Ruby: Handle response field arrays

@xeron on GitHub reported an issue whereby with a Rails 7.1 application
they were getting the following error

  2023/10/22 20:57:28 [error] 56#56 [unit] #8: Ruby: Wrong header entry 'value' from application
  2023/10/22 20:57:28 [error] 56#56 [unit] #8: Ruby: Failed to run ruby script

After some back and forth debugging it turns out rack was trying to send
back a header comprised of an array of values. E.g

  app = Proc.new do |env|
      ["200", {
          "Content-Type" => "text/plain",
          "X-Array-Header" => ["Item-1", "Item-2"],
      }, ["Hello World\n"]]
  end

  run app

It seems this became a possibility in rack v3.0[0]

So along with a header value type of T_STRING we need to also allow
T_ARRAY.

If we get a T_ARRAY we need to build up the header field using the given
values.

E.g

  "X-Array-Header" => ["Item-1", "", "Item-3", "Item-4"],

becomes

  X-Array-Header: Item-1; ; Item-3; Item-4

[0]: <https://github.com/rack/rack/blob/main/UPGRADE-GUIDE.md?plain=1#L26>

Reported-by: Ivan Larionov <xeron.oskom@gmail.com>
Closes: <https://github.com/nginx/unit/issues/974>
Link: <https://github.com/nginx/unit/pull/998>
Tested-by: Timo Stark <t.stark@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/ruby/nxt_ruby.c

@@ -889,13 +889,37 @@ nxt_ruby_hash_info(VALUE r_key, VALUE r_value, VALUE arg)
         goto fail;
     }
 
-    if (nxt_slow_path(TYPE(r_value) != T_STRING)) {
+    if (nxt_slow_path(TYPE(r_value) != T_STRING && TYPE(r_value) != T_ARRAY)) {
         nxt_unit_req_error(headers_info->req,
                            "Ruby: Wrong header entry 'value' from application");
 
         goto fail;
     }
 
+    if (TYPE(r_value) == T_ARRAY) {
+        int     i;
+        int     arr_len = RARRAY_LEN(r_value);
+        VALUE   item;
+        size_t  len = 0;
+
+        for (i = 0; i < arr_len; i++) {
+            item = rb_ary_entry(r_value, i);
+            if (TYPE(item) != T_STRING) {
+                nxt_unit_req_error(headers_info->req,
+                                   "Ruby: Wrong header entry in 'value' array "
+                                   "from application");
+                goto fail;
+            }
+
+            len += RSTRING_LEN(item) + 2;   /* +2 for '; ' */
+        }
+
+        headers_info->fields++;
+        headers_info->size += RSTRING_LEN(r_key) + len - 2;
+
+        return ST_CONTINUE;
+    }
+
     value = RSTRING_PTR(r_value);
     value_end = value + RSTRING_LEN(r_value);
 
@@ -941,11 +965,52 @@ nxt_ruby_hash_add(VALUE r_key, VALUE r_value, VALUE arg)
     headers_info = (void *) (uintptr_t) arg;
     rc = &headers_info->rc;
 
+    key_len = RSTRING_LEN(r_key);
+
+    if (TYPE(r_value) == T_ARRAY) {
+        int     i;
+        int     arr_len = RARRAY_LEN(r_value);
+        char    *field, *p;
+        VALUE   item;
+        size_t  len = 0;
+
+        for (i = 0; i < arr_len; i++) {
+            item = rb_ary_entry(r_value, i);
+
+            len += RSTRING_LEN(item) + 2;   /* +2 for '; ' */
+        }
+
+        field = nxt_unit_malloc(NULL, len);
+        if (field == NULL) {
+            goto fail;
+        }
+
+        p = field;
+
+        for (i = 0; i < arr_len; i++) {
+            item = rb_ary_entry(r_value, i);
+
+            p = nxt_cpymem(p, RSTRING_PTR(item), RSTRING_LEN(item));
+            p = nxt_cpymem(p, "; ", 2);
+        }
+
+        len -= 2;
+
+        *rc = nxt_unit_response_add_field(headers_info->req,
+                                          RSTRING_PTR(r_key), key_len,
+                                          field, len);
+        nxt_unit_free(NULL, field);
+
+        if (nxt_slow_path(*rc != NXT_UNIT_OK)) {
+            goto fail;
+        }
+
+        return ST_CONTINUE;
+    }
+
     value = RSTRING_PTR(r_value);
     value_end = value + RSTRING_LEN(r_value);
 
-    key_len = RSTRING_LEN(r_key);
-
     pos = value;
 
     for ( ;; ) {