Check vaults cannot get into deadlock

src/controller/VaultManager.sol

@@ -81,6 +81,10 @@ abstract contract VaultManager is BaseController, PriceFeedManager {
      * @notice Thrown when trying to set a vault as the main vault for an asset it already is
      */
     error Vault_AlreadyMainVaultForAsset();
+    /**
+     * @notice Thrown when the overall vaults settings are invalid
+     */
+    error Vault_InvalidVaultsSettings();
 
     /**
      * @notice Initializes the VaultManager with an empty vaults linked list
@@ -121,6 +125,7 @@ abstract contract VaultManager is BaseController, PriceFeedManager {
         _checkVaultSettings(settings);
         vaultSettings[vault] = settings;
         emit VaultSettingsUpdated(vault, settings.maxCapacity, settings.maxProportionality, settings.minProportionality);
+        _checkVaultsProportionalities();
 
         if (isMainVaultForAsset || _vaultFor[asset] == address(0)) {
             emit MainVaultForAssetUpdated(asset, _vaultFor[asset], vault);
@@ -146,6 +151,7 @@ abstract contract VaultManager is BaseController, PriceFeedManager {
 
         delete vaultSettings[vault];
         emit VaultSettingsUpdated(vault, 0, 0, 0);
+        _checkVaultsProportionalities();
 
         address asset = IControlledVault(vault).asset();
         if (_vaultFor[asset] == vault) {
@@ -173,6 +179,7 @@ abstract contract VaultManager is BaseController, PriceFeedManager {
         _checkVaultSettings(settings);
         vaultSettings[vault] = settings;
         emit VaultSettingsUpdated(vault, settings.maxCapacity, settings.maxProportionality, settings.minProportionality);
+        _checkVaultsProportionalities();
     }
 
     /**
@@ -234,6 +241,34 @@ abstract contract VaultManager is BaseController, PriceFeedManager {
         require(settings.minProportionality <= settings.maxProportionality, Vault_MinProportionalityNotLessThanMax());
     }
 
+    /**
+     * @dev Validates that the sum of any two vaults' maximum proportionalities is greater than or equal to 100%.
+     * This ensures that the protocol cannot get into a deadlock due to proportionality constraints.
+     */
+    function _checkVaultsProportionalities() internal view {
+        address[] memory allVaults = vaults();
+        uint256 count = allVaults.length;
+        if (count == 0) return; // No vaults to check
+        if (count == 1) {
+            // Single vault must be allowed to cover 100% of the assets
+            require(vaultSettings[allVaults[0]].maxProportionality == MAX_BPS, Vault_InvalidVaultsSettings());
+        } else if (count == 2) {
+            // Sum of both vaults' max proportionalities must be at least 120%
+            uint256 sum =
+                vaultSettings[allVaults[0]].maxProportionality + vaultSettings[allVaults[1]].maxProportionality;
+            require(sum >= 12_000, Vault_InvalidVaultsSettings());
+        } else {
+            // For 3 or more vaults, ensure that the sum of the two smallest max proportionalities is at least 100%
+            uint256 maxProportionality;
+            uint256 minMaxProportionality = MAX_BPS;
+            for (uint256 i; i < count; ++i) {
+                maxProportionality = vaultSettings[allVaults[i]].maxProportionality;
+                require(maxProportionality + minMaxProportionality >= MAX_BPS, Vault_InvalidVaultsSettings());
+                minMaxProportionality = minMaxProportionality.min(maxProportionality);
+            }
+        }
+    }
+
     /**
      * @notice Retrieves the asset managed by a given vault
      * @param vault The address of the vault

tests/harness/ControllerHarness.sol

@@ -51,6 +51,10 @@ contract ControllerHarness is Controller {
         return _vaultsOverview(calculateTotalValue);
     }
 
+    function exposed_checkVaultsProportionalities() external view {
+        _checkVaultsProportionalities();
+    }
+
     function exposed_maxDepositLimit(address vault) external view returns (uint256 max) {
         return _maxDepositLimit(_vaultsOverview(false), vault);
     }