diff --git a/lib/ajax/service.php b/lib/ajax/service.php
index e72f1650ef2..d3d23ab9d77 100644
--- a/lib/ajax/service.php
+++ b/lib/ajax/service.php
@@ -41,66 +41,15 @@ if ($requests === null) {
 }
 $responses = array();
 
-
 foreach ($requests as $request) {
     $response = array();
     $methodname = clean_param($request['methodname'], PARAM_ALPHANUMEXT);
     $index = clean_param($request['index'], PARAM_INT);
     $args = $request['args'];
 
-    try {
-        $externalfunctioninfo = external_function_info($methodname);
-
-        if (!$externalfunctioninfo->allowed_from_ajax) {
-            error_log('This external function is not available to ajax. Failed to call "' . $methodname . '"');
-            throw new moodle_exception('servicenotavailable', 'webservice');
-        }
-
-        // Do not allow access to write or delete webservices as a public user.
-        if ($externalfunctioninfo->loginrequired) {
-            if (defined('NO_MOODLE_COOKIES') && NO_MOODLE_COOKIES) {
-                error_log('Set "loginrequired" to false in db/service.php when calling entry point service-nologin.php. ' .
-                          'Failed to call "' . $methodname . '"');
-                throw new moodle_exception('servicenotavailable', 'webservice');
-            }
-            if (!isloggedin()) {
-                error_log('This external function is not available to public users. Failed to call "' . $methodname . '"');
-                throw new moodle_exception('servicenotavailable', 'webservice');
-            } else {
-                require_sesskey();
-            }
-        }
-
-        // Validate params, this also sorts the params properly, we need the correct order in the next part.
-        $callable = array($externalfunctioninfo->classname, 'validate_parameters');
-        $params = call_user_func($callable,
-                                 $externalfunctioninfo->parameters_desc,
-                                 $args);
-
-        // Execute - gulp!
-        $callable = array($externalfunctioninfo->classname, $externalfunctioninfo->methodname);
-        $result = call_user_func_array($callable,
-                                       array_values($params));
-
-        // Validate the return parameters.
-        if ($externalfunctioninfo->returns_desc !== null) {
-            $callable = array($externalfunctioninfo->classname, 'clean_returnvalue');
-            $result = call_user_func($callable, $externalfunctioninfo->returns_desc, $result);
-        }
-
-        $response['error'] = false;
-        $response['data'] = $result;
-        $responses[$index] = $response;
-    } catch (Exception $e) {
-        $jsonexception = get_exception_info($e);
-        unset($jsonexception->a);
-        if (!debugging('', DEBUG_DEVELOPER)) {
-            unset($jsonexception->debuginfo);
-            unset($jsonexception->backtrace);
-        }
-        $response['error'] = true;
-        $response['exception'] = $jsonexception;
-        $responses[$index] = $response;
+    $response = external_api::call_external_function($methodname, $args, true);
+    $responses[$index] = $response;
+    if ($response['error']) {
         // Do not process the remaining requests.
         break;
     }
diff --git a/lib/externallib.php b/lib/externallib.php
index 002d5727d22..22b07d3df77 100644
--- a/lib/externallib.php
+++ b/lib/externallib.php
@@ -35,98 +35,7 @@ defined('MOODLE_INTERNAL') || die();
  * @since Moodle 2.0
  */
 function external_function_info($function, $strictness=MUST_EXIST) {
-    global $DB, $CFG;
-
-    if (!is_object($function)) {
-        if (!$function = $DB->get_record('external_functions', array('name'=>$function), '*', $strictness)) {
-            return false;
-        }
-    }
-
-    // First try class autoloading.
-    if (!class_exists($function->classname)) {
-        // Fallback to explicit include of externallib.php.
-        $function->classpath = empty($function->classpath) ? core_component::get_component_directory($function->component).'/externallib.php' : $CFG->dirroot.'/'.$function->classpath;
-        if (!file_exists($function->classpath)) {
-            throw new coding_exception('Cannot find file with external function implementation: ' . $function->classname);
-        }
-        require_once($function->classpath);
-        if (!class_exists($function->classname)) {
-            throw new coding_exception('Cannot find external class');
-        }
-    }
-
-    $function->ajax_method = $function->methodname.'_is_allowed_from_ajax';
-    $function->parameters_method = $function->methodname.'_parameters';
-    $function->returns_method    = $function->methodname.'_returns';
-    $function->deprecated_method = $function->methodname.'_is_deprecated';
-
-    // make sure the implementaion class is ok
-    if (!method_exists($function->classname, $function->methodname)) {
-        throw new coding_exception('Missing implementation method of '.$function->classname.'::'.$function->methodname);
-    }
-    if (!method_exists($function->classname, $function->parameters_method)) {
-        throw new coding_exception('Missing parameters description');
-    }
-    if (!method_exists($function->classname, $function->returns_method)) {
-        throw new coding_exception('Missing returned values description');
-    }
-    if (method_exists($function->classname, $function->deprecated_method)) {
-        if (call_user_func(array($function->classname, $function->deprecated_method)) === true) {
-            $function->deprecated = true;
-        }
-    }
-    $function->allowed_from_ajax = false;
-
-    // fetch the parameters description
-    $function->parameters_desc = call_user_func(array($function->classname, $function->parameters_method));
-    if (!($function->parameters_desc instanceof external_function_parameters)) {
-        throw new coding_exception('Invalid parameters description');
-    }
-
-    // fetch the return values description
-    $function->returns_desc = call_user_func(array($function->classname, $function->returns_method));
-    // null means void result or result is ignored
-    if (!is_null($function->returns_desc) and !($function->returns_desc instanceof external_description)) {
-        throw new coding_exception('Invalid return description');
-    }
-
-    //now get the function description
-    //TODO MDL-31115 use localised lang pack descriptions, it would be nice to have
-    //      easy to understand descriptions in admin UI,
-    //      on the other hand this is still a bit in a flux and we need to find some new naming
-    //      conventions for these descriptions in lang packs
-    $function->description = null;
-    $servicesfile = core_component::get_component_directory($function->component).'/db/services.php';
-    if (file_exists($servicesfile)) {
-        $functions = null;
-        include($servicesfile);
-        if (isset($functions[$function->name]['description'])) {
-            $function->description = $functions[$function->name]['description'];
-        }
-        if (isset($functions[$function->name]['testclientpath'])) {
-            $function->testclientpath = $functions[$function->name]['testclientpath'];
-        }
-        if (isset($functions[$function->name]['type'])) {
-            $function->type = $functions[$function->name]['type'];
-        }
-        if (isset($functions[$function->name]['ajax'])) {
-            $function->allowed_from_ajax = $functions[$function->name]['ajax'];
-        } else if (method_exists($function->classname, $function->ajax_method)) {
-            if (call_user_func(array($function->classname, $function->ajax_method)) === true) {
-                debugging('External function ' . $function->ajax_method . '() function is deprecated.' .
-                          'Set ajax=>true in db/service.php instead.', DEBUG_DEVELOPER);
-                $function->allowed_from_ajax = true;
-            }
-        }
-        if (isset($functions[$function->name]['loginrequired'])) {
-            $function->loginrequired = $functions[$function->name]['loginrequired'];
-        } else {
-            $function->loginrequired = true;
-        }
-    }
-
-    return $function;
+    return external_api::external_function_info($function, $strictness);
 }
 
