lib: redirect handling by protocol handler

Adds a `follow()` callback to protocol handlers, so they may decide how
to act on a `newurl` after a request has been done. This is optional.

This moves the HTTP code for handling redirects from multi.c to http.c
where it should be. If we ever add a protocol with its own logic, it
would install its own follow function.

Closes #16075

lib/curl_rtmp.c

@@ -85,6 +85,7 @@ const struct Curl_handler Curl_handler_rtmp = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_RTMP,                            /* defport */
   CURLPROTO_RTMP,                       /* protocol */
   CURLPROTO_RTMP,                       /* family */
@@ -109,6 +110,7 @@ const struct Curl_handler Curl_handler_rtmpt = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_RTMPT,                           /* defport */
   CURLPROTO_RTMPT,                      /* protocol */
   CURLPROTO_RTMPT,                      /* family */
@@ -133,6 +135,7 @@ const struct Curl_handler Curl_handler_rtmpe = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_RTMP,                            /* defport */
   CURLPROTO_RTMPE,                      /* protocol */
   CURLPROTO_RTMPE,                      /* family */
@@ -157,6 +160,7 @@ const struct Curl_handler Curl_handler_rtmpte = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_RTMPT,                           /* defport */
   CURLPROTO_RTMPTE,                     /* protocol */
   CURLPROTO_RTMPTE,                     /* family */
@@ -181,6 +185,7 @@ const struct Curl_handler Curl_handler_rtmps = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_RTMPS,                           /* defport */
   CURLPROTO_RTMPS,                      /* protocol */
   CURLPROTO_RTMP,                       /* family */
@@ -205,6 +210,7 @@ const struct Curl_handler Curl_handler_rtmpts = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_RTMPS,                           /* defport */
   CURLPROTO_RTMPTS,                     /* protocol */
   CURLPROTO_RTMPT,                      /* family */

lib/dict.c

@@ -93,6 +93,7 @@ const struct Curl_handler Curl_handler_dict = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_DICT,                            /* defport */
   CURLPROTO_DICT,                       /* protocol */
   CURLPROTO_DICT,                       /* family */

lib/file.c

@@ -120,6 +120,7 @@ const struct Curl_handler Curl_handler_file = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   0,                                    /* defport */
   CURLPROTO_FILE,                       /* protocol */
   CURLPROTO_FILE,                       /* family */

lib/ftp.c

@@ -250,6 +250,7 @@ const struct Curl_handler Curl_handler_ftp = {
   ZERO_NULL,                       /* write_resp_hd */
   ZERO_NULL,                       /* connection_check */
   ZERO_NULL,                       /* attach connection */
+  ZERO_NULL,                       /* follow */
   PORT_FTP,                        /* defport */
   CURLPROTO_FTP,                   /* protocol */
   CURLPROTO_FTP,                   /* family */
@@ -282,6 +283,7 @@ const struct Curl_handler Curl_handler_ftps = {
   ZERO_NULL,                       /* write_resp_hd */
   ZERO_NULL,                       /* connection_check */
   ZERO_NULL,                       /* attach connection */
+  ZERO_NULL,                       /* follow */
   PORT_FTPS,                       /* defport */
   CURLPROTO_FTPS,                  /* protocol */
   CURLPROTO_FTP,                   /* family */

lib/gopher.c

@@ -79,6 +79,7 @@ const struct Curl_handler Curl_handler_gopher = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_GOPHER,                          /* defport */
   CURLPROTO_GOPHER,                     /* protocol */
   CURLPROTO_GOPHER,                     /* family */
@@ -104,6 +105,7 @@ const struct Curl_handler Curl_handler_gophers = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_GOPHER,                          /* defport */
   CURLPROTO_GOPHERS,                    /* protocol */
   CURLPROTO_GOPHER,                     /* family */

lib/http.c

@@ -64,6 +64,7 @@
 #include "http_negotiate.h"
 #include "http_aws_sigv4.h"
 #include "url.h"
+#include "urlapi-int.h"
 #include "share.h"
 #include "hostip.h"
 #include "dynhds.h"
@@ -145,6 +146,7 @@ const struct Curl_handler Curl_handler_http = {
   Curl_http_write_resp_hd,              /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  Curl_http_follow,                     /* follow */
   PORT_HTTP,                            /* defport */
   CURLPROTO_HTTP,                       /* protocol */
   CURLPROTO_HTTP,                       /* family */
@@ -174,6 +176,7 @@ const struct Curl_handler Curl_handler_https = {
   Curl_http_write_resp_hd,              /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  Curl_http_follow,                     /* follow */
   PORT_HTTPS,                           /* defport */
   CURLPROTO_HTTPS,                      /* protocol */
   CURLPROTO_HTTP,                       /* family */
@@ -1091,6 +1094,283 @@ static bool http_should_fail(struct Curl_easy *data, int httpcode)
   return data->state.authproblem;
 }
 
