Create local .m2 dir to cache deps between builds

Also deletes target directory as an AfterBuild step

fn/langs/java_maven.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"errors"
 )
 
 // JavaMavenLangHelper provides a set of helper methods for the build lifecycle of Java Maven projects
@@ -23,9 +24,9 @@ func (lh *JavaMavenLangHelper) HasPreBuild() bool {
 	return true
 }
 
-// PreBuild runs "mvn clean package" in the root of the project and by default expects a jar at `target/function.jar` as the
-// output.  We mount `$HOME/.m2` in order to get maven the proxy config specified in .m2/settings.xml, but it has the nice
-// side effect of making the users .m2/repository available.  Any new deps downloaded will be owned by root :(
+// PreBuild runs "mvn package" in the root of the project. A local .m2 directory is created the first time this is run
+// so that any pulled dependencies are cached in between builds. The local .m2 directory contains a hardlink to user's
+// own settings.xml file and this is mounted into the build container.
 func (lh *JavaMavenLangHelper) PreBuild() error {
 	wd, err := os.Getwd()
 	if err != nil {
@@ -33,16 +34,21 @@ func (lh *JavaMavenLangHelper) PreBuild() error {
 	}
 
 	if !exists(filepath.Join(wd, "pom.xml")) {
-		return fmt.Errorf("Could not find pom.xml - are you sure this is a maven project?")
+		return errors.New("Could not find pom.xml - are you sure this is a maven project?")
+	}
+
+	err = createLocalM2Dir(wd)
+	if err != nil {
+		return err
 	}
 
 	cmd := exec.Command(
 		"docker", "run",
 		"--rm",
 		"-v", wd+":/java", "-w", "/java",
-		"-v", os.Getenv("HOME")+"/.m2:/.m2",
+		"-v", wd+"/.m2:/root/.m2",
 		"maven:3.5-jdk-8-alpine",
-		"/bin/sh", "-c", "mvn -gs /.m2/settings.xml clean package",
+		"/bin/sh", "-c", "mvn package",
 	)
 
 	cmd.Stderr = os.Stderr
@@ -53,7 +59,50 @@ func (lh *JavaMavenLangHelper) PreBuild() error {
 	return nil
 }
 
-// AfterBuild should remove the (root-owned) target dir.
+// createLocalM2Dir creates a .m2 directory in the function's working directory and creates a hard link to the user's
+// settings.xml file.
+func createLocalM2Dir(wd string) error {
+	usersSettingsFile := filepath.Join(os.Getenv("HOME"), ".m2/settings.xml")
+	localSettingsFile := filepath.Join(wd, ".m2/settings.xml")
+
+	if exists(localSettingsFile) {
+		return nil
+	}
+
+	if !exists(usersSettingsFile) {
+		return fmt.Errorf("Unable to find user's settings.xml at %s", usersSettingsFile)
+	}
+
+	if !exists(filepath.Dir(localSettingsFile)) {
+		if err := os.Mkdir(filepath.Dir(localSettingsFile), 0755); err != nil {
+			return fmt.Errorf("Unable to create a local .m2 directory: %s", err)
+		}
+	}
+
+	return os.Link(usersSettingsFile, localSettingsFile)
+}
+
+// AfterBuild removes the target directory by mounting the working directory into a container and removingit. This is
+// done inside a container as the folder is owned by root.
 func (lh *JavaMavenLangHelper) AfterBuild() error {
+	wd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+
+	cmd := exec.Command(
+		"docker", "run",
+		"--rm",
+		"-v", wd+":/root",
+		"maven:3.5-jdk-8-alpine",
+		"/bin/sh", "-c", "rm -r /root/target",
+	)
+
+	cmd.Stderr = os.Stderr
+	cmd.Stdout = os.Stdout
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("Error occured trying to delete the target directory: %s", err)
+	}
+
 	return nil
 }