fallback option

pyodide_build/config.py

@@ -179,6 +179,7 @@ def to_env(self) -> dict[str, str]:
     "build_dependency_index_url": "BUILD_DEPENDENCY_INDEX_URL",
     # maintainer only
     "_f2c_fixes_wrapper": "_F2C_FIXES_WRAPPER",
+    "_build_dependency_fallback_to_pypi": "BUILD_DEPENDENCY_FALLBACK_TO_PYPI",
 }
 
 BUILD_VAR_TO_KEY = {v: k for k, v in BUILD_KEY_TO_VAR.items()}
@@ -195,6 +196,7 @@ def to_env(self) -> dict[str, str]:
     "build_dependency_index_url",
     # maintainer only
     "_f2c_fixes_wrapper",
+    "_build_dependency_fallback_to_pypi",
 }
 
 # Default configuration values.
@@ -214,6 +216,7 @@ def to_env(self) -> dict[str, str]:
     "build_dependency_index_url": "https://pypi.anaconda.org/pyodide-build/simple",
     # maintainer only
     "_f2c_fixes_wrapper": "",
+    "_build_dependency_fallback_to_pypi": "1",
 }
 
 # Default configs that are computed from other values (often from Makefile.envs)

pyodide_build/pypabuild.py

@@ -156,13 +156,19 @@ def _build_in_isolated_env(
                 install_reqs(env, builder.build_system_requires)
                 installed_build_system_requires = True
             except Exception:
+                pass
+
+        # Disabled for testing
+        if not installed_build_system_requires:
+            if common.to_bool(get_build_flag("BUILD_DEPENDENCY_FALLBACK_TO_PYPI")):
                 print(
                     f"Failed to install build dependencies from {index_url_for_cross_build}, falling back to default index url"
                 )
-
-        # Disabled for testing
-        # if not installed_build_system_requires:
-        #     install_reqs(env, builder.build_system_requires)
+                install_reqs(env, builder.build_system_requires)
+            else:
+                print(
+                    f"Failed to install build dependencies from {index_url_for_cross_build}, proceeding the build, but it will fail."
+                )
 
         try:
             build_reqs = builder.get_requires_for_build(