+CURLcode Curl_http_follow(struct Curl_easy *data, const char *newurl,
+                          followtype type)
+{
+  bool disallowport = FALSE;
+  bool reachedmax = FALSE;
+  char *follow_url = NULL;
+  CURLUcode uc;
+
+  DEBUGASSERT(type != FOLLOW_NONE);
+
+  if(type != FOLLOW_FAKE)
+    data->state.requests++; /* count all real follows */
+  if(type == FOLLOW_REDIR) {
+    if((data->set.maxredirs != -1) &&
+       (data->state.followlocation >= data->set.maxredirs)) {
+      reachedmax = TRUE;
+      type = FOLLOW_FAKE; /* switch to fake to store the would-be-redirected
+                             to URL */
+    }
+    else {
+      data->state.followlocation++; /* count redirect-followings, including
+                                       auth reloads */
+
+      if(data->set.http_auto_referer) {
+        CURLU *u;
+        char *referer = NULL;
+
+        /* We are asked to automatically set the previous URL as the referer
+           when we get the next URL. We pick the ->url field, which may or may
+           not be 100% correct */
+
+        if(data->state.referer_alloc) {
+          Curl_safefree(data->state.referer);
+          data->state.referer_alloc = FALSE;
+        }
+
+        /* Make a copy of the URL without credentials and fragment */
+        u = curl_url();
+        if(!u)
+          return CURLE_OUT_OF_MEMORY;
+
+        uc = curl_url_set(u, CURLUPART_URL, data->state.url, 0);
+        if(!uc)
+          uc = curl_url_set(u, CURLUPART_FRAGMENT, NULL, 0);
+        if(!uc)
+          uc = curl_url_set(u, CURLUPART_USER, NULL, 0);
+        if(!uc)
+          uc = curl_url_set(u, CURLUPART_PASSWORD, NULL, 0);
+        if(!uc)
+          uc = curl_url_get(u, CURLUPART_URL, &referer, 0);
+
+        curl_url_cleanup(u);
+
+        if(uc || !referer)
+          return CURLE_OUT_OF_MEMORY;
+
+        data->state.referer = referer;
+        data->state.referer_alloc = TRUE; /* yes, free this later */
+      }
+    }
+  }
+
+  if((type != FOLLOW_RETRY) &&
+     (data->req.httpcode != 401) && (data->req.httpcode != 407) &&
+     Curl_is_absolute_url(newurl, NULL, 0, FALSE)) {
+    /* If this is not redirect due to a 401 or 407 response and an absolute
+       URL: do not allow a custom port number */
+    disallowport = TRUE;
+  }
+
+  DEBUGASSERT(data->state.uh);
+  uc = curl_url_set(data->state.uh, CURLUPART_URL, newurl, (unsigned int)
+                    ((type == FOLLOW_FAKE) ? CURLU_NON_SUPPORT_SCHEME :
+                     ((type == FOLLOW_REDIR) ? CURLU_URLENCODE : 0) |
+                     CURLU_ALLOW_SPACE |
+                     (data->set.path_as_is ? CURLU_PATH_AS_IS : 0)));
+  if(uc) {
+    if(type != FOLLOW_FAKE) {
+      failf(data, "The redirect target URL could not be parsed: %s",
+            curl_url_strerror(uc));
+      return Curl_uc_to_curlcode(uc);
+    }
+
+    /* the URL could not be parsed for some reason, but since this is FAKE
+       mode, just duplicate the field as-is */
+    follow_url = strdup(newurl);
+    if(!follow_url)
+      return CURLE_OUT_OF_MEMORY;
+  }
+  else {
+    uc = curl_url_get(data->state.uh, CURLUPART_URL, &follow_url, 0);
+    if(uc)
+      return Curl_uc_to_curlcode(uc);
+
+    /* Clear auth if this redirects to a different port number or protocol,
+       unless permitted */
+    if(!data->set.allow_auth_to_other_hosts && (type != FOLLOW_FAKE)) {
+      char *portnum;
+      int port;
+      bool clear = FALSE;
+
+      if(data->set.use_port && data->state.allow_port)
+        /* a custom port is used */
+        port = (int)data->set.use_port;
+      else {
+        uc = curl_url_get(data->state.uh, CURLUPART_PORT, &portnum,
+                          CURLU_DEFAULT_PORT);
+        if(uc) {
+          free(follow_url);
+          return Curl_uc_to_curlcode(uc);
+        }
+        port = atoi(portnum);
+        free(portnum);
+      }
+      if(port != data->info.conn_remote_port) {
+        infof(data, "Clear auth, redirects to port from %u to %u",
+              data->info.conn_remote_port, port);
+        clear = TRUE;
+      }
+      else {
+        char *scheme;
+        const struct Curl_handler *p;
+        uc = curl_url_get(data->state.uh, CURLUPART_SCHEME, &scheme, 0);
+        if(uc) {
+          free(follow_url);
+          return Curl_uc_to_curlcode(uc);
+        }
+
+        p = Curl_get_scheme_handler(scheme);
+        if(p && (p->protocol != data->info.conn_protocol)) {
+          infof(data, "Clear auth, redirects scheme from %s to %s",
+                data->info.conn_scheme, scheme);
+          clear = TRUE;
+        }
+        free(scheme);
+      }
+      if(clear) {
+        Curl_safefree(data->state.aptr.user);
+        Curl_safefree(data->state.aptr.passwd);
+      }
+    }
+  }
+  DEBUGASSERT(follow_url);
+
+  if(type == FOLLOW_FAKE) {
+    /* we are only figuring out the new URL if we would have followed locations
+       but now we are done so we can get out! */
+    data->info.wouldredirect = follow_url;
+
+    if(reachedmax) {
