platform_build_soong/docs/best_practices.md

Build System Best Practices

Read only source tree

Never write to the source directory during the build, always write to $OUT_DIR. We expect to enforce this in the future.

If you want to verify / provide an update to a checked in generated source file, generate that file into $OUT_DIR during the build, fail the build asking the user to run a command (either a straight command, checked in script, generated script, etc) to explicitly copy that file from the output into the source tree.

Network access

Never access the network during the build. We expect to enforce this in the future, though there will be some level of exceptions for tools like distcc and goma.

Paths

Don't use absolute paths in Ninja files (with make's $(abspath) or similar), as that could trigger extra rebuilds when a source directory is moved.

Assume that the source directory is $PWD. If a script is going to change directories and needs to convert an input from a relative to absolute path, prefer to do that in the script.

Don't encode absolute paths in build intermediates or outputs. This would make it difficult to reproduce builds on other machines.

Don't assume that $OUT_DIR is out. The source and output trees are very large these days, so some people put these on different disks. There are many other uses as well.

Don't assume that $OUT_DIR is under $PWD, users can set it to a relative path or an absolute path.

$(shell) use in Android.mk files

Don't use $(shell) to write files, create symlinks, etc. We expect to enforce this in the future. Encode these as build rules in the build graph instead. This can be problematic in a number of ways:

• $(shell) calls run at the beginning of every build, at minimum this slows down build startup, but it can also trigger more build steps to run than are necessary, since these files will change more often than necessary.
• It's no longer possible for a stripped-down product configuration to opt-out of these created files. It's better to have actual rules and dependencies set up so that space isn't wasted, but the files are there when necessary.

Headers

LOCAL_COPY_HEADERS is deprecated. Soong modules cannot use these headers, and when the VNDK is enabled, System modules in Make cannot declare or use them either.

The set of global include paths provided by the build system is also being removed. They've been switched from using -isystem to -I already, and are removed entirely in some environments (vendor code when the VNDK is enabled).

Instead, use LOCAL_EXPORT_C_INCLUDE_DIRS/export_include_dirs. These allow access to the headers automatically if you link to the associated code.

If your library uses LOCAL_EXPORT_C_INCLUDE_DIRS/export_include_dirs, and the exported headers reference a library that you link to, use LOCAL_EXPORT_SHARED_LIBRARY_HEADERS/LOCAL_EXPORT_STATIC_LIBRARY_HEADERS/LOCAL_EXPORT_HEADER_LIBRARY_HEADERS (export_shared_lib_headers/export_static_lib_headers/export_header_lib_headers) to re-export the necessary headers to your users.

Don't use non-local paths in your LOCAL_EXPORT_C_INCLUDE_DIRS, use one of the LOCAL_EXPORT_*_HEADERS instead. Non-local exported include dirs are not supported in Soong. You may need to either move your module definition up a directory (for example, if you have ./src/ and ./include/, you probably want to define the module in ./Android.bp, not ./src/Android.bp), define a header library and re-export it, or move the headers into a more appropriate location.

Prefer to use header libraries (BUILD_HEADER_LIBRARY/ cc_library_headers) only if the headers are actually standalone, and do not have associated code. Sometimes there are headers that have header-only sections, but also define interfaces to a library. Prefer to split those header-only sections out to a separate header-only library containing only the header-only sections, and re-export that header library from the existing library. This will prevent accidentally linking more code than you need (slower at build and/or runtime), or accidentally not linking to a library that's actually necessary.

Prefer LOCAL_EXPORT_C_INCLUDE_DIRS over LOCAL_C_INCLUDES as well. Eventually we'd like to remove LOCAL_C_INCLUDES, though significant cleanup will be required first. This will be necessary to detect cases where modules are using headers that shouldn't be available to them -- usually due to the lack of ABI/API guarantees, but for various other reasons as well: layering violations, planned deprecations, potential optimizations like C++ modules, etc.

Use defaults over variables

Soong supports variable definitions in Android.bp files, but in many cases, it's better to use defaults modules like cc_defaults, java_defaults, etc.

• It moves more information next to the values -- that the array of strings will be used as a list of sources is useful, both for humans and automated tools. This is even more useful if it's used inside an architecture or target specific property.
• It can collect multiple pieces of information together into logical inheritable groups that can be selected with a single property.

Custom build tools

If writing multiple files from a tool, declare them all in the build graph.

• Make: Use .KATI_IMPLICIT_OUTPUTS
• Android.bp: Just add them to the out list in genrule
• Custom Soong Plugin: Add to Outputs or ImplicitOutputs

Declare all files read by the tool, either with a dependency if you can, or by writing a dependency file. Ninja supports a fairly limited set of dependency file formats. You can verify that the dependencies are read correctly with:

NINJA_ARGS="-t deps <output_file>" m

Prefer to list input files on the command line, otherwise we may not know to re-run your command when a new input file is added. Ninja does not treat a change in dependencies as something that would invalidate an action -- the command line would need to change, or one of the inputs would need to be newer than the output file. If you don't include the inputs in your command line, you may need to add the the directories to your dependency list or dependency file, so that any additions or removals from those directories would trigger your tool to be re-run. That can be more expensive than necessary though, since many editors will write temporary files into the same directory, so changing a README could trigger the directory's timestamp to be updated.

Only control output files based on the command line, not by an input file. We need to know which files will be created before any inputs are read, since we generate the entire build graph before reading source files, or running your tool. This comes up with Java based tools fairly often -- they'll generate different output files based on the classes declared in their input files. We've worked around these tools with the "srcjar" concept, which is just a jar file containing the generated sources. Our Java compilation tasks understand *.srcjar files, and will extract them before passing them on to the compiler.

Libraries in PRODUCT_PACKAGES

Most libraries aren't necessary to include in PRODUCT_PACKAGES, unless they're used dynamically via dlopen. If they're only used via LOCAL_SHARED_LIBRARIES / shared_libs, then those dependencies will trigger them to be installed when necessary. Adding unnecessary libraries into PRODUCT_PACKAGES will force them to always be installed, wasting space.