ABSPLITDBG: debug symbols in a something-dbg package

[cthbleachbit]

This PR introduces a new variable for use in defines: ABSPLITDBG. This variable controls whether a separate package should be built to ship debugging symbols. Here are the detailed semantics:

• A package great-pack will produce another deb package great-pack-dbg with the exact same version when ABSPLITDBG is specified. This package will contain debug symbols at /usr/lib/debug/.build-id/SH/A256_BUILD_ID.debug for each of ELF files shipped in the base package. The debug package only depends on the base package with the exact same version to minimize confusion from version mismatch of binaries and their symbols.
  • This implies that the version number of a debug package should strictly match its base package, and should be pushed to repo server in pairs.
• ABSPLITDBG has no effect when ABSTRIP is unset. Since ELFs will retain debug symbols, it makes no sense to ABSPLITDBG. autobuild3 currently produces a warning message and unsets ABSPLITDBG in this case.
• Setting ABSPLITDBG will append -ggdb to CFLAGS and CXXFLAGS so that ELFs built will actually contain meaningful debug symbols.

In RFC state:

• Techincally a package marked ABHOST=noarch cannot contain ELF, thus by extension should never produce a symbol package. Currently autobuild3 does not check against this case and will produce an empty symbol package.
  • Exception: optenv32 is marked as noarch but is allowed to contain ELF. This might require special handling.
• On the other hand, a non-noarch package should always has some sort of ELF file, thus by extension can always produce a debug package.
  • There are some exception to this rule.
  • Node, electron-based software and JavaScript related packages might produce unreasonably huge debug packages.
  • Software containing rust / go codes requires specific procedures and do not need stripped symbols anyway.
  • The base package in question must be compiled with -g to produce meaningful symbols, but some packages (say bzip2 and dhcp) are known to ignore CFLAGS, CXXFLAGS and other environment variables. An ELF without debugging symbol can be successfully stripped, but the symbol file produced won't be helpful at all.
  • Multiple subpackages built in succession need special attention. Although each 01-first, 02-second, 03-third and so on may specify different values for ABSPLITDBG, for 02-second and 03-third to produce meaningful debugging symbols, 01-first must be properly compiled with -g even when symbol package for 01 is not needed.
• I would advise against setting ABSPLITDBG by default.

[cthbleachbit]

The issues in RFC status above are currently resolved as following:

• For packages labeled noarch:
  • AB3 will disable symbol package generation by default.
  • This default behavior may be explicitly overriden by specifying ABSPLITDBG=1 in defines. AB3 will produce a warning message.
• For packages labeled otherwise:
  • AB3 will enable symbol package generation by default.
  • AB3 will warn about ELF without .debug_blah sections. Such ELF files won't produce debug symbol files.
• AB3 will check whether any symbol files are actually produced, and will not produce a symbol package if there is none.