Make specifying exts_defaultclass optional

It is NOT required when all extensions have either custom easyblocks or
an `easyblock` key in their option dict.
In those cases `exts_defaultclass` is redundant making it harder to
write the EasyConfig.
Check for a missing easyblock when actually using it so the error will
be just a bit later but still (almost) the same.

Author: Flamefire

easybuild/framework/easyblock.py

@@ -3086,8 +3086,10 @@ def init_ext_instances(self):
             # proper way: derive module path from specified class name
             default_class = exts_defaultclass
             default_class_modpath = get_module_path(default_class, generic=True)
+        elif exts_defaultclass is None:
+            default_class = default_class_modpath = None
         else:
-            error_msg = "Improper default extension class specification, should be string: %s (%s)"
+            error_msg = "Improper default extension class specification, should be string or None: %s (%s)"
             raise EasyBuildError(error_msg, exts_defaultclass, type(exts_defaultclass))
 
         exts_cnt = len(self.exts)
@@ -3139,8 +3141,11 @@ def init_ext_instances(self):
                                          "for extension %s: %s",
                                          class_name, mod_path, ext_name, err)
 
-            # fallback attempt: use default class
+            # fallback attempt: use default class if any
             if inst is None:
+                if not default_class:
+                    raise EasyBuildError("ERROR: No default extension class set for %s and no explicit or custom "
+                                         "easyblock found for extension %s", self.name, ext_name)
                 try:
                     cls = get_class_for(default_class_modpath, default_class)
                     self.log.debug("Obtained class %s for installing extension %s", cls, ext_name)
@@ -3176,10 +3181,6 @@ def extensions_step(self, fetch=False, install=True):
             self.log.debug("No extensions in exts_list")
             return
 
-        # we really need a default class
-        if not self.cfg['exts_defaultclass'] and install:
-            raise EasyBuildError("ERROR: No default extension class set for %s", self.name)
-
         start_progress_bar(PROGRESS_BAR_EXTENSIONS, len(self.cfg.get_ref('exts_list')))
 
         self.prepare_for_extensions()

test/framework/easyconfig.py

@@ -540,6 +540,46 @@ def test_exts_list(self):
         regex = re.compile('EBEXTSLISTPI.*"ext1-1.0,ext2-2.0,ext-PI-3.14,ext-pi-3.0')
         self.assertTrue(regex.search(modtxt), "Pattern '%s' found in: %s" % (regex.pattern, modtxt))
 
+    def test_extensions_default_class(self):
+        """Test that exts_defaultclass doesn't need to be specified if explicit one is present."""
+
+        init_config(build_options={'silent': True})
+
+        self.contents = textwrap.dedent("""
+            easyblock = "ConfigureMake"
+            name = "PI"
+            version = "3.14"
+            homepage = "http://example.com"
+            description = "test easyconfig"
+            toolchain = SYSTEM
+            exts_list = [
+               ("toy", "0.0"),  # Custom block by name
+               ("bar", "0.0", { # Explicit
+                    'easyblock': 'EB_toy',
+                    'sources': ['toy-%(version)s.tar.gz'],
+                }),
+            ]
+        """)
+        self.prep()
+        # Ensure source is found
+        toy_tar_gz = os.path.join(self.test_sourcepath, 'toy', 'toy-0.0.tar.gz')
+        copy_file(toy_tar_gz, self.test_prefix)
+        os.environ['EASYBUILD_SOURCEPATH'] = self.test_prefix
+        init_config(build_options={'silent': True})
+
+        ec = EasyConfig(self.eb_file)
+        eb = EasyBlock(ec)
+        eb.fetch_step()
+        with self.mocked_stdout_stderr():
+            eb.extensions_step()
+
+        pi_installdir = os.path.join(self.test_installpath, 'software', 'PI', '3.14')
+
+        # check whether files expected to be installed for both extensions are in place
+        self.assertExists(os.path.join(pi_installdir, 'bin', 'toy'))
+        self.assertExists(os.path.join(pi_installdir, 'lib', 'libtoy.a'))
+        self.assertExists(os.path.join(pi_installdir, 'lib', 'libbar.a'))
+
     def test_extensions_templates(self):
         """Test whether templates used in exts_list are resolved properly."""
 
@@ -562,7 +602,6 @@ def test_extensions_templates(self):
             'description = "test easyconfig"',
             'toolchain = SYSTEM',
             'dependencies = [("Python", "3.6.6")]',
-            'exts_defaultclass = "EB_Toy"',
             # bogus, but useful to check whether this get resolved
             'exts_default_options = {"source_urls": [PYPI_SOURCE]}',
             'exts_list = [',