sasl: Implement SASL authorisation identity via CURLOPT_SASL_AUTHZID

Added the ability for the calling program to specify the authorisation
identity (authzid), the identity to act as, in addition to the
authentication identity (authcid) and password when using SASL PLAIN
authentication.

Fixes #3653
Closes #3790

NOTE: This commit was cherry-picked and is part of a series of commits
that added the authzid feature for upcoming 7.66.0. The series was
temporarily reverted in db8ec1f so that it would not ship in a 7.65.x
patch release.

Closes https://github.com/curl/curl/pull/4186

lib/curl_sasl.c

@@ -370,8 +370,9 @@ CURLcode Curl_sasl_start(struct SASL *sasl, struct connectdata *conn,
       sasl->authused = SASL_MECH_PLAIN;
 
       if(force_ir || data->set.sasl_ir)
-        result = Curl_auth_create_plain_message(data, NULL, conn->user,
-                                                conn->passwd, &resp, &len);
+        result = Curl_auth_create_plain_message(data, conn->sasl_authzid,
+                                                conn->user, conn->passwd,
+                                                &resp, &len);
     }
     else if(enabledmechs & SASL_MECH_LOGIN) {
       mech = SASL_MECH_STRING_LOGIN;
@@ -453,8 +454,9 @@ CURLcode Curl_sasl_continue(struct SASL *sasl, struct connectdata *conn,
     *progress = SASL_DONE;
     return result;
   case SASL_PLAIN:
-    result = Curl_auth_create_plain_message(data, NULL, conn->user,
-                                            conn->passwd, &resp, &len);
+    result = Curl_auth_create_plain_message(data, conn->sasl_authzid,
+                                            conn->user, conn->passwd,
+                                            &resp, &len);
     break;
   case SASL_LOGIN:
     result = Curl_auth_create_login_message(data, conn->user, &resp, &len);

lib/setopt.c

@@ -2402,6 +2402,12 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
     break;
 #endif
 
+  case CURLOPT_SASL_AUTHZID:
+    /* Authorisation identity (identity to act as) */
+    result = Curl_setstropt(&data->set.str[STRING_SASL_AUTHZID],
+                            va_arg(param, char *));
+    break;
+
   case CURLOPT_SASL_IR:
     /* Enable/disable SASL initial response */
     data->set.sasl_ir = (0 != va_arg(param, long)) ? TRUE : FALSE;

lib/url.c

@@ -715,6 +715,7 @@ static void conn_free(struct connectdata *conn)
   Curl_safefree(conn->user);
   Curl_safefree(conn->passwd);
   Curl_safefree(conn->oauth_bearer);
+  Curl_safefree(conn->sasl_authzid);
   Curl_safefree(conn->options);
   Curl_safefree(conn->http_proxy.user);
   Curl_safefree(conn->socks_proxy.user);
@@ -3492,6 +3493,14 @@ static CURLcode create_conn(struct Curl_easy *data,
     }
   }
 
+  if(data->set.str[STRING_SASL_AUTHZID]) {
+    conn->sasl_authzid = strdup(data->set.str[STRING_SASL_AUTHZID]);
+    if(!conn->sasl_authzid) {
+      result = CURLE_OUT_OF_MEMORY;
+      goto out;
+    }
+  }
+
 #ifdef USE_UNIX_SOCKETS
   if(data->set.str[STRING_UNIX_SOCKET_PATH]) {
     conn->unix_domain_socket = strdup(data->set.str[STRING_UNIX_SOCKET_PATH]);

lib/urldata.h

@@ -875,7 +875,8 @@ struct connectdata {
   char *passwd;  /* password string, allocated */
   char *options; /* options string, allocated */
 
-  char *oauth_bearer; /* bearer token for OAuth 2.0, allocated */
+  char *oauth_bearer;     /* bearer token for OAuth 2.0, allocated */
+  char *sasl_authzid;     /* authorisation identity string, allocated */
 
   int httpversion;        /* the HTTP version*10 reported by the server */
   int rtspversion;        /* the RTSP version*10 reported by the server */
@@ -1498,6 +1499,7 @@ enum dupstring {
 #ifdef USE_ALTSVC
   STRING_ALTSVC,                /* CURLOPT_ALTSVC */
 #endif
+  STRING_SASL_AUTHZID,          /* CURLOPT_SASL_AUTHZID */
   /* -- end of zero-terminated strings -- */
 
   STRING_LASTZEROTERMINATED,