 /**
@@ -161,6 +70,195 @@ class external_api {
     /** @var stdClass context where the function calls will be restricted */
     private static $contextrestriction;
 
+    /**
+     * Returns detailed function information
+     *
+     * @param string|object $function name of external function or record from external_function
+     * @param int $strictness IGNORE_MISSING means compatible mode, false returned if record not found, debug message if more found;
+     *                        MUST_EXIST means throw exception if no record or multiple records found
+     * @return stdClass description or false if not found or exception thrown
+     * @since Moodle 2.0
+     */
+    public static function external_function_info($function, $strictness=MUST_EXIST) {
+        global $DB, $CFG;
+
+        if (!is_object($function)) {
+            if (!$function = $DB->get_record('external_functions', array('name' => $function), '*', $strictness)) {
+                return false;
+            }
+        }
+
+        // First try class autoloading.
+        if (!class_exists($function->classname)) {
+            // Fallback to explicit include of externallib.php.
+            if (empty($function->classpath)) {
+                $function->classpath = core_component::get_component_directory($function->component).'/externallib.php';
+            } else {
+                $function->classpath = $CFG->dirroot.'/'.$function->classpath;
+            }
+            if (!file_exists($function->classpath)) {
+                throw new coding_exception('Cannot find file with external function implementation');
+            }
+            require_once($function->classpath);
+            if (!class_exists($function->classname)) {
+                throw new coding_exception('Cannot find external class');
+            }
+        }
+
+        $function->ajax_method = $function->methodname.'_is_allowed_from_ajax';
+        $function->parameters_method = $function->methodname.'_parameters';
+        $function->returns_method    = $function->methodname.'_returns';
+        $function->deprecated_method = $function->methodname.'_is_deprecated';
+
+        // Make sure the implementaion class is ok.
+        if (!method_exists($function->classname, $function->methodname)) {
+            throw new coding_exception('Missing implementation method of '.$function->classname.'::'.$function->methodname);
+        }
+        if (!method_exists($function->classname, $function->parameters_method)) {
+            throw new coding_exception('Missing parameters description');
+        }
+        if (!method_exists($function->classname, $function->returns_method)) {
+            throw new coding_exception('Missing returned values description');
+        }
+        if (method_exists($function->classname, $function->deprecated_method)) {
+            if (call_user_func(array($function->classname, $function->deprecated_method)) === true) {
+                $function->deprecated = true;
+            }
+        }
+        $function->allowed_from_ajax = false;
+
+        // Fetch the parameters description.
+        $function->parameters_desc = call_user_func(array($function->classname, $function->parameters_method));
+        if (!($function->parameters_desc instanceof external_function_parameters)) {
+            throw new coding_exception('Invalid parameters description');
+        }
+
+        // Fetch the return values description.
+        $function->returns_desc = call_user_func(array($function->classname, $function->returns_method));
+        // Null means void result or result is ignored.
+        if (!is_null($function->returns_desc) and !($function->returns_desc instanceof external_description)) {
+            throw new coding_exception('Invalid return description');
+        }
+
+        // Now get the function description.
+
+        // TODO MDL-31115 use localised lang pack descriptions, it would be nice to have
+        // easy to understand descriptions in admin UI,
+        // on the other hand this is still a bit in a flux and we need to find some new naming
+        // conventions for these descriptions in lang packs.
+        $function->description = null;
+        $servicesfile = core_component::get_component_directory($function->component).'/db/services.php';
+        if (file_exists($servicesfile)) {
+            $functions = null;
+            include($servicesfile);
+            if (isset($functions[$function->name]['description'])) {
+                $function->description = $functions[$function->name]['description'];
+            }
+            if (isset($functions[$function->name]['testclientpath'])) {
+                $function->testclientpath = $functions[$function->name]['testclientpath'];
