Add `@CURRENT_SOURCE_DIR@` to `custom_command`

Describe the bug Continuing from #7511

There may be a possible regression with 0.57 where @CURRENT_SOURCE_DIR@ has been removed from custom_target. I realize that’s not in the manual, but removing CURRENT_SOURCE_DIR broke my build and I can’t figure out a working replacement.

0.56.2 generates this build.ninja

...
build proto/settings.pb.cc: CUSTOM_COMMAND ../proto/settings.proto | /usr/bin/protoc
 COMMAND = /usr/bin/protoc --proto_path=../proto --cpp_out=proto ../proto/settings.proto
 description = Generating$ settings_pb_cc$ with$ a$ custom$ command

build proto/settings.pb.h: CUSTOM_COMMAND ../proto/settings.proto | /usr/bin/protoc
 COMMAND = /usr/bin/protoc --proto_path=../proto --cpp_out=proto ../proto/settings.proto
 description = Generating$ settings_pb_h$ with$ a$ custom$ command
 ...

0.57 generates:

build proto/settings.pb.cc: CUSTOM_COMMAND ../proto/settings.proto | /usr/bin/protoc
COMMAND = /usr/bin/protoc --proto_path=@CURRENT_SOURCE_DIR@ --cpp_out=proto ../proto/settings.proto
description = Generating$ settings_pb_cc$ with$ a$ custom$ command

build proto/settings.pb.h: CUSTOM_COMMAND ../proto/settings.proto | /usr/bin/protoc
COMMAND = /usr/bin/protoc --proto_path=@CURRENT_SOURCE_DIR@ --cpp_out=proto ../proto/settings.proto
description = Generating$ settings_pb_h$ with$ a$ custom$ command

To Reproduce

meson.build

protobuf = dependency('protobuf', version: '>=3.0.0')
protoc = find_program('protoc')

# many thanks to @marcelhollerbach for his solution.
# https://github.com/mesonbuild/meson/issues/7511#issuecomment-665470520

settings_pb_cc = custom_target('settings_pb_cc',
  input: ['settings.proto'],
  output: ['@BASENAME@.pb.cc'],
  command: [
    protoc,
    '--proto_path=@CURRENT_SOURCE_DIR@',
    '--cpp_out=@OUTDIR@',
    '@INPUT@',
  ]
)

settings_pb_h = custom_target('settings_pb_h',
  input: ['settings.proto'],
  output: ['@BASENAME@.pb.h'],
  command: [
    protoc,
    '--proto_path=@CURRENT_SOURCE_DIR@',
    '--cpp_out=@OUTDIR@',
    '@INPUT@',
  ],
  install: true,
  install_dir: get_option('includedir') / meson.project_name(),
)

settings_incdir = include_directories('.')

libazabacheproto = library(meson.project_name() + 'proto', settings_pb_cc,
  version: meson.project_version(),
  dependencies: protobuf,
  # FIXME(mdegans): remove this when moving to a 20.04 rootfs with new protobuf
  # becuase of this bug: https://github.com/protocolbuffers/protobuf/issues/4628
  cpp_args: ['-Wno-unused-parameter'],
  install: true,
)

azabacheproto_dep = declare_dependency(
  link_with: libazabacheproto,
  include_directories: include_directories('.'),
  dependencies: protobuf,
)

# generate a separate .pc for just the protobuf code.
pkg = import('pkgconfig')
azabacheproto_pc = pkg.generate(libazabacheproto,
  description: 'Azabache Camera Metadata Library',
  url: project_url,
  # for consistency with existing cmake install:
  install_dir: get_option('datadir') / 'pkgconfig',
  subdirs: meson.project_name(),
)

Any setttings.proto should do.

As a workaround, i tried '--proto_path=' + meson.current_source_dir(), but this doesn’t seem to do the same thing. protoc does not like absolute paths, apparently:

[1/56] Generating settings_pb_cc with a custom command
FAILED: proto/settings.pb.cc 
/usr/bin/protoc --proto_path=/home/user/Projects/lib-azabache/proto --cpp_out=proto ../proto/settings.proto
../proto/settings.proto: File does not reside within any path specified using --proto_path (or -I).  You must specify a --proto_path which encompasses this file.  Note that the proto_path must be an exact prefix of the .proto file names -- protoc is too dumb to figure out when two paths (e.g. absolute and relative) are equivalent (it's harder than you think).
[2/56] Generating settings_pb_h with a custom command
FAILED: proto/settings.pb.h 
/usr/bin/protoc --proto_path=/home/user/Projects/lib-azabache/proto --cpp_out=proto ../proto/settings.proto
../proto/settings.proto: File does not reside within any path specified using --proto_path (or -I).  You must specify a --proto_path which encompasses this file.  Note that the proto_path must be an exact prefix of the .proto file names -- protoc is too dumb to figure out when two paths (e.g. absolute and relative) are equivalent (it's harder than you think).
[3/56] Compiling C++ object src/libazabache-0.1.so.p/encoder.cpp.o
FAILED: src/libazabache-0.1.so.p/encoder.cpp.o 

Expected behavior @CURRENT_SOURCE_DIR in custom_target to be replaced with the crurrent source dir, or for there to be a usable replacement. Per the referenced issue, generator was unsuitable for this task. I’m using custom target, actually two of them, since I couldn’t figure out a cleaner way.

system parameters

• plain, native build
• Ubuntu 18.04 aarch64 (Tegra)
• Python 3.6.9
• meson 0.57
• ninja 1.8.1

[xclaesse]

@mdegans Indeed, also notice how replace_paths() does not exist in vs2010backend.py, so those never worked there.

[xclaesse]

There are millions of subtle inconsistencies in what is supported in add_*_script(), run_target(), custom_target(), and generator(). I’m working on refactoring them all to use as much as possible the same code path there: https://github.com/mesonbuild/meson/pull/8342.

That refactor is for 0.58, not sure we want a “fix” for 0.57 since it was never documented…