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cmake: fix problem that go_library is never rebuilt. #2954

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Jul 19, 2017
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cmake: fix problem that go_library is never rebuilt. #2954

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22 changes: 21 additions & 1 deletion cmake/generic.cmake
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Original file line number Diff line number Diff line change
Expand Up @@ -290,8 +290,22 @@ function(go_library TARGET_NAME)
set(${TARGET_NAME}_LIB_NAME "${CMAKE_STATIC_LIBRARY_PREFIX}${TARGET_NAME}${CMAKE_STATIC_LIBRARY_SUFFIX}" CACHE STRING "output library name for target ${TARGET_NAME}")
endif()

# Add dummy code to support `make target_name` under Terminal Command
set(dummyfile ${CMAKE_CURRENT_BINARY_DIR}/${TARGET_NAME}_dummy.c)

# This custom command will always run since it depends on a not
# existing file.
add_custom_command(
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Naive question: why add a custom command since we already have file(WRITE ${dummyfile} "const char * dummy = \"${dummyfile}\";") below?

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@helinwang helinwang Jul 19, 2017 •
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From my understanding file will create a file when cmake is called, during the Makefile generation process. But it will never be called when make is called. This has the problem that after the first time building, go_library never gets build again even the source code changed.

In contrast, the custom command is called when make is called, forcing the rebuild.

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I see. Thanks! No rebuild is due to we build go libs by add_custom_command , so make will not track the source file changes.

OUTPUT dummy_rebulid_${TARGET_NAME}
COMMAND cmake -E touch ${dummyfile}
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@gangliao gangliao Jul 19, 2017 •
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Is that being built every time? Even if there no changes?

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@helinwang helinwang Jul 19, 2017 •
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Yes. I thought it's better to rebuild even there is no change than not rebuild when there is change.

It's hard to track if the library source to be built changed or not, since the files may depend on other files that have changed located in other directories.

The good news is Go compiler caches the build result for all dependencies when they have not changed.

)
# Create a custom target that depends on the custom command output
# file, so the custom command can be referenced as a dependency by
# `add_dependencies`.
add_custom_target(rebuild_${TARGET_NAME}
DEPENDS dummy_rebulid_${TARGET_NAME}
)

# Add dummy code to support `make target_name` under Terminal Command
file(WRITE ${dummyfile} "const char * dummy = \"${dummyfile}\";")
if (go_library_SHARED OR go_library_shared)
add_library(${TARGET_NAME} SHARED ${dummyfile})
Expand All @@ -302,6 +316,12 @@ function(go_library TARGET_NAME)
add_dependencies(${TARGET_NAME} ${go_library_DEPS})
endif(go_library_DEPS)

# The "source file" of the library is `${dummyfile}` which never
# change, so the target will never rebuild. Make the target depends
# on the custom command that touches the library "source file", so
# rebuild will always happen.
add_dependencies(${TARGET_NAME} rebuild_${TARGET_NAME})

set(${TARGET_NAME}_LIB_PATH "${CMAKE_CURRENT_BINARY_DIR}/${${TARGET_NAME}_LIB_NAME}" CACHE STRING "output library path for target ${TARGET_NAME}")

file(GLOB GO_SOURCE RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}" "*.go")
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