+            }
+            if (isset($functions[$function->name]['type'])) {
+                $function->type = $functions[$function->name]['type'];
+            }
+            if (isset($functions[$function->name]['ajax'])) {
+                $function->allowed_from_ajax = $functions[$function->name]['ajax'];
+            } else if (method_exists($function->classname, $function->ajax_method)) {
+                if (call_user_func(array($function->classname, $function->ajax_method)) === true) {
+                    debugging('External function ' . $function->ajax_method . '() function is deprecated.' .
+                              'Set ajax=>true in db/service.php instead.', DEBUG_DEVELOPER);
+                    $function->allowed_from_ajax = true;
+                }
+            }
+            if (isset($functions[$function->name]['loginrequired'])) {
+                $function->loginrequired = $functions[$function->name]['loginrequired'];
+            } else {
+                $function->loginrequired = true;
+            }
+        }
+
+        return $function;
+    }
+
+    /**
+     * Call an external function validating all params/returns correctly.
+     *
+     * Note that an external function may modify the state of the current page, so this wrapper
+     * saves and restores tha PAGE and COURSE global variables before/after calling the external function.
+     *
+     * @param string $function A webservice function name.
+     * @param array $args Params array (named params)
+     * @param boolean $ajaxonly If true, an extra check will be peformed to see if ajax is required.
+     * @return array containing keys for error (bool), exception and data.
+     */
+    public static function call_external_function($function, $args, $ajaxonly=false) {
+        global $PAGE, $COURSE, $CFG, $SITE;
+
+        require_once($CFG->libdir . "/pagelib.php");
+
+        $externalfunctioninfo = self::external_function_info($function);
+
+        $currentpage = $PAGE;
+        $currentcourse = $COURSE;
+        $response = array();
+
+        try {
+
+            $PAGE = new moodle_page();
+            $COURSE = clone($SITE);
+
+            if ($ajaxonly && !$externalfunctioninfo->allowed_from_ajax) {
+                throw new moodle_exception('servicenotavailable', 'webservice');
+            }
+
+            // Do not allow access to write or delete webservices as a public user.
+            if ($externalfunctioninfo->loginrequired) {
+                if (defined('NO_MOODLE_COOKIES') && NO_MOODLE_COOKIES && !PHPUNIT_TEST) {
+                    throw new moodle_exception('servicenotavailable', 'webservice');
+                }
+                if (!isloggedin()) {
+                    throw new moodle_exception('servicenotavailable', 'webservice');
+                } else {
+                    require_sesskey();
+                }
+            }
+
+            // Validate params, this also sorts the params properly, we need the correct order in the next part.
+            $callable = array($externalfunctioninfo->classname, 'validate_parameters');
+            $params = call_user_func($callable,
+                                     $externalfunctioninfo->parameters_desc,
+                                     $args);
+
+            // Execute - gulp!
+            $callable = array($externalfunctioninfo->classname, $externalfunctioninfo->methodname);
+            $result = call_user_func_array($callable,
+                                           array_values($params));
+
+            // Validate the return parameters.
+            if ($externalfunctioninfo->returns_desc !== null) {
+                $callable = array($externalfunctioninfo->classname, 'clean_returnvalue');
+                $result = call_user_func($callable, $externalfunctioninfo->returns_desc, $result);
+            }
+
+            $response['error'] = false;
+            $response['data'] = $result;
+        } catch (Exception $e) {
+            $exception = get_exception_info($e);
+            unset($exception->a);
+            if (!debugging('', DEBUG_DEVELOPER)) {
+                unset($exception->debuginfo);
+                unset($exception->backtrace);
+            }
+            $response['error'] = true;
+            $response['exception'] = $exception;
+            // Do not process the remaining requests.
+        }
+
+        $PAGE = $currentpage;
+        $COURSE = $currentcourse;
+
+        return $response;
+    }
+
     /**
      * Set context restriction for all following subsequent function calls.
      *
@@ -359,7 +457,7 @@ class external_api {
      * @since Moodle 2.0
      */
     public static function validate_context($context) {
-        global $CFG;
+        global $CFG, $PAGE;
 
