easy: allow connect-only handle reuse with easy_perform

- Detach and disconnect an attached connection before performing.

Prior to this change it was not possible to safely reuse an easy handle
with an attached connection in a second call to curl_easy_perform. The
only known case of this is a connect-only type handle where the
connection was detached when curl_easy_perform returned, only to be
reattached by either curl_easy_send/recv.

This commit effectively reverts 2f8ecd5d and be82a360, the latter of
which treated the reuse as an error. Prior to that change undefined
behavior may occur in such a case.

Bug: https://curl.se/mail/lib-2025-01/0044.html
Reported-by: Aleksander Mazur

Closes https://github.com/curl/curl/pull/16008

tests/libtest/lib556.c

@@ -41,6 +41,9 @@ CURLcode test(char *URL)
 {
   CURLcode res;
   CURL *curl;
+#ifdef LIB696
+  int transfers = 0;
+#endif
 
   if(curl_global_init(CURL_GLOBAL_ALL) != CURLE_OK) {
     fprintf(stderr, "curl_global_init() failed\n");
@@ -58,6 +61,10 @@ CURLcode test(char *URL)
   test_setopt(curl, CURLOPT_CONNECT_ONLY, 1L);
   test_setopt(curl, CURLOPT_VERBOSE, 1L);
 
+#ifdef LIB696
+again:
+#endif
+
   res = curl_easy_perform(curl);
 
   if(!res) {
@@ -87,8 +94,12 @@ CURLcode test(char *URL)
 
       if(nread) {
         /* send received stuff to stdout */
-        if(!write(STDOUT_FILENO, buf, nread))
+        if((size_t)write(STDOUT_FILENO, buf, nread) != nread) {
+          fprintf(stderr, "write() failed: errno %d (%s)\n",
+                  errno, strerror(errno));
+          res = TEST_ERR_FAILURE;
           break;
+        }
       }
 
     } while((res == CURLE_OK && nread) || (res == CURLE_AGAIN));
@@ -98,12 +109,10 @@ CURLcode test(char *URL)
   }
 
 #ifdef LIB696
-  /* attempt to use the handle again */
-  test_setopt(curl, CURLOPT_URL, URL);
-  test_setopt(curl, CURLOPT_CONNECT_ONLY, 1L);
-  test_setopt(curl, CURLOPT_VERBOSE, 1L);
-
-  res = curl_easy_perform(curl);
+  ++transfers;
+  /* perform the transfer a second time */
+  if(!res && transfers == 1)
+    goto again;
 #endif
 
 test_cleanup: