fix(platforms): extra_or_secret keeps blank-string YAML values where the old readers did

dingtalk _extra_get, mattermost _extra_or_env and slack _extra_or_env_flag/_channel_set fell
through to env only on None, so `allowed_channels: ""` / `free_response_channels: ""` meant
"no whitelist" rather than "use the env CSV". The shared reader treated blank as unset and
silently widened those to the env value. New `blank_is_unset=False` knob restores the old
semantics at those seven call sites; the default (blank = unset) stays for the readers whose
old body was `extra.get(k) or env`.

gateway/platforms/_shared.py

@@ -86,16 +86,19 @@ def platform_gate_env(name: str, default: str = "") -> str:
     return (os.getenv(name) or default).strip()
 
 
-def extra_or_secret(extra: Optional[dict], key: str, env: str, default: Any = "") -> Any:
+def extra_or_secret(extra: Optional[dict], key: str, env: str, default: Any = "",
+                    *, blank_is_unset: bool = True) -> Any:
     """``config.extra[key]`` when set, else the scoped env var ``env`` (else ``default``).
 
     ``extra`` is the per-profile truth under multiplexing (the YAML→env bridge is skipped for a
     secondary profile), so it is consulted first; the env read goes through ``get_scoped_secret``.
-    "Unset" is ``None`` or a blank string — an explicit ``False``/``0`` in YAML is a real value
-    (``require_mention: false`` must not fall through to the env default).
+    An explicit ``False``/``0`` in YAML is always a real value (``require_mention: false`` must not
+    fall through to the env default). A blank string is unset by default; readers whose YAML key
+    means "clear it" (``allowed_channels: ""`` = no whitelist, not "use the env CSV") pass
+    ``blank_is_unset=False`` so only a missing/``None`` key falls through.
     """
     value = (extra or {}).get(key)
-    if value is None or (isinstance(value, str) and not value.strip()):
+    if value is None or (blank_is_unset and isinstance(value, str) and not value.strip()):
         return get_scoped_secret(env, default)
     return value
 

plugins/platforms/dingtalk/adapter.py

@@ -254,11 +254,11 @@ class DingTalkAdapter(BasePlatformAdapter):
 
     def _csv_setting(self, key: str, env_name: str) -> Set[str]:
         """List/CSV setting from config.extra[key], falling back to the env var."""
-        return _csv_set(_extra_or_secret(self.config.extra, key, env_name))
+        return _csv_set(_extra_or_secret(self.config.extra, key, env_name, blank_is_unset=False))
 
     def _dingtalk_require_mention(self) -> bool:
         """Whether group chats require an explicit bot trigger."""
-        configured = _extra_or_secret(self.config.extra, "require_mention", "DINGTALK_REQUIRE_MENTION", "false")
+        configured = _extra_or_secret(self.config.extra, "require_mention", "DINGTALK_REQUIRE_MENTION", "false", blank_is_unset=False)
         return configured.lower() in _TRUTHY if isinstance(configured, str) else bool(configured)
 
     def _dingtalk_allowed_chats(self) -> Set[str]:

plugins/platforms/mattermost/adapter.py

@@ -495,14 +495,14 @@ class MattermostAdapter(BasePlatformAdapter):
         """Mention-gate a non-DM post; return the cleaned text, or None to ignore it. allowed_channels is a
         whitelist checked first (@mentions elsewhere are ignored); require_mention (default true) is
         bypassed in free_response_channels."""
-        allowed_channels = _channel_id_set(_extra_or_secret(self.config.extra, "allowed_channels", "MATTERMOST_ALLOWED_CHANNELS"))
+        allowed_channels = _channel_id_set(_extra_or_secret(self.config.extra, "allowed_channels", "MATTERMOST_ALLOWED_CHANNELS", blank_is_unset=False))
         if allowed_channels and channel_id not in allowed_channels:
             logger.debug("Mattermost: ignoring message in non-allowed channel: %s", channel_id)
             return None
-        require_mention = str(_extra_or_secret(self.config.extra, "require_mention", "MATTERMOST_REQUIRE_MENTION", "true")
+        require_mention = str(_extra_or_secret(self.config.extra, "require_mention", "MATTERMOST_REQUIRE_MENTION", "true", blank_is_unset=False)
                               ).lower() not in {"false", "0", "no"}
         free_channels = _channel_id_set(
-            _extra_or_secret(self.config.extra, "free_response_channels", "MATTERMOST_FREE_RESPONSE_CHANNELS"))
+            _extra_or_secret(self.config.extra, "free_response_channels", "MATTERMOST_FREE_RESPONSE_CHANNELS", blank_is_unset=False))
         mention_patterns = [f"@{self._bot_username}", f"@{self._bot_user_id}"]
         has_mention = any(pattern.lower() in message_text.lower() for pattern in mention_patterns)
         if require_mention and channel_id not in free_channels and not has_mention:

plugins/platforms/slack/adapter.py

@@ -5963,7 +5963,7 @@ class SlackAdapter(BasePlatformAdapter):
 
     def _extra_or_env_flag(self, key: str, env_var: str, *, strip: bool = False) -> bool:
         """Opt-in boolean: ``config.extra[key]`` wins, else ``env_var`` (default false)."""
-        configured = _extra_or_secret(self.config.extra, key, env_var, "false")
+        configured = _extra_or_secret(self.config.extra, key, env_var, "false", blank_is_unset=False)
         if isinstance(configured, str):
             if strip:
                 configured = configured.strip()
@@ -5997,7 +5997,7 @@ class SlackAdapter(BasePlatformAdapter):
         self, key: str, env_var: str, *, coerce_scalar: bool = False) -> set:
         """Channel-ID set from ``config.extra[key]`` (list or CSV) else ``env_var`` CSV.
         ``coerce_scalar`` accepts non-str scalars (a bare numeric YAML value loads as int)."""
-        raw = _extra_or_secret(self.config.extra, key, env_var, "")
+        raw = _extra_or_secret(self.config.extra, key, env_var, "", blank_is_unset=False)
         if isinstance(raw, list):
             return {str(part).strip() for part in raw if str(part).strip()}
         if coerce_scalar:

tests/gateway/test_shared_platform_boilerplate.py

@@ -108,6 +108,9 @@ def test_extra_or_secret_honours_explicit_false_but_not_blank(monkeypatch):
     assert shared.extra_or_secret({"require_mention": False}, "require_mention", "X", "true") is False
     assert shared.extra_or_secret({"require_mention": ""}, "require_mention", "X", "true") == "env:true"
     assert shared.extra_or_secret(None, "require_mention", "X", "true") == "env:true"
+    # Readers where a blank YAML value means "cleared" (channel whitelists) keep it as a value.
+    assert shared.extra_or_secret({"allowed_channels": ""}, "allowed_channels", "X", "", blank_is_unset=False) == ""
+    assert shared.extra_or_secret({}, "allowed_channels", "X", "", blank_is_unset=False) == "env:"
 
 
 def test_external_fallback_consults_profile_scope_only_when_unscoped(monkeypatch):