+      failf(data, "Maximum (%ld) redirects followed", data->set.maxredirs);
+      return CURLE_TOO_MANY_REDIRECTS;
+    }
+    return CURLE_OK;
+  }
+
+  if(disallowport)
+    data->state.allow_port = FALSE;
+
+  if(data->state.url_alloc)
+    Curl_safefree(data->state.url);
+
+  data->state.url = follow_url;
+  data->state.url_alloc = TRUE;
+  Curl_req_soft_reset(&data->req, data);
+  infof(data, "Issue another request to this URL: '%s'", data->state.url);
+
+  /*
+   * We get here when the HTTP code is 300-399 (and 401). We need to perform
+   * differently based on exactly what return code there was.
+   *
+   * News from 7.10.6: we can also get here on a 401 or 407, in case we act on
+   * an HTTP (proxy-) authentication scheme other than Basic.
+   */
+  switch(data->info.httpcode) {
+    /* 401 - Act on a WWW-Authenticate, we keep on moving and do the
+       Authorization: XXXX header in the HTTP request code snippet */
+    /* 407 - Act on a Proxy-Authenticate, we keep on moving and do the
+       Proxy-Authorization: XXXX header in the HTTP request code snippet */
+    /* 300 - Multiple Choices */
+    /* 306 - Not used */
+    /* 307 - Temporary Redirect */
+  default:  /* for all above (and the unknown ones) */
+    /* Some codes are explicitly mentioned since I have checked RFC2616 and
+     * they seem to be OK to POST to.
+     */
+    break;
+  case 301: /* Moved Permanently */
+    /* (quote from RFC7231, section 6.4.2)
+     *
+     * Note: For historical reasons, a user agent MAY change the request
+     * method from POST to GET for the subsequent request. If this
+     * behavior is undesired, the 307 (Temporary Redirect) status code
+     * can be used instead.
+     *
+     * ----
+     *
+     * Many webservers expect this, so these servers often answers to a POST
+     * request with an error page. To be sure that libcurl gets the page that
+     * most user agents would get, libcurl has to force GET.
+     *
+     * This behavior is forbidden by RFC1945 and the obsolete RFC2616, and
+     * can be overridden with CURLOPT_POSTREDIR.
+     */
+    if((data->state.httpreq == HTTPREQ_POST
+        || data->state.httpreq == HTTPREQ_POST_FORM
+        || data->state.httpreq == HTTPREQ_POST_MIME)
+       && !(data->set.keep_post & CURL_REDIR_POST_301)) {
+      infof(data, "Switch from POST to GET");
+      data->state.httpreq = HTTPREQ_GET;
+      Curl_creader_set_rewind(data, FALSE);
+    }
+    break;
+  case 302: /* Found */
+    /* (quote from RFC7231, section 6.4.3)
+     *
+     * Note: For historical reasons, a user agent MAY change the request
+     * method from POST to GET for the subsequent request. If this
+     * behavior is undesired, the 307 (Temporary Redirect) status code
+     * can be used instead.
+     *
+     * ----
+     *
+     * Many webservers expect this, so these servers often answers to a POST
+     * request with an error page. To be sure that libcurl gets the page that
+     * most user agents would get, libcurl has to force GET.
+     *
+     * This behavior is forbidden by RFC1945 and the obsolete RFC2616, and
+     * can be overridden with CURLOPT_POSTREDIR.
+     */
+    if((data->state.httpreq == HTTPREQ_POST
+        || data->state.httpreq == HTTPREQ_POST_FORM
+        || data->state.httpreq == HTTPREQ_POST_MIME)
+       && !(data->set.keep_post & CURL_REDIR_POST_302)) {
+      infof(data, "Switch from POST to GET");
+      data->state.httpreq = HTTPREQ_GET;
+      Curl_creader_set_rewind(data, FALSE);
+    }
+    break;
+
+  case 303: /* See Other */
+    /* 'See Other' location is not the resource but a substitute for the
+     * resource. In this case we switch the method to GET/HEAD, unless the
+     * method is POST and the user specified to keep it as POST.
+     * https://github.com/curl/curl/issues/5237#issuecomment-614641049
+     */
+    if(data->state.httpreq != HTTPREQ_GET &&
+       ((data->state.httpreq != HTTPREQ_POST &&
+         data->state.httpreq != HTTPREQ_POST_FORM &&
+         data->state.httpreq != HTTPREQ_POST_MIME) ||
+        !(data->set.keep_post & CURL_REDIR_POST_303))) {
+      data->state.httpreq = HTTPREQ_GET;
+      infof(data, "Switch to %s",
+            data->req.no_body ? "HEAD" : "GET");
+    }
+    break;
+  case 304: /* Not Modified */
+    /* 304 means we did a conditional request and it was "Not modified".
+     * We should not get any Location: header in this response!
+     */
+    break;
+  case 305: /* Use Proxy */
+    /* (quote from RFC2616, section 10.3.6):
+     * "The requested resource MUST be accessed through the proxy given
+     * by the Location field. The Location field gives the URI of the
+     * proxy. The recipient is expected to repeat this single request
+     * via the proxy. 305 responses MUST only be generated by origin
+     * servers."
+     */
+    break;
+  }
+  Curl_pgrsTime(data, TIMER_REDIRECT);
+  Curl_pgrsResetTransferSizes(data);
+
+  return CURLE_OK;
+}
+
 /*
  * Curl_compareheader()
  *

lib/http.h

@@ -42,6 +42,18 @@ typedef enum {
   HTTPREQ_HEAD
 } Curl_HttpReq;
 
+
+/* When redirecting transfers. */
+typedef enum {
+  FOLLOW_NONE,  /* not used within the function, just a placeholder to
+                   allow initing to this */
+  FOLLOW_FAKE,  /* only records stuff, not actually following */
+  FOLLOW_RETRY, /* set if this is a request retry as opposed to a real
+                   redirect following */
+  FOLLOW_REDIR /* a full true redirect */
+} followtype;
+
+
 #ifndef CURL_DISABLE_HTTP
 
 #if defined(USE_HTTP3)
@@ -103,6 +115,10 @@ CURLcode Curl_http_input_auth(struct Curl_easy *data, bool proxy,
                               const char *auth);
 CURLcode Curl_http_auth_act(struct Curl_easy *data);
 
+/* follow a redirect or not */
+CURLcode Curl_http_follow(struct Curl_easy *data, const char *newurl,
+                          followtype type);
+
 /* If only the PICKNONE bit is set, there has been a round-trip and we
    selected to use no auth at all. Ie, we actively select no auth, as opposed
    to not having one selected. The other CURLAUTH_* defines are present in the

lib/imap.c

@@ -134,6 +134,7 @@ const struct Curl_handler Curl_handler_imap = {
   ZERO_NULL,                        /* write_resp_hd */
   ZERO_NULL,                        /* connection_check */
   ZERO_NULL,                        /* attach connection */
+  ZERO_NULL,                        /* follow */
   PORT_IMAP,                        /* defport */
   CURLPROTO_IMAP,                   /* protocol */
   CURLPROTO_IMAP,                   /* family */
@@ -164,6 +165,7 @@ const struct Curl_handler Curl_handler_imaps = {
   ZERO_NULL,                        /* write_resp_hd */
   ZERO_NULL,                        /* connection_check */
   ZERO_NULL,                        /* attach connection */
+  ZERO_NULL,                        /* follow */
   PORT_IMAPS,                       /* defport */
   CURLPROTO_IMAPS,                  /* protocol */
   CURLPROTO_IMAP,                   /* family */

lib/ldap.c

@@ -187,6 +187,7 @@ const struct Curl_handler Curl_handler_ldap = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_LDAP,                            /* defport */
   CURLPROTO_LDAP,                       /* protocol */
   CURLPROTO_LDAP,                       /* family */
@@ -216,6 +217,7 @@ const struct Curl_handler Curl_handler_ldaps = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_LDAPS,                           /* defport */
   CURLPROTO_LDAPS,                      /* protocol */
   CURLPROTO_LDAP,                       /* family */

lib/mqtt.c

@@ -92,6 +92,7 @@ const struct Curl_handler Curl_handler_mqtt = {
   ZERO_NULL,                          /* write_resp_hd */
   ZERO_NULL,                          /* connection_check */
   ZERO_NULL,                          /* attach connection */
+  ZERO_NULL,                          /* follow */
   PORT_MQTT,                          /* defport */
   CURLPROTO_MQTT,                     /* protocol */
   CURLPROTO_MQTT,                     /* family */