         if (empty($context)) {
             throw new invalid_parameter_exception('Context does not exist');
@@ -382,10 +480,10 @@ class external_api {
             }
         }
 
-        if ($context->contextlevel >= CONTEXT_COURSE) {
-            list($context, $course, $cm) = get_context_info_array($context->id);
-            require_login($course, false, $cm, false, true);
-        }
+        $PAGE->reset_theme_and_output();
+        list($unused, $course, $cm) = get_context_info_array($context->id);
+        require_login($course, false, $cm, false, true);
+        $PAGE->set_context($context);
     }
 
     /**
diff --git a/lib/pagelib.php b/lib/pagelib.php
index 33466f59f2d..4145f8e7fe7 100644
--- a/lib/pagelib.php
+++ b/lib/pagelib.php
@@ -981,7 +981,6 @@ class moodle_page {
             }
             return;
         }
-
         // Ideally we should set context only once.
         if (isset($this->_context) && $context->id !== $this->_context->id) {
             $current = $this->_context->contextlevel;
@@ -993,11 +992,7 @@ class moodle_page {
             } else {
                 // We do not want devs to do weird switching of context levels on the fly because we might have used
                 // the context already such as in text filter in page title.
-                // This is explicitly allowed for webservices though which may
-                // call "external_api::validate_context on many contexts in a single request.
-                if (!WS_SERVER) {
-                    debugging("Coding problem: unsupported modification of PAGE->context from {$current} to {$context->contextlevel}");
-                }
+                debugging("Coding problem: unsupported modification of PAGE->context from {$current} to {$context->contextlevel}");
             }
         }
 
@@ -1560,6 +1555,22 @@ class moodle_page {
         $this->_wherethemewasinitialised = debug_backtrace();
     }
 
