[cmake-developers] [CMake 0015775]: Ninja will unnecessarily relink on windows if a library exports no symbols

[Mantis Bug Tracker]

The following issue has been SUBMITTED. 
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http://public.kitware.com/Bug/view.php?id=15775 
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Reported By:                James Johnston
Assigned To:                
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Project:                    CMake
Issue ID:                   15775
Category:                   CMake
Reproducibility:            always
Severity:                   minor
Priority:                   normal
Status:                     new
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Date Submitted:             2015-10-07 18:34 EDT
Last Modified:              2015-10-07 18:34 EDT
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Summary:                    Ninja will unnecessarily relink on windows if a
library exports no symbols
Description: 
This is very similar to the issue reported by Nils Gladitz:
0015666: Ninja may unnecessarily relink on windows
http://public.kitware.com/Bug/view.php?id=15666

However, my test case does not work on CMake 3.4.0-rc1 either.  In this
situation, the issue arises when the library exports no symbols.

My test case is almost identical to the one from Nils, except notice that
test.cpp is now a blank file: I don't export any symbols.  If the project does
not export any symbols, the linker *will not emit a LIB file at all.*

It initially sounds non-sensical, but not exporting any symbols is not as
uncommon as one might think; some examples:

 * It is common that a .NET C++/CLR project will not export unmanaged symbols,
as it is exporting directly through the DLL (i.e. no LIB file).
 * A DLL being used for the purpose of storing Win32 resources would not export
any symbols.
 * Some 3rd-party libraries (e.g. FLANN) happily seem to link an empty
library...

Steps to Reproduce: 
My test case, as you can see it's basically the same as Nils in
http://public.kitware.com/Bug/view.php?id=15666 except no symbols:

CMakeLists.txt
--------------

# Using CMake 3.4.0-rc1 here:
cmake_minimum_required(VERSION 3.4)

project(Foo CXX)

if(NOT EXISTS test.cpp)
    # Unlike Nils, I emit no symbols at all here:
    file(WRITE test.cpp "")
endif()

add_custom_target(touch
    COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_SOURCE_DIR}/test.cpp
)

add_library(foo SHARED test.cpp)

Procedure:
----------

1.  Configure with Ninja from a Visual C++ 2008 command prompt.  (Other VC++
versions will likely exhibit the same issue).
2.  ninja   # builds as expected
3.  ninja   # continues to build
4.  ninja -d explain

ninja explain: output foo.lib doesn't exist
ninja explain: foo.dll is dirty
[1/1] Linking CXX shared library foo.dll
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Issue History 
Date Modified    Username       Field                    Change               
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2015-10-07 18:34 James Johnston New Issue                                    
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