feat: portable rpc server builds use fork/threading less (#1231)

Now the `NotThreaded` cabal flag can be used to
omit the `-threaded` flag when building the rpc
servers, which reduces the number of `fork`
invocations when running the servers.

cryptol-remote-api/Dockerfile

@@ -91,8 +91,10 @@ ENV LANG=C.UTF-8 \
     LC_ALL=C.UTF-8
 COPY cabal.GHC-${GHCVER}.config cabal.project.freeze
 
+RUN if ${PORTABILITY}; then SERVERS_NOT_THREADED="true"; fi
+
 RUN cabal v2-update && \
-    cabal v2-build -j cryptol-remote-api:exe:cryptol-remote-api cryptol-remote-api:exe:cryptol-eval-server && \
+    cabal v2-build -j ${SERVERS_NOT_THREADED:+-fNotThreaded} cryptol-remote-api:exe:cryptol-remote-api cryptol-remote-api:exe:cryptol-eval-server && \
     mkdir -p rootfs/usr/local/bin && \
     cp $(cabal v2-exec which cryptol-remote-api) rootfs/usr/local/bin && \
     cp $(cabal v2-exec which cryptol-eval-server) rootfs/usr/local/bin

cryptol-remote-api/cryptol-remote-api.cabal

@@ -12,6 +12,11 @@ flag static
   default: False
   description: Create a statically-linked binary
 
+flag NotThreaded
+  default: false
+  manual: true
+  description: Omit the -threaded ghc flag
+
 common warnings
   ghc-options:
     -Wall
@@ -79,8 +84,10 @@ executable cryptol-remote-api
   import:              deps, warnings, errors
   main-is:             Main.hs
   hs-source-dirs:      cryptol-remote-api
-  ghc-options:
-    -threaded -rtsopts -with-rtsopts=-xb0x200000000
+  if flag(NotThreaded)
+    ghc-options: -rtsopts -with-rtsopts=-xb0x200000000
+  else
+    ghc-options: -threaded -rtsopts -with-rtsopts=-xb0x200000000
 
   build-depends:
     cryptol-remote-api,
@@ -93,8 +100,10 @@ executable cryptol-eval-server
   import:              deps, warnings, errors
   main-is:             Main.hs
   hs-source-dirs:      cryptol-eval-server
-  ghc-options:
-    -threaded -rtsopts
+  if flag(NotThreaded)
+    ghc-options: -rtsopts -with-rtsopts=-xb0x200000000
+  else
+    ghc-options: -threaded -rtsopts -with-rtsopts=-xb0x200000000
 
   build-depends:
     cryptol-remote-api,