[opencv4] Fix build failure when dnn feature is not enabled

[WentsingNee]

Given the following manifest (when dnn feature is not included):

"dependencies": [
  {
    "name": "opencv4",
    "version>=": "4.8.0#18",
    "default-features": false,
    "features": [
      "png"
    ]
  }
]

it failed and here is what vcpkg/buildtrees/opencv4/config-x64-linux-out.log said:

...
CMake Error at /home/peter/open-source/git/vcpkg/scripts/buildsystems/vcpkg.cmake:859 (_find_package):
  Could not find a package configuration file provided by "flatbuffers" with
  any of the following names:

    flatbuffersConfig.cmake
    flatbuffers-config.cmake

  Add the installation prefix of "flatbuffers" to CMAKE_PREFIX_PATH or set
  "flatbuffers_DIR" to a directory containing one of the above files.  If
  "flatbuffers" provides a separate development package or SDK, be sure it
  has been installed.
Call Stack (most recent call first):
  CMakeLists.txt:759 (find_package)
...

This problem is introduced by commit 6d2c9714 contributed by @jimwang118

In the patch vcpkg/ports/opencv4/0023-fix-no-flatbuffers.patch, Jim disabled the procedure of Flatbuffers detection introduced by the previous patch 0017-fix-flatbuffers.patch, which has correctly fixed the flatbuffers-related issues.

...
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -756,7 +756,7 @@ include(cmake/OpenCVFindLibsVideo.cmake)
 include(cmake/OpenCVFindLibsPerf.cmake)
 include(cmake/OpenCVFindLAPACK.cmake)
 include(cmake/OpenCVFindProtobuf.cmake)
-include(cmake/OpenCVDetectFlatbuffers.cmake)
+find_package(flatbuffers CONFIG REQUIRED)
...

In vcpkg/ports/opencv4/vcpkg.json we can see, flatbuffers is only the dependency of feature dnn:

  "dependencies": [
...
    "dnn": {
      "description": "Enable dnn module",
      "dependencies": [
        "flatbuffers",
        {
          "name": "flatbuffers",
          "host": true,
          "default-features": false
        },
        "protobuf"
      ]
    },
...

and when dnn is not enabled, it broke down at the line find_package(flatbuffers CONFIG REQUIRED)

And this change is unnecessary either:

-  list(APPEND libs ocv.3rdparty.flatbuffers)
+  list(APPEND libs flatbuffers::flatbuffers)

Thus, we'd better to revert those changes.

[WentsingNee]

I tested it locally without installing the dnn feature, and the error you mentioned did not occur. Installation command: vcpkg install opencv4[core,png] result:

port version 15 is no problem. Bug is introduced since port version 16 you committed

[jimwang118]

I suggest adding constraints to the patch that determine when using dnn to find flatbuffers through the patch. Removing the patch may trigger the original CI error.
...
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -756,7 +756,7 @@ include(cmake/OpenCVFindLibsVideo.cmake)
include(cmake/OpenCVFindLibsPerf.cmake)
include(cmake/OpenCVFindLAPACK.cmake)
include(cmake/OpenCVFindProtobuf.cmake)
-include(cmake/OpenCVDetectFlatbuffers.cmake)
if(BUILD_opencv_dnn)
+find_package(flatbuffers CONFIG REQUIRED)
endif()
...

[WentsingNee]

But I don't think that it's a good way to use find_package(flatbuffers) instead of include(cmake/OpenCVDetectFlatbuffers.cmake) because their script is better to handle with some corner cases.

Eg.

if(BUILD_opencv_dnn)

We shouldn't assume that dnn feature depends on flatbuffers or there is ONLY dnn feature depends on flatbuffers (i.e. if in the future, dnn is not depend on flatbuffers anymore or there is an another feature depends on flatbuffers), it will trigger errors again.

[WentsingNee]

May I have ask why CI failed if we remove the patch? Does it related to something like caches?

[WentsingNee]

I' ve seen that there are some commits removed the patches. It doesn't seem that vcpkg project disallow to remove port's patches

[WentsingNee]

And your patches is the last one applied, remove it will not cause other patches applied failed

[jimwang118]

And your patches is the last one applied, remove it will not cause other patches applied failed

Ok, I will test it locally and if there are no problems, I will mark it as reviewed.

[jimwang118]

Compile test pass with following triplets:

x64-windows
x64-windows-static