Address review comments

go/private/extensions.bzl

@@ -132,6 +132,12 @@ def _default_go_sdk_name(*, module, multi_version, tag_type, index):
     )
 
 def _toolchain_prefix(index, name):
+    """Prefixes the given name with the index, padded with zeros to ensure lexicographic sorting.
+
+    Examples:
+      _toolchain_prefix(   2, "foo") == "_0002_foo_"
+      _toolchain_prefix(2000, "foo") == "_2000_foo_"
+    """
     return "_{}_{}_".format(_left_pad_zero(index, _TOOLCHAIN_INDEX_PAD_LENGTH), name)
 
 def _left_pad_zero(index, length):

go/private/sdk.bzl

@@ -228,14 +228,7 @@ def go_toolchains_build_file_content(
         sdk_repos,
         sdk_types,
         sdk_versions):
-    if len({len(l): None for l in [
-        prefixes,
-        geese,
-        goarchs,
-        sdk_repos,
-        sdk_types,
-        sdk_versions,
-    ]}) != 1:
+    if not _have_same_length(prefixes, geese, goarchs, sdk_repos, sdk_types, sdk_versions):
         fail("all lists must have the same length")
 
     loads = [
@@ -622,6 +615,11 @@ def _version_string(v):
         v = v[:-1]
     return ".".join([str(n) for n in v]) + suffix
 
+def _have_same_length(*lists):
+    if not lists:
+        fail("expected at least one list")
+    return len({len(l): None for l in lists}) == 1
+
 def go_register_toolchains(version = None, nogo = None, go_version = None, experiments = None):
     """See /go/toolchains.rst#go-register-toolchains for full documentation."""
     if not version: