Skip interpolation checks for non-selected alternative sections

btest

@@ -3123,28 +3123,14 @@ if __name__ == "__main__":
     Config = getcfgparser(defaults)
     Config.read(btest_cfg, encoding="utf-8")
 
-    # Make sure at this point that any Python-level %()s substitution patterns
-    # resolve. Cleaner to error now than in mid-processing when we retrieve an
-    # option. We use itemsNoDefaults() for this since falling back on defaults
-    # pulls in all of os.environ, some of which may well not be formatted for
-    # Python substitution.
-    try:
-        for section in Config.sections():
-            for _, _ in Config.itemsNoDefaults(section):
-                pass
-    except configparser.InterpolationError as err:
-        error(
-            f"{btest_cfg} contains invalid Python configparser interpolation in {err.section}.{err.option}: {err}"
-        )
-
     defaults["baselinedir"] = getOption("BaselineDir", defaults["baselinedir"])
 
     min_version = getOption("MinVersion", None)
     if min_version:
         validate_version_requirement(min_version, VERSION)
 
     if Options.alternatives:
-        # Preprocess to split into list. "-" refers to the default setup, as a
+        # Preprocess to split into list. "-" refers to the default alternative, as a
         # shorthand for "default", to allow combination with select alternatives.
         Options.alternatives = [alt.strip() for alt in Options.alternatives.split(",")]
         Options.alternatives = [
@@ -3188,6 +3174,34 @@ if __name__ == "__main__":
             None,
             transform=lambda x: normalize_path(x),
         )
+    else:
+        Options.alternatives = []
+
+    # Make sure at this point that any Python-level %()s substitution patterns
+    # resolve. Cleaner to error now than in mid-processing when we retrieve an
+    # option. We use itemsNoDefaults() for this since falling back on defaults
+    # pulls in all of os.environ, some of which may well not be formatted for
+    # Python substitution.
+    try:
+        for section in Config.sections():
+            # Skip interpolating sections of alternatives that aren't selected
+            # via -a / --alternatives. They aren't needed to run tests
+            # and aren't expected to properly interpolatable.
+            if (
+                section.startswith("environment-")
+                or section.startswith("filter-")
+                or section.startswith("substitution-")
+            ):
+                tag = section.split("-", 1)[-1]
+                if tag not in Options.alternatives:
+                    continue
+
+            for _, _ in Config.itemsNoDefaults(section):
+                pass
+    except configparser.InterpolationError as err:
+        error(
+            f"{btest_cfg} contains invalid Python configparser interpolation in {err.section}.{err.option}: {err}"
+        )
 
     orig_cwd = pathlib.Path.cwd()
     os.chdir(TestBase)

testing/Baseline/tests.btest-cfg-skip-non-selected-env/output

@@ -0,0 +1,13 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+X default
+X default
+test ... ok
+all 1 tests successful
+X default
+X installation
+Y installation
+X default
+X installation
+Y installation
+test [installation] ... ok
+all 1 tests successful

testing/Baseline/tests.btest-cfg-skip-non-selected-env/test.out

@@ -0,0 +1,6 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+default
+MYTEST_X=Xdefault
+installation
+MYTEST_X=Xinstallation
+MYTEST_Y=Yinstallation

testing/tests/btest-cfg-skip-non-selected-env.test

@@ -0,0 +1,29 @@
+# %TEST-DOC: Validates that a environment-<tag> section isn't interpolated/evaluated
+# it is not not selected.
+#
+# %TEST-EXEC: echo "default" >>test.out
+# %TEST-EXEC: btest -dt test >output 2>&1
+# %TEST-EXEC: echo "installation" >>test.out
+# %TEST-EXEC: btest -dt -a installation test >>output 2>&1
+# %TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff output
+# %TEST-EXEC: btest-diff test.out
+
+# %TEST-START-FILE btest.cfg
+[DEFAULT]
+val1 = abcd
+
+[btest]
+option1=%(val1)s
+
+[environment]
+MYTEST_X=`echo X default >&2; echo Xdefault`
+
+[environment-installation]
+MYTEST_X=`echo X installation >&2; echo Xinstallation`
+MYTEST_Y=`echo Y installation >&2; echo Yinstallation`
+# %TEST-END-FILE
+
+# %TEST-START-FILE test
+# # This writes into the outer test directory.
+# @TEST-EXEC: env | grep MYTEST | sort >> ../../test.out
+# %TEST-END-FILE