+    /**
+     * Reset the theme and output for a new context. This only makes sense from
+     * external::validate_context(). Do not cheat.
+     *
+     * @return string the name of the theme that should be used on this page.
+     */
+    public function reset_theme_and_output() {
+        global $COURSE, $SITE;
+
+        $COURSE = clone($SITE);
+        $this->_theme = null;
+        $this->_wherethemewasinitialised = null;
+        $this->_course = null;
+        $this->_context = null;
+    }
+
     /**
      * Work out the theme this page should use.
      *
diff --git a/lib/tests/externallib_test.php b/lib/tests/externallib_test.php
index b3b0198046c..e19dd2f6ec4 100644
--- a/lib/tests/externallib_test.php
+++ b/lib/tests/externallib_test.php
@@ -354,6 +354,46 @@ class core_externallib_testcase extends advanced_testcase {
         // The extra course passed is not returned.
         $this->assertArrayNotHasKey($c4->id, $courses);
     }
+
+
+    public function test_call_external_function() {
+        global $PAGE, $COURSE;
+
+        $this->resetAfterTest(true);
+
+        // Call some webservice functions and verify they are correctly handling $PAGE and $COURSE.
+        // First test a function that calls validate_context outside a course.
+        $this->setAdminUser();
+        $category = $this->getDataGenerator()->create_category();
+        $params = array(
+            'contextid' => context_coursecat::instance($category->id)->id,
+            'name' => 'aaagrrryyy',
+            'idnumber' => '',
+            'description' => ''
+        );
+        $cohort1 = $this->getDataGenerator()->create_cohort($params);
+        $cohort2 = $this->getDataGenerator()->create_cohort();
+
+        $beforepage = $PAGE;
+        $beforecourse = $COURSE;
+        $params = array('cohortids' => array($cohort1->id, $cohort2->id));
+        $result = external_api::call_external_function('core_cohort_get_cohorts', $params);
+
+        $this->assertSame($beforepage, $PAGE);
+        $this->assertSame($beforecourse, $COURSE);
+
+        // Now test a function that calls validate_context inside a course.
+        $course = $this->getDataGenerator()->create_course();
+
+        $beforepage = $PAGE;
+        $beforecourse = $COURSE;
+        $params = array('courseid' => $course->id, 'options' => array());
+        $result = external_api::call_external_function('core_enrol_get_enrolled_users', $params);
+
+        $this->assertSame($beforepage, $PAGE);
+        $this->assertSame($beforecourse, $COURSE);
+    }
+
 }
 
 /*
diff --git a/lib/upgrade.txt b/lib/upgrade.txt
index cb4b8aa0f14..43a5ad0e8d5 100644
--- a/lib/upgrade.txt
+++ b/lib/upgrade.txt
@@ -3,6 +3,8 @@ information provided here is intended especially for developers.
 
 === 3.1 ===
 
+* External functions that are not calling external_api::validate_context are buggy and will now generate
+  exceptions. Previously they were only generating warnings in the webserver error log.
 * The moodle/blog:associatecourse and moodle/blog:associatemodule capabilities has been removed.
 * The following functions has been finally deprecated and can not be used any more:
     - profile_display_badges()
diff --git a/mod/forum/tests/externallib_test.php b/mod/forum/tests/externallib_test.php
index a7ce2b040fa..371b664962d 100644
--- a/mod/forum/tests/externallib_test.php
+++ b/mod/forum/tests/externallib_test.php
@@ -346,10 +346,6 @@ class mod_forum_external_testcase extends externallib_advanced_testcase {
         $discussions = mod_forum_external::get_forum_discussions(array($forum1->id, $forum2->id));
         $discussions = external_api::clean_returnvalue(mod_forum_external::get_forum_discussions_returns(), $discussions);
         $this->assertEquals($expecteddiscussions, $discussions);
-        // Some debugging is going to be produced, this is because we switch PAGE contexts in the get_forum_discussions function,
-        // the switch happens when the validate_context function is called inside a foreach loop.
-        // See MDL-41746 for more information.
-        $this->assertDebuggingCalled();
 
         // Remove the users post from the qanda forum and ensure they can still see the discussion.
         $DB->delete_records('forum_posts', array('id' => $discussion2reply1->id));
@@ -365,7 +361,6 @@ class mod_forum_external_testcase extends externallib_advanced_testcase {
         } catch (moodle_exception $e) {
             $this->assertEquals('nopermissions', $e->errorcode);
         }
-        $this->assertDebuggingCalled();
 
         // Unenrol user from second course.
         $enrol->unenrol_user($instance2, $user1->id);
@@ -857,8 +852,6 @@ class mod_forum_external_testcase extends externallib_advanced_testcase {
             mod_forum_external::add_discussion_post($discussion->firstpost, 'some subject', 'some text here...');
             $this->fail('Exception expected due to invalid permissions for posting.');
         } catch (moodle_exception $e) {
-            // Expect debugging since we are switching context, and this is something WS_SERVER mode don't like.
-            $this->assertDebuggingCalled();
             $this->assertEquals('nopostforum', $e->errorcode);
         }
 
diff --git a/user/tests/externallib_test.php b/user/tests/externallib_test.php
index ebbb290089a..6172453b15d 100644
--- a/user/tests/externallib_test.php
+++ b/user/tests/externallib_test.php
@@ -818,6 +818,7 @@ class core_user_externallib_testcase extends externallib_advanced_testcase {
         $device2['pushid'] = "0987654321";
         $device2['appid'] = "other.app.com";
 
+        $this->setAdminUser();
         // Create a user device using the external API function.
         core_user_external::add_user_device($device['appid'], $device['name'], $device['model'], $device['platform'],
                                             $device['version'], $device['pushid'], $device['uuid']);