Static builds: use an environment variable rather than files laying around

Thanks @gasche for the hint about this hidden dune feature :)

Author: AltGr

.github/workflows/static-builds.yml

@@ -52,8 +52,7 @@ jobs:
           opam switch create . ocaml-base-compiler 'dune<2' --deps-only
       - name: Build the binaries
         run: |
-          ./scripts/setup-static-build.sh
-          opam exec -- make
+          opam exec -- make LINKING_MODE=static
           otool -L _build/install/default/bin/*
       - name: Archive static binaries
         uses: actions/upload-artifact@v2

scripts/setup-static-build.sh

@@ -11,8 +11,8 @@ tar c $(git ls-files) | \
   docker run --rm -i \
     ocamlpro/ocaml:4.05 \
     sh -uexc \
-      'tar x >&2 && ./scripts/setup-static-build.sh &&
+      'tar x >&2 &&
        opam switch create . ocaml-system "dune<2" --deps-only >&2 &&
-       opam exec make >&2 &&
+       opam exec make LINKING_MODE=static >&2 &&
        tar c -hC _build/install/default/bin .' | \
   tar vx

scripts/static-build.sh

@@ -56,13 +56,13 @@
 
 (rule
   (targets linking_main.sexp)
-  (mode    fallback)
-  (action  (with-stdout-to %{targets} (echo "()"))))
+  (action  (with-stdout-to %{targets}
+    (run ./linking_flags.sh %{env:LINKING_MODE=dynamic} laolao_stubs threads camlrun))))
 (rule
   (targets linking_client.sexp)
-  (mode    fallback)
-  (action  (with-stdout-to %{targets} (echo "()"))))
+  (action  (with-stdout-to %{targets}
+    (run ./linking_flags.sh %{env:LINKING_MODE=dynamic} threads camlrun))))
 (rule
   (targets linking_server.sexp)
-  (mode    fallback)
-  (action  (with-stdout-to %{targets} (echo "()"))))
+  (action  (with-stdout-to %{targets}
+    (run ./linking_flags.sh %{env:LINKING_MODE=dynamic} laolao_stubs threadsnat))))

src/main/dune

@@ -0,0 +1,61 @@
+#!/bin/sh
+set -ue
+
+# This script is called by dune to generate the linking flags for static builds
+# (on the limited set of supported platforms). It only returns an empty set of
+# flags for the default dynamic linking mode.
+
+LC_ALL=C
+
+echo ";; generated by $0"
+
+case "$1" in
+    dynamic) echo "()"; exit 0;;
+    static) ;;
+    *) echo "Invalid linking mode '$1'. Usage: $0 dynamic|static [extra-libs]" >&2; exit 2
+esac
+
+shift
+EXTRA_LIBS="$*"
+
+## Static linking configuration ##
+
+# The linked C libraries list may need updating on changes to the dependencies.
+#
+# To get the correct list for manual linking, the simplest way is to set the
+# flags to `-verbose`, while on the normal `autolink` mode, then extract them
+# from the gcc command-line.
+
+case $(uname -s) in
+    Linux)
+        case $(. /etc/os-release && echo $ID) in
+            alpine)
+                COMMON_LIBS="camlstr base_stubs ssl_threads_stubs ssl crypto cstruct_stubs lwt_unix_stubs bigarray unix c"
+                # `m` and `pthreads` are built-in musl
+                echo '(-noautolink'
+                echo ' -cclib -Wl,-Bstatic'
+                echo ' -cclib -static-libgcc'
+                for l in $EXTRA_LIBS $COMMON_LIBS; do
+                    echo " -cclib -l$l"
+                done
+                echo ' -cclib -static)'
+                ;;
+            *)
+                echo "Error: static linking is only supported in Alpine, to avoids glibc constraints" >&2
+                exit 3
+        esac
+        ;;
+    Darwin)
+        COMMON_LIBS="camlstr base_stubs ssl_threads_stubs /usr/local/opt/openssl/lib/libssl.a /usr/local/opt/openssl/lib/libcrypto.a cstruct_stubs lwt_unix_stubs bigarray unix pthread"
+        echo '(-noautolink'
+        for l in $EXTRA_LIBS $COMMON_LIBS; do
+            if [ "${l%.a}" != "${l}" ]; then echo " -cclib $l"
+            else echo " -cclib -l$l"
+            fi
+        done
+        echo ')'
+        ;;
+    *)
+        echo "Static linking is not supported for your platform. See $0 to contribute." >&2
+        exit 3
+esac