cmake: incorrectly reporting MSVC as using ccache #31771

1. 
   fanquake commented at 2:35 pm on January 31, 2025: member

   Mentioned by @stickies-v. The CMake configure currently outputs that MSVC builds are using ccache, i.e (https://github.com/bitcoin/bitcoin/actions/runs/13063954360/job/36452895056#step:8:2374):

   0Treat compiler warnings as errors ..... ON
   1Use ccache for compiling .............. ON
   

   However that’s a bug, as MSVC is currently excluded entirely from using ccache: https://github.com/bitcoin/bitcoin/blob/8fa10edcd1706a1f0dc9d8c3adbc8efa3c7755bf/cmake/ccache.cmake#L4-L6

2. fanquake added this to the milestone 29.0 on Jan 31, 2025
3. 
   vijayabhaskar78 commented at 10:40 am on February 2, 2025: none

   Hey, can I contribute to this issue
4. 
   hebasto commented at 10:44 am on February 2, 2025: member

   @vijayabhaskar78

   Hey, can I contribute to this issue

   Sure! No need to ask :)

5. hebasto added the label Build system on Feb 2, 2025
6. 
   vijayabhaskar78 commented at 10:59 am on February 2, 2025: none

   @hebasto sir Proposing a fix for #31771:

   Set USE_CCACHE=OFF for MSVC in cmake/ccache.cmake.
   Update status reporting in CMakeLists.txt.
   

   Questions:

   Does this align with project conventions?
   Any edge cases (MinGW/cross-compile) to test?

7. 
   purpleKarrot commented at 6:36 pm on February 2, 2025: contributor

   I don’t understand the motivation for setting CMAKE_<lang>_COMPILER_LAUNCHER from inside the project. Ideally, the project should not interfere with compiler launchers at all.

   This gives individual users as well as CI instances the highest flexibility.

   Assuming I have ENV{CMAKE_CXX_COMPILER_LAUNCHER} set locally to use sccache. What is the expected behavior when cmake/ccache.cmake invokes list(APPEND CMAKE_CXX_COMPILER_LAUNCHER ${CCACHE_EXECUTABLE})? Will it use sccache, ccache, or both?

8. 
   vijayabhaskar78 commented at 6:53 pm on February 2, 2025: none

   @purpleKarrot Should we remove the CMAKE_*_COMPILER_LAUNCHER assignments from ccache.cmake to prioritize user/CI flexibility? This would:

   Avoid conflicts with external launchers like sccache set via ENV{CMAKE_CXX_COMPILER_LAUNCHER}.
   Follow CMake best practices by letting users control launchers externally.
   Prevent invalid command chains (e.g., sccache;ccache;cl.exe) when multiple launchers are set.
   

   If agreed, I can update the PR to delete these lines and add documentation instead

9. 
   vijayabhaskar78 commented at 6:58 pm on February 2, 2025: none

   I’m a newbie to this repository. Can you help me to understand it?
10. 
    purpleKarrot commented at 7:08 pm on February 2, 2025: contributor

    @vijayabhaskar78, I am a newbe in this repository too, but I am not a newbie with CMake ;-) My recommendation would be to remove cmake/ccache.cmake for the reason you wrote. But let the original authors of that file chime in.
11. 
    vijayabhaskar78 commented at 7:28 pm on February 2, 2025: none

    OK let us see
12. 
    vijayabhaskar78 commented at 12:03 pm on February 3, 2025: none

    @fanquake @hebasto @purpleKarrot

    Problem Summary

    • False Reporting: CMake incorrectly reports ccache as “ON” for MSVC builds despite excluding it in cmake/ccache.cmake.
    • Launcher Conflicts: Forced CMAKE_*_COMPILER_LAUNCHER assignments override user/CI settings (e.g., sccache).

    Proposed Fix

    1. Delete cmake/ccache.cmake

    0rm cmake/ccache.cmake
    

    • Why? Removes forced ccache logic conflicting with external launchers.
    • Impact: Users/CI regain full control via CMAKE_CXX_COMPILER_LAUNCHER.

    2. Update Status Message

    0if(DEFINED ENV{CMAKE_CXX_COMPILER_LAUNCHER})  
    1  message(STATUS "Compiler launcher .............. $ENV{CMAKE_CXX_COMPILER_LAUNCHER}")  
    2else()  
    3  message(STATUS "Use ccache for compiling .............. NO")  
    4endif()
    

    • Result: Accurately reflects whether a launcher (e.g., ccache) is active.

    3. Document MSVC + ccache Workflow

    Add to doc/build-windows.md:

    Caching MSVC Builds

    • Step 1: Use Ninja:

      0-G "Ninja"
      

    • Step 2: Configure launcher:

      0cmake -B build -DCMAKE_CXX_COMPILER_LAUNCHER=ccache
      

    • Step 3: Use /Z7 (not /Zi) for debug flags.

    Why This Approach?

    • Flexibility: No forced ccache/generator logic. Users/CI control launchers via CMake/env vars.
    • CMake Compliance: Follows CMake best practices.
    • Simplification: Deletes 50+ lines of error-prone code.

    Feedback Requested

    • @purpleKarrot: Does this address your concerns about launcher flexibility?
    • Maintainers (@fanquake, @hebasto): Any objections to deleting ccache.cmake?
13. vijayabhaskar78 referenced this in commit 26a927fd80 on Feb 6, 2025
14. 
    s373nZ commented at 8:51 pm on February 18, 2025: none

    I don’t understand the motivation for setting CMAKE_<lang>_COMPILER_LAUNCHER from inside the project.

    I’m also ignorantly curious about this. Is it primarily a requirement from the feature parity list with the prior autotools?

    https://gist.github.com/hebasto/2ef97d3a726bfce08ded9df07f7dab5e

Contributors
fanquake vijayabhaskar78 hebasto purpleKarrot s373nZ

Labels
Build system

Milestone
29.0