lib/multi.c

@@ -1855,289 +1855,13 @@ static void multi_posttransfer(struct Curl_easy *data)
  * This function DOES NOT FREE the given url.
  */
 static CURLcode multi_follow(struct Curl_easy *data,
-                             char *newurl,    /* the Location: string */
+                             const struct Curl_handler *handler,
+                             const char *newurl, /* the Location: string */
                              followtype type) /* see transfer.h */
 {
-#ifdef CURL_DISABLE_HTTP
+  if(handler && handler->follow)
-  (void)data;
+    return handler->follow(data, newurl, type);
-  (void)newurl;
-  (void)type;
-  /* Location: following will not happen when HTTP is disabled */
   return CURLE_TOO_MANY_REDIRECTS;
-#else
-
-  /* Location: redirect */
-  bool disallowport = FALSE;
-  bool reachedmax = FALSE;
-  CURLUcode uc;
-
-  DEBUGASSERT(type != FOLLOW_NONE);
-
-  if(type != FOLLOW_FAKE)
-    data->state.requests++; /* count all real follows */
-  if(type == FOLLOW_REDIR) {
-    if((data->set.maxredirs != -1) &&
-       (data->state.followlocation >= data->set.maxredirs)) {
-      reachedmax = TRUE;
-      type = FOLLOW_FAKE; /* switch to fake to store the would-be-redirected
-                             to URL */
-    }
-    else {
-      data->state.followlocation++; /* count redirect-followings, including
-                                       auth reloads */
-
-      if(data->set.http_auto_referer) {
-        CURLU *u;
-        char *referer = NULL;
-
-        /* We are asked to automatically set the previous URL as the referer
-           when we get the next URL. We pick the ->url field, which may or may
-           not be 100% correct */
-
-        if(data->state.referer_alloc) {
-          Curl_safefree(data->state.referer);
-          data->state.referer_alloc = FALSE;
-        }
-
-        /* Make a copy of the URL without credentials and fragment */
-        u = curl_url();
-        if(!u)
-          return CURLE_OUT_OF_MEMORY;
-
-        uc = curl_url_set(u, CURLUPART_URL, data->state.url, 0);
-        if(!uc)
-          uc = curl_url_set(u, CURLUPART_FRAGMENT, NULL, 0);
-        if(!uc)
-          uc = curl_url_set(u, CURLUPART_USER, NULL, 0);
-        if(!uc)
-          uc = curl_url_set(u, CURLUPART_PASSWORD, NULL, 0);
-        if(!uc)
-          uc = curl_url_get(u, CURLUPART_URL, &referer, 0);
-
-        curl_url_cleanup(u);
-
-        if(uc || !referer)
-          return CURLE_OUT_OF_MEMORY;
-
-        data->state.referer = referer;
-        data->state.referer_alloc = TRUE; /* yes, free this later */
-      }
-    }
-  }
-
-  if((type != FOLLOW_RETRY) &&
-     (data->req.httpcode != 401) && (data->req.httpcode != 407) &&
-     Curl_is_absolute_url(newurl, NULL, 0, FALSE)) {
-    /* If this is not redirect due to a 401 or 407 response and an absolute
-       URL: do not allow a custom port number */
-    disallowport = TRUE;
-  }
-
-  DEBUGASSERT(data->state.uh);
-  uc = curl_url_set(data->state.uh, CURLUPART_URL, newurl, (unsigned int)
-                    ((type == FOLLOW_FAKE) ? CURLU_NON_SUPPORT_SCHEME :
-                     ((type == FOLLOW_REDIR) ? CURLU_URLENCODE : 0) |
-                     CURLU_ALLOW_SPACE |
-                     (data->set.path_as_is ? CURLU_PATH_AS_IS : 0)));
-  if(uc) {
-    if(type != FOLLOW_FAKE) {
-      failf(data, "The redirect target URL could not be parsed: %s",
-            curl_url_strerror(uc));
-      return Curl_uc_to_curlcode(uc);
-    }
-
-    /* the URL could not be parsed for some reason, but since this is FAKE
-       mode, just duplicate the field as-is */
-    newurl = strdup(newurl);
-    if(!newurl)
-      return CURLE_OUT_OF_MEMORY;
-  }
-  else {
-    uc = curl_url_get(data->state.uh, CURLUPART_URL, &newurl, 0);
-    if(uc)
-      return Curl_uc_to_curlcode(uc);
-
-    /* Clear auth if this redirects to a different port number or protocol,
-       unless permitted */
-    if(!data->set.allow_auth_to_other_hosts && (type != FOLLOW_FAKE)) {
-      char *portnum;
-      int port;
-      bool clear = FALSE;
-
-      if(data->set.use_port && data->state.allow_port)
-        /* a custom port is used */
-        port = (int)data->set.use_port;
-      else {
-        uc = curl_url_get(data->state.uh, CURLUPART_PORT, &portnum,
-                          CURLU_DEFAULT_PORT);
-        if(uc) {
-          free(newurl);
-          return Curl_uc_to_curlcode(uc);
-        }
-        port = atoi(portnum);
-        free(portnum);
-      }
-      if(port != data->info.conn_remote_port) {
-        infof(data, "Clear auth, redirects to port from %u to %u",
-              data->info.conn_remote_port, port);
-        clear = TRUE;
-      }
-      else {
-        char *scheme;
-        const struct Curl_handler *p;
-        uc = curl_url_get(data->state.uh, CURLUPART_SCHEME, &scheme, 0);
-        if(uc) {
-          free(newurl);
-          return Curl_uc_to_curlcode(uc);
-        }
-
-        p = Curl_get_scheme_handler(scheme);
-        if(p && (p->protocol != data->info.conn_protocol)) {
-          infof(data, "Clear auth, redirects scheme from %s to %s",
-                data->info.conn_scheme, scheme);
-          clear = TRUE;
-        }
-        free(scheme);
-      }
-      if(clear) {
-        Curl_safefree(data->state.aptr.user);
