tool: fix retries in parallel mode

Verify that curl's --retry behaviour works

- test 502 in serial, works
- test 502 in parallel, hangs forver, test skipped by default
- test 401, no retries done

Fixes #20669
Reported-by: BlackFuffey on github
Closes #21206

src/tool_operate.c

@@ -1723,6 +1723,29 @@ static CURLcode parallel_event(struct parastate *s)
                            &s->still_running);
 
   while(!s->mcode && (s->still_running || s->more_transfers)) {
+
+    if(!s->still_running && s->more_transfers) {
+      curl_off_t nxfers;
+      /* None are running, but there may be more ready to run. Try
+       * to add more when possible. */
+      s->result = add_parallel_transfers(s->multi, s->share,
+                                         &s->more_transfers,
+                                         &s->added_transfers);
+      if(s->result)
+        break;
+      if(s->more_transfers) {
+        /* There are still more transfers, so not all were added. If we
+         * did not add *any*, it means there is nothing to do until time
+         * passes and a transfer becomes ready to add.
+         * Avoid busy looping by taking a small break. */
+        CURLMcode mcode = curl_multi_get_offt(
+          s->multi, CURLMINFO_XFERS_CURRENT, &nxfers);
+        if(!mcode && !nxfers) {
+          /* None have been added, must be waiting on a timeout */
+          curlx_wait_ms(500);
+        }
+      }
+    }
 #if DEBUG_UV
     curl_mfprintf(tool_stderr, "parallel_event: uv_run(), "
                   "mcode=%d, %d running, %d more\n",
@@ -1919,6 +1942,13 @@ static CURLcode parallel_transfers(CURLSH *share)
           s->wrapitup_processed = TRUE;
         }
       }
+      else if(s->more_transfers) {
+        s->result = add_parallel_transfers(s->multi, s->share,
+                                           &s->more_transfers,
+                                           &s->added_transfers);
+        if(s->result)
+          break;
+      }
 
       s->mcode = curl_multi_poll(s->multi, NULL, 0, 1000, NULL);
       if(!s->mcode)

tests/http/test_05_errors.py

@@ -143,3 +143,40 @@ class TestErrors:
         ])
         assert r.exit_code == 60, f'{r}'
         assert r.stats[0]['errormsg'] != 'CURL_DBG_SOCK_FAIL_IPV6: failed to open socket'
+
+    # Get, retry on 502
+    def test_05_06_retry_502(self, env: Env, httpd, nghttpx):
+        proto = 'http/1.1'
+        curl = CurlClient(env=env)
+        url = f'https://{env.authority_for(env.domain1, proto)}/curltest/tweak?status=502'
+        r = curl.http_download(urls=[url], alpn_proto=proto, extra_args=[
+            '--retry', '2', '--retry-all-errors', '--retry-delay', '1',
+        ])
+        r.check_response(http_status=502)
+        assert r.stats[0]['num_retries'] == 2, f'{r}'
+        # curious, since curl does the retries, it finds the previous
+        # connection in the cache and reports that no connects were done
+        assert r.stats[0]['num_connects'] == 0, f'{r}'
+
+    # Get, retry on 502 in parallel mode
+    def test_05_07_retry_502_parallel(self, env: Env, httpd, nghttpx):
+        proto = 'http/1.1'
+        curl = CurlClient(env=env)
+        url = f'https://{env.authority_for(env.domain1, proto)}/curltest/tweak?status=502'
+        r = curl.http_download(urls=[url], alpn_proto=proto, extra_args=[
+            '--retry', '2', '--retry-all-errors', '--retry-delay', '1', '--parallel'
+        ])
+        r.check_response(http_status=502)
+        assert r.stats[0]['num_retries'] == 2, f'{r}'
+
+    # Get, retry on 401, not happening
+    def test_05_08_retry_401(self, env: Env, httpd, nghttpx):
+        proto = 'http/1.1'
+        curl = CurlClient(env=env)
+        url = f'https://{env.authority_for(env.domain1, proto)}/curltest/tweak?status=401'
+        r = curl.http_download(urls=[url], alpn_proto=proto, extra_args=[
+            '--retry', '2', '--retry-all-errors', '--retry-delay', '1'
+        ])
+        r.check_response(http_status=401)
+        # No retries on a 401
+        assert r.stats[0]['num_retries'] == 0, f'{r}'