Introduce IERS_A_URL_MIRROR for when IERS_A_URL is down

CHANGES.rst

@@ -1782,6 +1782,9 @@ astropy.units
 astropy.utils
 ^^^^^^^^^^^^^
 
+- Fix failures caused by IERS_A_URL being unavailable by introducing
+  IERS_A_URL_MIRROR. [#8308]
+
 astropy.visualization
 ^^^^^^^^^^^^^^^^^^^^^
 

astropy/utils/iers/iers.py

@@ -29,13 +29,14 @@
            'IERS', 'IERS_B', 'IERS_A', 'IERS_Auto',
            'FROM_IERS_B', 'FROM_IERS_A', 'FROM_IERS_A_PREDICTION',
            'TIME_BEFORE_IERS_RANGE', 'TIME_BEYOND_IERS_RANGE',
-           'IERS_A_FILE', 'IERS_A_URL', 'IERS_A_README',
+           'IERS_A_FILE', 'IERS_A_URL', 'IERS_A_URL_MIRROR', 'IERS_A_README',
            'IERS_B_FILE', 'IERS_B_URL', 'IERS_B_README',
            'IERSRangeError', 'IERSStaleWarning']
 
 # IERS-A default file name, URL, and ReadMe with content description
 IERS_A_FILE = 'finals2000A.all'
 IERS_A_URL = 'http://maia.usno.navy.mil/ser7/finals2000A.all'
+IERS_A_URL_MIRROR = 'http://toshi.nofs.navy.mil/ser7/finals2000A.all'
 IERS_A_README = get_pkg_data_filename('data/ReadMe.finals2000A')
 
 # IERS-B default file name, URL, and ReadMe with content description
@@ -96,6 +97,9 @@ class Conf(_config.ConfigNamespace):
     iers_auto_url = _config.ConfigItem(
         IERS_A_URL,
         'URL for auto-downloading IERS file data.')
+    iers_auto_url_mirror = _config.ConfigItem(
+        IERS_A_URL_MIRROR,
+        'Mirror URL for auto-downloading IERS file data.')
     remote_timeout = _config.ConfigItem(
         10.0,
         'Remote timeout downloading IERS file data (seconds).')
@@ -399,8 +403,8 @@ class IERS_A(IERS):
     Notes
     -----
     The IERS A file is not part of astropy.  It can be downloaded from
-    ``iers.IERS_A_URL``.  See ``iers.__doc__`` for instructions on how to use
-    it in ``Time``, etc.
+    ``iers.IERS_A_URL`` or ``iers.IERS_A_URL_MIRROR``. See ``iers.__doc__``
+    for instructions on use in ``Time``, etc.
     """
 
     iers_table = None
@@ -621,27 +625,40 @@ def open(cls):
             cls.iers_table = IERS.open()
             return cls.iers_table
 
+        all_urls = (conf.iers_auto_url, conf.iers_auto_url_mirror)
+
         if cls.iers_table is not None:
 
             # If the URL has changed, we need to redownload the file, so we
             # should ignore the internally cached version.
 
-            if cls.iers_table.meta.get('data_url') == conf.iers_auto_url:
+            if cls.iers_table.meta.get('data_url') in all_urls:
                 return cls.iers_table
 
-        try:
-            filename = download_file(conf.iers_auto_url, cache=True)
-        except Exception as err:
+        dl_success = False
+        err_list = []
+
+        for url in all_urls:
+            try:
+                filename = download_file(url, cache=True)
+            except Exception as err:
+                err_list.append(str(err))
+            else:
+                dl_success = True
+                break
+
+        if not dl_success:
             # Issue a warning here, perhaps user is offline.  An exception
             # will be raised downstream when actually trying to interpolate
             # predictive values.
             warn(AstropyWarning('failed to download {}, using local IERS-B: {}'
-                                .format(conf.iers_auto_url, str(err))))
+                                .format(' and '.join(all_urls),
+                                        ';'.join(err_list))))  # noqa
             cls.iers_table = IERS.open()
             return cls.iers_table
 
         cls.iers_table = cls.read(file=filename)
-        cls.iers_table.meta['data_url'] = str(conf.iers_auto_url)
+        cls.iers_table.meta['data_url'] = str(url)
 
         return cls.iers_table
 
@@ -693,13 +710,24 @@ def _refresh_table_as_needed(self, mjd):
             raise ValueError('IERS auto_max_age configuration value must be larger than 10 days')
 
         if (max_input_mjd > predictive_mjd and
-               now_mjd - predictive_mjd > auto_max_age):
+                now_mjd - predictive_mjd > auto_max_age):
+
+            all_urls = (conf.iers_auto_url, conf.iers_auto_url_mirror)
+            dl_success = False
+            err_list = []
 