-        Curl_safefree(data->state.aptr.passwd);
-      }
-    }
-  }
-
-  if(type == FOLLOW_FAKE) {
-    /* we are only figuring out the new URL if we would have followed locations
-       but now we are done so we can get out! */
-    data->info.wouldredirect = newurl;
-
-    if(reachedmax) {
-      failf(data, "Maximum (%ld) redirects followed", data->set.maxredirs);
-      return CURLE_TOO_MANY_REDIRECTS;
-    }
-    return CURLE_OK;
-  }
-
-  if(disallowport)
-    data->state.allow_port = FALSE;
-
-  if(data->state.url_alloc)
-    Curl_safefree(data->state.url);
-
-  data->state.url = newurl;
-  data->state.url_alloc = TRUE;
-  Curl_req_soft_reset(&data->req, data);
-  infof(data, "Issue another request to this URL: '%s'", data->state.url);
-
-  /*
-   * We get here when the HTTP code is 300-399 (and 401). We need to perform
-   * differently based on exactly what return code there was.
-   *
-   * News from 7.10.6: we can also get here on a 401 or 407, in case we act on
-   * an HTTP (proxy-) authentication scheme other than Basic.
-   */
-  switch(data->info.httpcode) {
-    /* 401 - Act on a WWW-Authenticate, we keep on moving and do the
-       Authorization: XXXX header in the HTTP request code snippet */
-    /* 407 - Act on a Proxy-Authenticate, we keep on moving and do the
-       Proxy-Authorization: XXXX header in the HTTP request code snippet */
-    /* 300 - Multiple Choices */
-    /* 306 - Not used */
-    /* 307 - Temporary Redirect */
-  default:  /* for all above (and the unknown ones) */
-    /* Some codes are explicitly mentioned since I have checked RFC2616 and
-     * they seem to be OK to POST to.
-     */
-    break;
-  case 301: /* Moved Permanently */
-    /* (quote from RFC7231, section 6.4.2)
-     *
-     * Note: For historical reasons, a user agent MAY change the request
-     * method from POST to GET for the subsequent request. If this
-     * behavior is undesired, the 307 (Temporary Redirect) status code
-     * can be used instead.
-     *
-     * ----
-     *
-     * Many webservers expect this, so these servers often answers to a POST
-     * request with an error page. To be sure that libcurl gets the page that
-     * most user agents would get, libcurl has to force GET.
-     *
-     * This behavior is forbidden by RFC1945 and the obsolete RFC2616, and
-     * can be overridden with CURLOPT_POSTREDIR.
-     */
-    if((data->state.httpreq == HTTPREQ_POST
-        || data->state.httpreq == HTTPREQ_POST_FORM
-        || data->state.httpreq == HTTPREQ_POST_MIME)
-       && !(data->set.keep_post & CURL_REDIR_POST_301)) {
-      infof(data, "Switch from POST to GET");
-      data->state.httpreq = HTTPREQ_GET;
-      Curl_creader_set_rewind(data, FALSE);
-    }
-    break;
-  case 302: /* Found */
-    /* (quote from RFC7231, section 6.4.3)
-     *
-     * Note: For historical reasons, a user agent MAY change the request
-     * method from POST to GET for the subsequent request. If this
-     * behavior is undesired, the 307 (Temporary Redirect) status code
-     * can be used instead.
-     *
-     * ----
-     *
-     * Many webservers expect this, so these servers often answers to a POST
-     * request with an error page. To be sure that libcurl gets the page that
-     * most user agents would get, libcurl has to force GET.
-     *
-     * This behavior is forbidden by RFC1945 and the obsolete RFC2616, and
-     * can be overridden with CURLOPT_POSTREDIR.
-     */
-    if((data->state.httpreq == HTTPREQ_POST
-        || data->state.httpreq == HTTPREQ_POST_FORM
-        || data->state.httpreq == HTTPREQ_POST_MIME)
-       && !(data->set.keep_post & CURL_REDIR_POST_302)) {
-      infof(data, "Switch from POST to GET");
-      data->state.httpreq = HTTPREQ_GET;
-      Curl_creader_set_rewind(data, FALSE);
-    }
-    break;
-
-  case 303: /* See Other */
-    /* 'See Other' location is not the resource but a substitute for the
-     * resource. In this case we switch the method to GET/HEAD, unless the
-     * method is POST and the user specified to keep it as POST.
-     * https://github.com/curl/curl/issues/5237#issuecomment-614641049
-     */
-    if(data->state.httpreq != HTTPREQ_GET &&
-       ((data->state.httpreq != HTTPREQ_POST &&
-         data->state.httpreq != HTTPREQ_POST_FORM &&
-         data->state.httpreq != HTTPREQ_POST_MIME) ||
-        !(data->set.keep_post & CURL_REDIR_POST_303))) {
-      data->state.httpreq = HTTPREQ_GET;
-      infof(data, "Switch to %s",
-            data->req.no_body ? "HEAD" : "GET");
-    }
-    break;
-  case 304: /* Not Modified */
-    /* 304 means we did a conditional request and it was "Not modified".
-     * We should not get any Location: header in this response!
-     */
-    break;
-  case 305: /* Use Proxy */
-    /* (quote from RFC2616, section 10.3.6):
-     * "The requested resource MUST be accessed through the proxy given
-     * by the Location field. The Location field gives the URI of the
-     * proxy. The recipient is expected to repeat this single request
-     * via the proxy. 305 responses MUST only be generated by origin
-     * servers."
-     */
-    break;
-  }
-  Curl_pgrsTime(data, TIMER_REDIRECT);
-  Curl_pgrsResetTransferSizes(data);
-
-  return CURLE_OK;
-#endif /* CURL_DISABLE_HTTP */
 }
 
