I have run into path ordering issues when setting up a Houdini environment a couple of times. For example when adding htoa, redshift and similar plugins to HOUDINI_PATH. The issue seemed to be related to menu items, which plugins can add to Houdini UI, but I have never managed to get to the bottom of it. I would randomly shuffle paths order in my environment variables until all errors went away :)

Recently I was revisiting som older configs and I've stumbled upon this once again. Therefore I have decided to finally investigate it a little bit.

Errors I would get would look like this:

Error while parsing a menu definition file
 'X:/packages/qLib/0.2.207/MainMenuCommon.xml':
Cannot find element 'help_menu' needed for ordering of 'qLib_menu'.
Cannot find element 'export_files' needed for ordering of 'file_menu_sep_ql_recover'.
Cannot find element 'saveastype_strip' needed for ordering of 'file_menu_sep_ql_save_pre'.
Error while parsing a menu definition file
 'X:/packages/GameDevToolset/19.0.455/MainMenuCommon.xml':
Could not find the parent element 'preferences_submenu' for 'set_external_editor'.
Cannot set order: element 'set_external_editor' has no parent.
Could not find the parent element 'windows_menu' for 'external_python_source_editor'.
Cannot set order: element 'external_python_source_editor' has no parent.
Error while parsing a menu definition file
 'X:/packages/qLib/0.2.207/NetworkViewMenu.xml':
Cannot find element 'help_menu' needed for ordering of 'qLib_network_view_menu'.
Error while parsing a menu definition file
 'X:/packages/GameDevToolset/19.0.455/NetworkViewMenu.xml':
Cannot find element 'help_menu' needed for ordering of 'labs_network_view_menu'.

In this case I am adding qLib and GameDevToolset tools into Houdini. Houdini would load, but menus would be in wrong order or wouldn't show at all.

I suspected what was the issue: custom menu items can be set as children of Houdini menus. But Houdini menu items must load first, so that child items can find their parent item. Therefore I paid close attention to have Houdini default paths at the start of HOUDINI_PATH (e.g. with the & special character), so that they would be loaded before the additional tools would be loaded. This setup however still resulted in the errors above.

After a few attempts I looked into docs and checked results of hconfig and finally found the reason. All Houdini environment variables are read left to right, but HOUDINI_MENU_PATH loads in reverse order.

From docs:

Houdini searches the $HOUDINI_MENU_PATH in reverse order, first looking for MainMenuCommon.xml and then for MainMenuMaster.xml files.

And hconfig -ap:

HOUDINI_PATH := "C:/PROGRA~1/SIDEEF~1/HOUDIN~1.455/packages/kinefx;&;X:/packages/GameDevToolset/19.0.455;X:/packages/qLib/0.2.207"
    The path of directories where Houdini looks for configuration files.

    Directories searched (in order) are:
        1) "$HFS/packages/kinefx"
        2) "$HIP"
        3) "$HOUDINI_USER_PREF_DIR"
        4) "$HFS/houdini"
        5) "$HFS/bin"
        6) "X:/packages/GameDevToolset/19.0.455"
        7) "X:/packages/qLib/0.2.207"

HOUDINI_MENU_PATH := "@"
    The search path for XML menu files, such as the main menu, the node
    RMB menu, parameter menu, and shelf menu.
    Note, the paths are scanned in reverse order, i.e., the files from
    directories further in the path are loaded first, and files from
    directories earlier in the path are loaded later. This ensures that
    the default menus are loaded first, which allows custom files to
    modify their configuration.

    Default path: '@'
        Where @ is replaced with HOUDINI_PATH
    Directories searched (in order) are:
        1) "$HFS/packages/kinefx"
        2) "$HIP"
        3) "$HOUDINI_USER_PREF_DIR"
        4) "$HFS/houdini"
        5) "$HFS/bin"
        6) "X:/packages/GameDevToolset/19.0.455"
        7) "X:/packages/qLib/0.2.207"

So this arguably unintuitive decision is diverting from the expected behavior, probably to catch user mistakes. The issue is that when not set, HOUDINI_MENU_PATH's default @ value is inheriting HOUDINI_PATH, which is not reversed.

Though many tools prefer pointing HOUDINI_PATH to an expected folder structure, where other paths can find their things (menus, assets, scripts etc) for the simplicity of config. I like that idea too as it requires setting only one path, instead of dozens.

It's not a big deal and was easy to fix once I've found the reason. In my opinion however it violates the principle of least astonishment. Possible fix at SideFX side could involve switching order (but being a minor backward incompatible change), or updating error messages to indicate the likely cause of the issue, like

Cannot find element 'help_menu' needed for ordering of 'xyz'. Note that paths in HOUDINI_MENU_PATH are read in reversed order.

For completeness I am adding the correct config values:

HOUDINI_PATH := "C:/PROGRA~1/SIDEEF~1/HOUDIN~1.455/packages/kinefx;X:/packages/GameDevToolset/19.0.455;X:/packages/qLib/0.2.207;&"
    The path of directories where Houdini looks for configuration files.

    Directories searched (in order) are:
        1) "$HFS/packages/kinefx"
        2) "X:/packages/GameDevToolset/19.0.455"
        3) "X:/packages/qLib/0.2.207"
        4) "$HIP"
        5) "$HOUDINI_USER_PREF_DIR"
        6) "$HFS/houdini"
        7) "$HFS/bin"

HOUDINI_MENU_PATH := "@"
    The search path for XML menu files, such as the main menu, the node
    RMB menu, parameter menu, and shelf menu.
    Note, the paths are scanned in reverse order, i.e., the files from
    directories further in the path are loaded first, and files from
    directories earlier in the path are loaded later. This ensures that
    the default menus are loaded first, which allows custom files to
    modify their configuration.

    Default path: '@'
        Where @ is replaced with HOUDINI_PATH
    Directories searched (in order) are:
        1) "$HFS/packages/kinefx"
        2) "X:/packages/GameDevToolset/19.0.455"
        3) "X:/packages/qLib/0.2.207"
        4) "$HIP"
        5) "$HOUDINI_USER_PREF_DIR"
        6) "$HFS/houdini"
        7) "$HFS/bin"