             # Get the latest version
-            try:
-                clear_download_cache(conf.iers_auto_url)
-                filename = download_file(conf.iers_auto_url, cache=True)
-            except Exception as err:
+            for url in all_urls:
+                try:
+                    clear_download_cache(url)
+                    filename = download_file(url, cache=True)
+                except Exception as err:
+                    err_list.append(str(err))
+                else:
+                    dl_success = True
+                    break
+
+            if not dl_success:
                 # Issue a warning here, perhaps user is offline.  An exception
                 # will be raised downstream when actually trying to interpolate
                 # predictive values.
@@ -708,10 +736,11 @@ def _refresh_table_as_needed(self, mjd):
                                     'not covered by the available IERS file.  See the '
                                     '"IERS data access" section of the astropy documentation '
                                     'for additional information on working offline.'
-                                    .format(conf.iers_auto_url, str(err))))
+                                    .format(' and '.join(all_urls), ';'.join(err_list))))
                 return
 
             new_table = self.__class__.read(file=filename)
+            new_table.meta['data_url'] = str(url)
 
             # New table has new values?
             if new_table['MJD'][-1] > self['MJD'][-1]:

astropy/utils/iers/tests/test_iers.py

@@ -200,15 +200,16 @@ def test_interpolate_error_formatting(self):
         IERS_Auto._check_interpolate_indices() is formatted correctly.
         """
         with iers.conf.set_temp('iers_auto_url', self.iers_a_url_1):
-            with iers.conf.set_temp('auto_max_age', self.ame):
-                with pytest.raises(ValueError) as err:
-                    iers_table = iers.IERS_Auto.open()
-                    with warnings.catch_warnings():
-                        # Ignoring this if it comes up -- IERS_Auto predictive
-                        # values are older than 30.0 days but downloading the
-                        # latest table did not find newer values
-                        warnings.simplefilter('ignore', iers.IERSStaleWarning)
-                        iers_table.ut1_utc(self.t.jd1, self.t.jd2)
+            with iers.conf.set_temp('iers_auto_url_mirror', self.iers_a_url_1):
+                with iers.conf.set_temp('auto_max_age', self.ame):
+                    with pytest.raises(ValueError) as err:
+                        iers_table = iers.IERS_Auto.open()
+                        with warnings.catch_warnings():
+                            # Ignoring this if it comes up -- IERS_Auto predictive
+                            # values are older than 30.0 days but downloading the
+                            # latest table did not find newer values
+                            warnings.simplefilter('ignore', iers.IERSStaleWarning)
+                            iers_table.ut1_utc(self.t.jd1, self.t.jd2)
         assert str(err.value) == iers.INTERPOLATE_ERROR.format(self.ame)
 
     def test_auto_max_age_none(self):

docs/utils/iers.rst

@@ -81,6 +81,9 @@ of the automatic IERS downloading:
   iers_auto_url:
     URL for auto-downloading IERS file data
 
+  iers_auto_url_mirror:
+    Mirror URL for auto-downloading IERS file data.
+
   remote_timeout:
     Remote timeout downloading IERS file data (seconds)
 
@@ -156,7 +159,7 @@ part of astropy and can be used for transformations.  For example::
   '2010-01-01 00:00:00.114'
 
 Instead of local copies of IERS files, one can also download them, using
-``iers.IERS_A_URL`` and ``iers.IERS_B_URL``::
+``iers.IERS_A_URL`` (or ``iers.IERS_A_URL_MIRROR``) and ``iers.IERS_B_URL``::
 
   >>> iers_a = iers.IERS_A.open(iers.IERS_A_URL)  # doctest: +SKIP