 static CURLMcode state_performing(struct Curl_easy *data,
@@ -2236,6 +1960,7 @@ static CURLMcode state_performing(struct Curl_easy *data,
     multi_done(data, result, TRUE);
   }
   else if(data->req.done && !Curl_cwriter_is_paused(data)) {
+    const struct Curl_handler *handler = data->conn->handler;
 
     /* call this even if the readwrite function returned error */
     multi_posttransfer(data);
@@ -2256,7 +1981,7 @@ static CURLMcode state_performing(struct Curl_easy *data,
         follow = FOLLOW_RETRY;
       (void)multi_done(data, CURLE_OK, FALSE);
       /* multi_done() might return CURLE_GOT_NOTHING */
-      result = multi_follow(data, newurl, follow);
+      result = multi_follow(data, handler, newurl, follow);
       if(!result) {
         multistate(data, MSTATE_SETUP);
         rc = CURLM_CALL_MULTI_PERFORM;
@@ -2271,7 +1996,7 @@ static CURLMcode state_performing(struct Curl_easy *data,
         free(newurl);
         newurl = data->req.location;
         data->req.location = NULL;
-        result = multi_follow(data, newurl, FOLLOW_FAKE);
+        result = multi_follow(data, handler, newurl, FOLLOW_FAKE);
         if(result) {
           *stream_errorp = TRUE;
           result = multi_done(data, result, TRUE);
@@ -2380,6 +2105,7 @@ static CURLMcode state_do(struct Curl_easy *data,
        * unexpectedly died. If possible, send the connection back to the
        * CONNECT phase so we can try again.
        */
+      const struct Curl_handler *handler = data->conn->handler;
       char *newurl = NULL;
       followtype follow = FOLLOW_NONE;
       CURLcode drc;
@@ -2399,7 +2125,7 @@ static CURLMcode state_do(struct Curl_easy *data,
       if(newurl) {
         if(!drc || (drc == CURLE_SEND_ERROR)) {
           follow = FOLLOW_RETRY;
-          drc = multi_follow(data, newurl, follow);
+          drc = multi_follow(data, handler, newurl, follow);
           if(!drc) {
             multistate(data, MSTATE_SETUP);
             rc = CURLM_CALL_MULTI_PERFORM;

lib/openldap.c

@@ -134,6 +134,7 @@ const struct Curl_handler Curl_handler_ldap = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_LDAP,                            /* defport */
   CURLPROTO_LDAP,                       /* protocol */
   CURLPROTO_LDAP,                       /* family */
@@ -163,6 +164,7 @@ const struct Curl_handler Curl_handler_ldaps = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_LDAPS,                           /* defport */
   CURLPROTO_LDAPS,                      /* protocol */
   CURLPROTO_LDAP,                       /* family */

lib/pop3.c

@@ -138,6 +138,7 @@ const struct Curl_handler Curl_handler_pop3 = {
   ZERO_NULL,                        /* write_resp_hd */
   ZERO_NULL,                        /* connection_check */
   ZERO_NULL,                        /* attach connection */
+  ZERO_NULL,                        /* follow */
   PORT_POP3,                        /* defport */
   CURLPROTO_POP3,                   /* protocol */
   CURLPROTO_POP3,                   /* family */
@@ -168,6 +169,7 @@ const struct Curl_handler Curl_handler_pop3s = {
   ZERO_NULL,                        /* write_resp_hd */
   ZERO_NULL,                        /* connection_check */
   ZERO_NULL,                        /* attach connection */
+  ZERO_NULL,                        /* follow */
   PORT_POP3S,                       /* defport */
   CURLPROTO_POP3S,                  /* protocol */
   CURLPROTO_POP3,                   /* family */

lib/rtsp.c

@@ -117,6 +117,7 @@ const struct Curl_handler Curl_handler_rtsp = {
   ZERO_NULL,                            /* write_resp_hd */
   rtsp_conncheck,                       /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  Curl_http_follow,                     /* follow */
   PORT_RTSP,                            /* defport */
   CURLPROTO_RTSP,                       /* protocol */
   CURLPROTO_RTSP,                       /* family */

lib/smb.c

@@ -273,6 +273,7 @@ const struct Curl_handler Curl_handler_smb = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_SMB,                             /* defport */
   CURLPROTO_SMB,                        /* protocol */
   CURLPROTO_SMB,                        /* family */
@@ -301,6 +302,7 @@ const struct Curl_handler Curl_handler_smbs = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_SMBS,                            /* defport */
   CURLPROTO_SMBS,                       /* protocol */
   CURLPROTO_SMB,                        /* family */

lib/smtp.c

@@ -135,6 +135,7 @@ const struct Curl_handler Curl_handler_smtp = {
   ZERO_NULL,                        /* write_resp_hd */
   ZERO_NULL,                        /* connection_check */
   ZERO_NULL,                        /* attach connection */
+  ZERO_NULL,                        /* follow */
   PORT_SMTP,                        /* defport */
   CURLPROTO_SMTP,                   /* protocol */
   CURLPROTO_SMTP,                   /* family */
@@ -165,6 +166,7 @@ const struct Curl_handler Curl_handler_smtps = {
   ZERO_NULL,                        /* write_resp_hd */
   ZERO_NULL,                        /* connection_check */
   ZERO_NULL,                        /* attach connection */
+  ZERO_NULL,                        /* follow */
   PORT_SMTPS,                       /* defport */
   CURLPROTO_SMTPS,                  /* protocol */
   CURLPROTO_SMTP,                   /* family */

