build: add root dir to CMAKE_PREFIX_PATH

Nix patches cmake to remove the root directory `/` from
`CMAKE_SYSTEM_PREFIX_PATH`:
https://github.com/NixOS/nixpkgs/blob/428b49b28ebc8938a6d9f6c540d32d7a06713972/pkgs/by-name/cm/cmake/001-search-path.diff#L10

Without this, and when using the toolchain for depends builds, cmake's
`find_path()` and `find_package()` do not know where to find
dependencies, causing issues like:
bitcoin/bitcoin#32428

Adding this path back via CMAKE_PREFIX_PATH is harmless on other
systems, and fixes the toolchain for Nix users.

We append the `/` dir a maximum of once, as the toolchain may be called
repeatedly during builds.

Co-authored-by: Russell Yanofsky <[email protected]>
Co-authored-by: josibake <[email protected]>

Author: 3 people

depends/toolchain.cmake.in

@@ -92,6 +92,22 @@ set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
 set(QT_TRANSLATIONS_DIR "${CMAKE_CURRENT_LIST_DIR}/translations")
 
+# The following is only necessary when using cmake from Nix or NixOS, because
+# Nix patches cmake to remove the root directory `/` from
+# CMAKE_SYSTEM_PREFIX_PATH. Adding it back is harmless on other platforms and
+# necessary on Nix because without it cmake find_path, find_package, etc
+# functions do not know where to look in CMAKE_FIND_ROOT_PATH for dependencies
+# (https://github.com/bitcoin/bitcoin/issues/32428).
+#
+# TODO: longer term, it may be possible to use a dependency provider, which
+# would bring the find_package calls completely under our control, making this
+# patch unnecessary.
+#
+# Make sure we only append once, as this file may be called repeatedly.
+if(NOT "/" IN_LIST CMAKE_PREFIX_PATH)
+  list(APPEND CMAKE_PREFIX_PATH "/")
+endif()
+
 if(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND NOT CMAKE_HOST_APPLE)
   # The find_package(Qt ...) function internally uses find_library()
   # calls for all dependencies to ensure their availability.