Introduce BP_KEEP_NODE_BUILD_CACHE to safeguard existing behaviour

Author: c0d1ngm0nk3y

README.md

@@ -40,9 +40,10 @@ file that looks like the following:
 
 ## Configuration
 
-| Environment Variable          | Description                                                                                                                                                               |
-|-------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
-| `$BP_NPM_VERSION`     | If set, this custom version of `npm` will be used instead of the one provided by the `nodejs` installation.                                                                       |
+| Environment Variable        | Description                                                                                                                                                               |
+|-----------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `$BP_NPM_VERSION`           | If set, this custom version of `npm` will be used instead of the one provided by the `nodejs` installation.                                                               |
+| `$BP_KEEP_NODE_BUILD_CACHE` | If set to `true` (default `false`), the folder `node_modules/.cache` will not be removed after the build, but will be readonly at runtime.                                |
 
 ## Usage
 

build.go

@@ -284,16 +284,19 @@ func Build(entryResolver EntryResolver,
 					return packit.BuildResult{}, err
 				}
 
-				linkName := filepath.Join(layer.Path, "node_modules", ".cache")
-				err = os.RemoveAll(linkName)
-				if err != nil {
-					return packit.BuildResult{}, err
-				}
+				keepBuildCache, _ := environment.Lookup("BP_KEEP_NODE_BUILD_CACHE")
+				if keepBuildCache != "true" {
+					linkName := filepath.Join(layer.Path, "node_modules", ".cache")
+					err = os.RemoveAll(linkName)
+					if err != nil {
+						return packit.BuildResult{}, err
+					}
 
-				cacheFolder := filepath.Join(os.TempDir(), NODE_MODULES_CACHE)
-				err = os.Symlink(cacheFolder, linkName)
-				if err != nil {
-					return packit.BuildResult{}, err
+					cacheFolder := filepath.Join(os.TempDir(), NODE_MODULES_CACHE)
+					err = os.Symlink(cacheFolder, linkName)
+					if err != nil {
+						return packit.BuildResult{}, err
+					}
 				}
 
 				if build {

buildpack.toml

@@ -32,6 +32,11 @@ api = "0.7"
     name = "BP_NPM_VERSION"
     description = "configures a custom npm version"
 
+	[[metadata.configurations]]
+    name = "BP_KEEP_NODE_BUILD_CACHE"
+    default = "false"
+    description = "keep the 'node_modules/.cache' folder after the build (will be readonly at runtime)'"
+
   [[metadata.configurations]]
 	  name = "NODE_HOME"
     description = "path the Node.js installation"
@@ -45,5 +50,6 @@ api = "0.7"
     default = "error"
     description = "configures npm output verbosity"
 
+
 [[stacks]]
   id = "*"