lib/telnet.c

@@ -190,6 +190,7 @@ const struct Curl_handler Curl_handler_telnet = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_TELNET,                          /* defport */
   CURLPROTO_TELNET,                     /* protocol */
   CURLPROTO_TELNET,                     /* family */

lib/tftp.c

@@ -185,6 +185,7 @@ const struct Curl_handler Curl_handler_tftp = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_TFTP,                            /* defport */
   CURLPROTO_TFTP,                       /* protocol */
   CURLPROTO_TFTP,                       /* family */

lib/transfer.h

@@ -33,15 +33,6 @@ void Curl_init_CONNECT(struct Curl_easy *data);
 
 CURLcode Curl_pretransfer(struct Curl_easy *data);
 
-typedef enum {
-  FOLLOW_NONE,  /* not used within the function, just a placeholder to
-                   allow initing to this */
-  FOLLOW_FAKE,  /* only records stuff, not actually following */
-  FOLLOW_RETRY, /* set if this is a request retry as opposed to a real
-                   redirect following */
-  FOLLOW_REDIR /* a full true redirect */
-} followtype;
-
 CURLcode Curl_sendrecv(struct Curl_easy *data, struct curltime *nowp);
 int Curl_single_getsock(struct Curl_easy *data,
                         struct connectdata *conn, curl_socket_t *socks);

lib/urldata.h

@@ -676,6 +676,12 @@ struct Curl_handler {
   /* attach() attaches this transfer to this connection */
   void (*attach)(struct Curl_easy *data, struct connectdata *conn);
 
+  /* return CURLE_OK if a redirect to `newurl` should be followed,
+     CURLE_TOO_MANY_REDIRECTS otherwise. May alter `data` to change
+     the way the follow request is performed. */
+  CURLcode (*follow)(struct Curl_easy *data, const char *newurl,
+                     followtype type);
+
   int defport;            /* Default port. */
   curl_prot_t protocol;  /* See CURLPROTO_* - this needs to be the single
                             specific protocol bit */

lib/vssh/libssh.c

@@ -161,6 +161,7 @@ const struct Curl_handler Curl_handler_scp = {
   ZERO_NULL,                    /* write_resp_hd */
   ZERO_NULL,                    /* connection_check */
   ZERO_NULL,                    /* attach connection */
+  ZERO_NULL,                    /* follow */
   PORT_SSH,                     /* defport */
   CURLPROTO_SCP,                /* protocol */
   CURLPROTO_SCP,                /* family */
@@ -189,6 +190,7 @@ const struct Curl_handler Curl_handler_sftp = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_SSH,                             /* defport */
   CURLPROTO_SFTP,                       /* protocol */
   CURLPROTO_SFTP,                       /* family */

lib/vssh/libssh2.c

@@ -139,6 +139,7 @@ const struct Curl_handler Curl_handler_scp = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ssh_attach,                           /* attach */
+  ZERO_NULL,                            /* follow */
   PORT_SSH,                             /* defport */
   CURLPROTO_SCP,                        /* protocol */
   CURLPROTO_SCP,                        /* family */
@@ -169,6 +170,7 @@ const struct Curl_handler Curl_handler_sftp = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ssh_attach,                           /* attach */
+  ZERO_NULL,                            /* follow */
   PORT_SSH,                             /* defport */
   CURLPROTO_SFTP,                       /* protocol */
   CURLPROTO_SFTP,                       /* family */

lib/vssh/wolfssh.c

@@ -95,6 +95,7 @@ const struct Curl_handler Curl_handler_scp = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_SSH,                             /* defport */
   CURLPROTO_SCP,                        /* protocol */
   PROTOPT_DIRLOCK | PROTOPT_CLOSEACTION
@@ -125,6 +126,7 @@ const struct Curl_handler Curl_handler_sftp = {
   ZERO_NULL,                            /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  ZERO_NULL,                            /* follow */
   PORT_SSH,                             /* defport */
   CURLPROTO_SFTP,                       /* protocol */
   CURLPROTO_SFTP,                       /* family */

lib/ws.c

@@ -1334,6 +1334,7 @@ const struct Curl_handler Curl_handler_ws = {
   Curl_http_write_resp_hd,              /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  Curl_http_follow,                     /* follow */
   PORT_HTTP,                            /* defport */
   CURLPROTO_WS,                         /* protocol */
   CURLPROTO_HTTP,                       /* family */
@@ -1360,6 +1361,7 @@ const struct Curl_handler Curl_handler_wss = {
   Curl_http_write_resp_hd,              /* write_resp_hd */
   ZERO_NULL,                            /* connection_check */
   ZERO_NULL,                            /* attach connection */
+  Curl_http_follow,                     /* follow */
   PORT_HTTPS,                           /* defport */
   CURLPROTO_WSS,                        /* protocol */
   CURLPROTO_HTTP,                       /* family */