UP(size + 2), sizeof(char*));
         if (!c)
                 return -ENOMEM;
 
-        c[n] = value;
-        c[n+1] = NULL;
+        c[size] = value;
+        c[size+1] = NULL;
 
         *l = c;
+        if (n)
+                *n = size + 1;
         return 0;
 }
 
@@ -484,10 +490,10 @@ int strv_insert(char ***l, size_t position, char *value) {
         return free_and_replace(*l, c);
 }
 
-int strv_consume(char ***l, char *value) {
+int strv_consume_with_size(char ***l, size_t *n, char *value) {
         int r;
 
-        r = strv_push(l, value);
+        r = strv_push_with_size(l, n, value);
         if (r < 0)
                 free(value);
 
@@ -529,7 +535,7 @@ int strv_prepend(char ***l, const char *value) {
         return strv_consume_prepend(l, v);
 }
 
-int strv_extend(char ***l, const char *value) {
+int strv_extend_with_size(char ***l, size_t *n, const char *value) {
         char *v;
 
         if (!value)
@@ -539,7 +545,7 @@ int strv_extend(char ***l, const char *value) {
         if (!v)
                 return -ENOMEM;
 
-        return strv_consume(l, v);
+        return strv_consume_with_size(l, n, v);
 }
 
 int strv_extend_front(char ***l, const char *value) {