cc-wrapper: Make hygienic
See the added comments for what exactly has been done.
This commit is contained in:
John Ericson
@@ -1,21 +1,111 @@
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# CC Wrapper hygiene
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#
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# For at least cross compilation, we need to depend on multiple cc-wrappers at
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# once---specifically up to one per sort of dependency. This follows from having
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# different tools targeting different platforms, and different flags for those
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# tools. For example:
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#
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# # Flags for compiling (whether or not linking) C code for the...
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# NIX_BUILD_CFLAGS_COMPILE # ...build platform
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# NIX_CFLAGS_COMPILE # ...host platform
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# NIX_TARGET_CFLAGS_COMPILE # ...target platform
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#
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# Notice that these platforms are the 3 *relative* to the package using
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# cc-wrapper, not absolute like `x86_64-pc-linux-gnu`.
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#
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# The simplest solution would be to have separate cc-wrappers per (3 intended
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# use-cases * n absolute concrete platforms). For the use-case axis, we would
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# @-splice in 'BUILD_' '' 'TARGET_' to use the write environment variables when
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# building the cc-wrapper, and likewise prefix the binaries' names so they didn't
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# clobber each other on the PATH. But the need for 3x cc-wrappers, along with
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# non-standard name prefixes, is annoying and liable to break packages' build
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# systems.
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#
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# Instead, we opt to have just one cc-wrapper per absolute platform. Matching
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# convention, the binaries' names can just be prefixed with their target
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# platform. On the other hand, that means packages will depend on not just
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# multiple cc-wrappers, but the exact same cc-wrapper derivation multiple ways.
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# That means the exact same cc-wrapper derivation must be able to avoid
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# conflicting with itself, despite the fact that `setup-hook.sh`, the `addCvars`
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# function, and `add-flags.sh` are all communicating with each other with
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# environment variables. Yuck.
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#
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# The basic strategy is:
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#
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# - Everyone exclusively *adds information* to relative-platform-specific
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# environment variables, like `NIX_TARGET_CFLAGS_COMPILE`, to communicate
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# with the wrapped binaries.
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#
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# - The wrapped binaries will exclusively *read* cc-wrapper-derivation-specific
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# environment variables distinguished with with `infixSalt`, like
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# `NIX_@infixSalt@_CFLAGS_COMPILE`.
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#
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# - `add-flags`, beyond its old task of reading extra flags stuck inside the
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# cc-wrapper derivation, will convert the relative-platform-specific
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# variables to cc-wrapper-derivation-specific variables. This conversion is
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# the only time all but one of the cc-wrapper-derivation-specific variables
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# are set.
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#
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# This ensures the flow of information is exclusive from
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# relative-platform-specific variables to cc-wrapper-derivation-specific
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# variables. This allows us to support the general case of a many--many relation
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# between relative platforms and cc-wrapper derivations.
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#
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# For more details, read the individual files where the mechanisms used to
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# accomplish this will be individually documented.
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# It's fine that any other cc-wrapper will redefine this. Bash functions close
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# over no state, and there's no @-substitutions within, so any redefined
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# function is guaranteed to be exactly the same.
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ccWrapper_addCVars () {
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# The `depOffset` describes how the platforms of the dependencies are slid
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# relative to the depending package. It is brought into scope of the
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# environment hook defined as the role of the dependency being applied.
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case $depOffset in
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-1) local role='BUILD_' ;;
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0) local role='' ;;
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1) local role='TARGET_' ;;
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*) echo "cc-wrapper: Error: Cannot be used with $depOffset-offset deps, " >2;
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return 1 ;;
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esac
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if [[ -d "$1/include" ]]; then
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export NIX_CFLAGS_COMPILE+=" ${ccIncludeFlag:--isystem} $1/include"
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export NIX_${role}CFLAGS_COMPILE+=" ${ccIncludeFlag:--isystem} $1/include"
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fi
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if [[ -d "$1/lib64" && ! -L "$1/lib64" ]]; then
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export NIX_LDFLAGS+=" -L$1/lib64"
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export NIX_${role}LDFLAGS+=" -L$1/lib64"
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fi
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if [[ -d "$1/lib" ]]; then
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export NIX_LDFLAGS+=" -L$1/lib"
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export NIX_${role}LDFLAGS+=" -L$1/lib"
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fi
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if [[ -d "$1/Library/Frameworks" ]]; then
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export NIX_CFLAGS_COMPILE+=" -F$1/Library/Frameworks"
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export NIX_${role}CFLAGS_COMPILE+=" -F$1/Library/Frameworks"
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fi
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}
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# Since the same cc-wrapper derivation can be depend on in multiple ways, we
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# need to accumulate *each* role (i.e. target platform relative the depending
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# derivation) in which the cc-wrapper derivation is used.
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# `NIX_CC_WRAPPER_@infixSalt@_TARGET_*` tracks this (needs to be an exported env
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# var so can't use fancier data structures).
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#
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# We also need to worry about what role is being added on *this* invocation of
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# setup-hook, which `role` tracks.
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if [ -n "${crossConfig:-}" ]; then
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export NIX_CC_WRAPPER_@infixSalt@_TARGET_BUILD=1
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role="BUILD_"
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else
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export NIX_CC_WRAPPER_@infixSalt@_TARGET_HOST=1
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role=""
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fi
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# Eventually the exact sort of env-hook we create will depend on the role. This
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# is because based on what relative platform we are targeting, we use different
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# dependencies.
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envHooks+=(ccWrapper_addCVars)
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# Note 1: these come *after* $out in the PATH (see setup.sh).
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@@ -41,16 +131,12 @@ if [ -n "@coreutils_bin@" ]; then
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addToSearchPath _PATH @coreutils_bin@/bin
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fi
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if [ -z "${crossConfig:-}" ]; then
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ENV_PREFIX=""
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else
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ENV_PREFIX="BUILD_"
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fi
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# Export tool environment variables so various build systems use the right ones.
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export NIX_${ENV_PREFIX}CC=@out@
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export NIX_${role}CC=@out@
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export ${ENV_PREFIX}CC=@named_cc@
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export ${ENV_PREFIX}CXX=@named_cxx@
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export ${role}CC=@named_cc@
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export ${role}CXX=@named_cxx@
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for CMD in \
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cpp \
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@@ -59,9 +145,9 @@ do
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if
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PATH=$_PATH type -p "@binPrefix@$CMD" > /dev/null
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then
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export "${ENV_PREFIX}$(echo "$CMD" | tr "[:lower:]" "[:upper:]")=@binPrefix@${CMD}";
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export "${role}$(echo "$CMD" | tr "[:lower:]" "[:upper:]")=@binPrefix@${CMD}";
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fi
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done
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# No local scope available for sourced files
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unset ENV_PREFIX
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# No local scope in sourced file
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unset role
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