Sahil Takiar created IMPALA-8101:
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Summary: Thrift 11 compilation and Thrift ext-data-source
compilation are always run
Key: IMPALA-8101
URL: https://issues.apache.org/jira/browse/IMPALA-8101
Project: IMPALA
Issue Type: Task
Reporter: Sahil Takiar
Assignee: Sahil Takiar
[~tarmstrong] pointed out that after IMPALA-7924 the build output started
displaying lines such as: "Running thrift 11 compiler on..." even during builds
when Thrift files were not modified.
I dug a bit deeper and found the following:
* This seems to be happening for Thrift compilation of {{ext-data-source}}
files as well (e.g. ExternalDataSource.thrift, Types.thrift, etc.); "Running
thrift compiler for ext-data-source on..." is always printed
* The issue is that the [custom
command|https://cmake.org/cmake/help/v3.8/command/add_custom_command.html] for
ext-data-source and Thrift 11 compilation specify an {{OUTPUT}} file that does
not exist (and is not generated by Thrift)
* According to the CMake docs "if the command does not actually create the
{{OUTPUT}} then the rule will always run" - so Thrift compilation will run
during every build
* The issue is that you don't really know what files Thrift is going to
generate without actually looking into the Thrift file and understanding Thrift
internals
* For C++ and Python there is a workaround; for C++ Thrift always generates a
file \{THRIFT_FILE_NAME}_types.h (similar situation for Python); however, for
Java no such file necessarily exists (ext-data-source only does Java gen)
** This is how regular Thrift compilation works (e.g. compilation of
beeswax.thrift, ImpalaService.thrift, etc.); which is why we don't see the
issue for regular Thrift compilation
A solution for Thrift 11 compilation is to just add generated Python files to
the {{OUTPUT}} for the custom_command.
A solution for Thrift compilation of ext-data-source seems trickier, so open to
suggestions.
Ideally, Thrift would be provide a way to return the list of files generated
from a .thrift file, without actually generating the files, but I don't see a
way to do that.
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