How to make derived header-only libraries?

[rnburn]

I'm using bazel to build an apache module. I have dependencies set up like this

configure_make(
    name = "apr",
    configure_options = [
        "--enable-shared=no",
        "--with-pic",
    ],
    lib_source = "@org_apache_apr//:all",
    static_libraries = ["libapr-1.a"],
)
...
configure_make(
    name = "apache_httpd",
    configure_options = [
        "--with-apr=$EXT_BUILD_DEPS/apr",
        "--with-apr-util=$EXT_BUILD_DEPS/apr_util",
        "--with-pcre=$EXT_BUILD_DEPS/pcre",
        "--enable-mods-static='proxy proxy_balancer unixd authz_core slotmem_shm'",
    ],
    lib_source = "@org_apache_httpd//:all",
    out_bin_dir = ".",
    binaries = [
        "bin/httpd",
        "bin/apachectl",
    ],
    deps = [
        ":apr",
        ":apr_util",
        ":pcre",
    ],
    visibility = [
        "//visibility:public",
    ],
)

When I build a .so apache module, I need to include the apr dependency headers but not link to libapr-1.a because those symbols are resolved when the module is loaded into the apache binary. Is there a way that I can make a derived cc_library from :apr that only has the headers?

I'm currently adding an additional configure_make like this

configure_make(
    name = "apr_headers",
    configure_options = [
        "--enable-shared=no",
        "--with-pic",
    ],
    lib_source = "@org_apache_apr//:all",
    out_include_dir = "include/apr-1",
    headers_only = True,
)

But that hardly seems optimal as I have to configure_make apr twice.

[irengrig]

Hi @rnburn, you can have only one configure_make target, with different paths to select only headers or headers and libraries together.
Here is the example how I was able to select headers:
`
filegroup(
name = "httpd_dir",
srcs = [
":apache_httpd",
],
output_group = "gen_dir"
)
genrule(
name = "select_headers",
srcs = [":httpd_dir"],
cmd = "cp -r $(location httpd_dir)/include $(location out_dir)",
outs = ["out_dir"]
)

genrule(
name = "list_headers",
srcs = [":select_headers"],
cmd = "ls $(location select_headers) > $(location list.txt)",
outs = ["list.txt"]
)
`

• I used list_headers only to make select_headers to be built and demonstrate it works, you do not need it in the real build.
• You can also have a custom rule for filtering and do everything with Starlark, because with genrule it is platform-bound.
• Unfortunately, it seems to not work just woth filegroup like: srcs = glob([":target/subdir"]) - probably, glob syntax will always look into the current directory where the BUILD file is and not at the target...

[rnburn]

Thanks @irengrig. I'm still having some trouble getting this to work. Here's the setup I have to produce a cpython_header_only library to use with a C++ python module

configure_make(
    name = "openssl",
    configure_command = "config",
    configure_options = [
        "no-shared",
    ],
    lib_source = "@com_github_openssl_openssl//:all",
    static_libraries = [
        "libcrypto.a",
        "libssl.a",
    ],
)

configure_make(
    name = "cpython",
    configure_options = [
        "--with-openssl=$EXT_BUILD_DEPS/openssl",
        # Hack to work around https://github.com/bazelbuild/rules_foreign_cc/issues/239
        "CFLAGS='-Dredacted=\"redacted\"'",
    ],
    lib_source = "@com_github_python_cpython//:all",
    out_include_dir = "include/python3.7m",
    binaries = [
        "python3",
    ],
    visibility = [
        "//visibility:public",
    ],
    deps = [
        ":openssl",
    ],
)

filegroup(
    name = "cpython_files",
    srcs = [
        ":cpython",
    ],
    visibility = [
        "//visibility:public",
    ],
    output_group = "gen_dir",
)

genrule(
    name = "cpython_includes",
    srcs = [
        ":cpython_files",
    ],
    cmd = "cp -r $(location cpython_files)/include/python3.7m $(location include)",
    outs = ["include"],
)

cc_library(
    name = "cpython_header_only",
    srcs = [
        ":cpython_includes",
    ],
    copts = ["-isystem include"],
)

But I get the error

root@193498bdc412:/src# bazel build //...
ERROR: /src/3rd_party/BUILD:65:12: in srcs attribute of cc_library rule //3rd_party:cpython_header_only: '//3rd_party:cpython_includes' does not produce any cc_library srcs files (expected .cc, .cpp, .cxx, .c++, .C, .c, .h, .hh, .hpp, .ipp, .hxx, .inc, .inl, .H, .S, .s, .asm, .a, .lib, .pic.a, .lo, .lo.lib, .pic.lo, .so, .dylib, .dll, .o, .obj or .pic.o)
ERROR: Analysis of target '//3rd_party:cpython_header_only' failed; build aborted: Analysis of target '//3rd_party:cpython_header_only' failed; build aborted
INFO: Elapsed time: 0.638s
INFO: 0 processes.

Any ideas?

[rnburn]

I was able to get things to work with this variant

cc_library(
    name = "cpython_header_only",
    hdrs = [
        ":cpython_includes",
    ],
    copts = ["-isystem include"],
)

though not sure why that fixed the problem

[rnburn]

Another curious error

If I use this version (which works better when the project is used externally bazelbuild/bazel#2670 )

cc_library(
    name = "cpython_header_only",
    hdrs = [
        ":cpython_includes",
    ],
    includes = ["include"],
)

I get

ERROR: /src/src/lib/BUILD:9:1: undeclared inclusion(s) in rule '//src/lib:bridge_tracer_lib':
this rule is missing dependency declarations for the following files included by 'src/lib/python_object_wrapper.cpp':
  'bazel-out/k8-fastbuild/genfiles/3rd_party/include/Python.h'
  'bazel-out/k8-fastbuild/genfiles/3rd_party/include/patchlevel.h'
...

[rnburn]

@irengrig - this approach doesn't quite work. It still links in dependencies. This is my setup for a cpython_header_only library.

But when I build I get

bazel build -s //:bridge_tracer.so
...
/usr/bin/gcc -shared -o bazel-out/k8-fastbuild/bin/bridge_tracer.so -Wl,-S '-fuse-ld=gold' -Wl,-no-as-needed -Wl,-z,relro,-z,now -B/usr/bin -pass-exit-codes -Wl,@bazel-out/k8-fastbuild/bin/bridge_tracer.so-2.params

And bazel-out/k8-fastbuild/bin/bridge_tracer.so-2.params has the cpython dependencies linked in

cat bazel-out/k8-fastbuild/bin/bridge_tracer.so-2.params
...
-whole-archive
bazel-out/k8-fastbuild/genfiles/3rd_party/openssl/lib/libssl.a
-no-whole-archive

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[github-actions]

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