CMake: Make build system of installed Qt more relocatable

Aka handle CMAKE_INSTALL_PREFIX in a more relocatable way.

The following story inspired this change.

If a user wants to build a Qt repo into a different install prefix
than the usual Qt one, this will fail configuration because we
look for various things like syncqt, qdoc, etc relative to
CMAKE_INSTALL_PREFIX, which will now point to a different location
where none of the above tools are located.

The intent for such a use case is to support building Qt packages with
Conan, which sets a random install prefix when configuring a repo.

The idea is to derive the qt prefix dynamically from the
QtBuildInternals package location. Essentially it's a reverse relative
path from the QtBuildInternalsConfig.cmake file to the install prefix
that was specified when initially configuring qtbase.

Once the dynamic prefix is computed (so we know where the possibly
relocated Qt is), we can find tools like syncqt and qdoc.

This is an initial attempt to support a use case like that.

More design work will probably needed in case if tools / libs need to
be found in a location different than the Qt install prefix (so
support for multiple install prefixes / search paths).

An example of such a case would be when building qtdeclarative and
qtquickcontrols2 as Conan packages in one go. Most likely the
qmltyperegistrar tool will be located in the random install prefix
set by Conan, so building qtquickcontrols2 might fail due to not
finding the tool in the original Qt install prefix.

As to the implementation details, the change does the following:
- Dynamically computes and sets the
  QT_BUILD_INTERNALS_RELOCATABLE_INSTALL_PREFIX variable when
  find_package()'ing QtBuildInternals. It's an absolute path
  pointing to where the relocated Qt is.

- When building qtbase this variable is not yet available (due
  to QtBuildInternalsExtra not existing), in that case we set
  the variable to the absolute path of CMAKE_INSTALL_PREFIX
  (but only for the initial qtbase configuration).

- Remove QT_BUILD_INTERNALS_ORIGINAL_INSTALL_PREFIX which was used
  for standalone tests purposes. It's not needed now that we compute
  the location of the Qt prefix dynamically.

- The Unixy qt-cmake and qt-cmake-private shell scripts now
  use a relative path to find the toolchain file we created.

- The toolchain file also dynamically computes the location of the Qt
  packages, and adds them to CMAKE_PREFIX_PATH.

- A lot of existing CMAKE_INSTALL_PREFIX uses are replaced with
  QT_BUILD_INTERNALS_RELOCATABLE_INSTALL_PREFIX. This includes finding
  tool locations, mkspecs dir, path environment setup for tools, etc.

- Some places still use CMAKE_PREFIX_PATH in the following cases
  - When determining paths while configuring qtbase (valid cases)
  - When I wasn't sure what the behavior should be, so I left them
    as-is (an example is documentation generation, do we want to
    install it into the random Conan prefix, or into the main prefix?
    Currently it installs in the random prefix).

Note that relocating a Qt installation does not work for non-prefix /
non-installed builds, due to hardcoded paths to include directories
and libraries in generated FooTargets.cmake files.

Task-number: QTBUG-83999
Change-Id: I87d6558729db93121b1715771034b03ce3295923
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

1 files changed, 4 insertions, 4 deletions

diff --git a/cmake/QtSetup.cmake b/cmake/QtSetup.cmake
index 6bc3c74f69..39142fa332 100644
--- a/cmake/QtSetup.cmake
+++ b/cmake/QtSetup.cmake
@@ -51,8 +51,8 @@ set(CMAKE_C_VISIBILITY_PRESET hidden)
 set(CMAKE_CXX_VISIBILITY_PRESET hidden)
 set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
 
-# Detect non-prefix builds, either when the install prefix is set to the binary dir
-# or when enabling developer builds and no prefix is specified.
+# Detect non-prefix builds: either when the qtbase install prefix is set to the binary dir
+# or when a developer build is explicitly enabled and no install prefix is specified.
 # This detection only happens when building qtbase, and later is propagated via the generated
 # QtBuildInternalsExtra.cmake file.
 if (PROJECT_NAME STREQUAL "QtBase" AND NOT QT_BUILD_STANDALONE_TESTS)
@@ -62,8 +62,8 @@ if (PROJECT_NAME STREQUAL "QtBase" AND NOT QT_BUILD_STANDALONE_TESTS)
         set(__qt_will_install_value OFF)
         # Handle non-prefix builds by setting the CMake install prefix to point to qtbase's build
         # dir.
-        # While building another repo (like qtsvg), the CMAKE_INSTALL_PREFIX or CMAKE_PREFIX_PATH
-        # (either work) should be set on the command line to point to the qtbase build dir.
+        # While building another repo (like qtsvg) the CMAKE_PREFIX_PATH
+        # should be set on the command line to point to the qtbase build dir.
         set(CMAKE_INSTALL_PREFIX ${QtBase_BINARY_DIR} CACHE PATH
             "Install path prefix, prepended onto install directories." FORCE)
     else()