Make the microgenerator run under Linux

generateapis.sh

@@ -38,14 +38,6 @@ fetch_github_repos() {
       --config core.eol=lf
   fi
 
-  if [ -d "gapic-generator-csharp" ]
-  then
-    git -C gapic-generator-csharp pull -q
-    git -C gapic-generator-csharp submodule update
-  else
-    git clone --recursive https://github.com/googleapis/gapic-generator-csharp
-  fi
-
   if [[ "$SYNTHTOOL_GOOGLEAPIS" == "" ]]
   then
     if [ -d "googleapis" ]
@@ -237,6 +229,7 @@ generate_api() {
 # Entry point
 
 install_protoc
+install_microgenerator
 install_grpc
 fetch_github_repos
 GAPIC_GENERATOR_VERSION=$(cat gapic-generator/version.txt)
@@ -246,10 +239,6 @@ GAPIC_GENERATOR_VERSION=$(cat gapic-generator/version.txt)
 # won't be using Gradle at all by the end of 2018, with any luck...
 (cd gapic-generator; ./gradlew shadowJar --warning-mode=none)
 
-# Build the microgenerator
-(cd gapic-generator-csharp; dotnet publish -c Release --self-contained --runtime=win-x64 Google.Api.Generator)
-declare -r GAPIC_PLUGIN=gapic-generator-csharp/Google.Api.Generator/bin/Release/netcoreapp2.2/win-x64/publish/Google.Api.Generator.exe
-
 OUTDIR=tmp
 rm -rf $OUTDIR
 mkdir $OUTDIR

toolversions.sh

@@ -90,6 +90,38 @@ install_protoc() {
   chmod +x $PROTOC
 }
 
+install_microgenerator() {
+  # TODO: Use a specific tag, or even a NuGet package eventually
+  if [ -d "gapic-generator-csharp" ]
+  then
+    git -C gapic-generator-csharp pull -q
+    git -C gapic-generator-csharp submodule update
+  else
+    git clone --recursive https://github.com/googleapis/gapic-generator-csharp
+  fi
+
+  case "$OSTYPE" in
+    linux*)
+      declare -r RUNTIME=linux-x64
+      declare -r EXTENSION=
+      ;;
+    darwin*)
+      echo "Microgenerator not currently supported on MacOSX. Ask jonskeet@ for help."
+      exit 1
+      ;;
+    win* | msys* | cygwin*)
+      declare -r RUNTIME=win-x64
+      declare -r EXTENSION=.exe
+      ;;
+    *)
+      echo "Unknown OSTYPE: $OSTYPE"
+      exit 1
+  esac
+  (cd gapic-generator-csharp; dotnet publish -c Release --self-contained --runtime=$RUNTIME Google.Api.Generator)
+
+  export GAPIC_PLUGIN=$REPO_ROOT/gapic-generator-csharp/Google.Api.Generator/bin/Release/netcoreapp2.2/$RUNTIME/publish/Google.Api.Generator$EXTENSION
+}
+
 install_grpc() {
   install_nuget_package Grpc.Tools $GRPC_VERSION
   chmod +x $GRPC_PLUGIN