minor: Allow safely checking for repo in GrgitService

Service creation throws if the repo does not exist. Instead we should
defer this to when you get the Grgit instance from the service.
Additionally, we add a findGrgit() that catches exceptions so a user can
safely check if a repo is present for that service.

Fixes #362

grgit-gradle/src/compatTest/groovy/org/ajoberstar/grgit/gradle/GrgitServicePluginCompatTest.groovy

@@ -49,6 +49,23 @@ class DoStuffTask extends DefaultTask {
         println service.get().grgit.describe()
     }
 }
+
+tasks.register("doStuffSafe", DoStuffSafeTask, grgitService.service)
+
+class DoStuffSafeTask extends DefaultTask {
+    private final Provider<GrgitService> service
+
+    @Inject
+    DoStuffSafeTask(Provider<GrgitService> service) {
+        this.service = service
+        usesService(service)
+    }
+
+    @TaskAction
+    void execute() {
+        println service.get().findGrgit().map { it.describe() }.orElse(null)
+    }
+}
 """
     }
 
@@ -61,6 +78,16 @@ class DoStuffTask extends DefaultTask {
         result.task(':doStuff').outcome == TaskOutcome.FAILED
     }
 
+    def 'with no repo, accessing service with safe method works'() {
+        given:
+        // nothing
+        when:
+        def result = build('doStuffSafe', '--quiet', '--no-configuration-cache')
+        then:
+        result.task(':doStuffSafe')?.outcome == TaskOutcome.SUCCESS
+        result.output.normalize() == 'null\n'
+    }
+
     def 'with repo, plugin opens the repo as grgit'() {
         given:
         Grgit git = Grgit.init(dir: projectDir)

grgit-gradle/src/main/java/org/ajoberstar/grgit/gradle/GrgitService.java

@@ -1,6 +1,7 @@
 package org.ajoberstar.grgit.gradle;
 
 import java.io.File;
+import java.util.Optional;
 
 import javax.inject.Inject;
 
@@ -9,6 +10,7 @@
 import org.gradle.api.logging.Logger;
 import org.gradle.api.logging.Logging;
 import org.gradle.api.provider.Property;
+import org.gradle.api.provider.ProviderFactory;
 import org.gradle.api.services.BuildService;
 import org.gradle.api.services.BuildServiceParameters;
 
@@ -23,39 +25,55 @@ public interface Params extends BuildServiceParameters {
     Property<Boolean> getInitIfNotExists();
   }
 
-  private final Grgit grgit;
-
   @Inject
-  public GrgitService() {
+  public GrgitService(ProviderFactory providers) {
+    getGrgitProperty().set(providers.provider(this::makeGrgit));
+    getGrgitProperty().disallowChanges();
+    getGrgitProperty().finalizeValueOnRead();
+  }
+
+  protected abstract Property<Grgit> getGrgitProperty();
+
+  public Grgit getGrgit() {
+    return getGrgitProperty().get();
+  }
+
+  public Optional<Grgit> findGrgit() {
+    try {
+      return Optional.of(getGrgit());
+    } catch (Exception e) {
+      logger.info("Failed to make grgit service.", e);
+      return Optional.empty();
+    }
+  }
+
+  @Override
+  public void close() throws Exception {
+    findGrgit().ifPresent(grgit -> {
+      logger.info("Closing Git repo: {}", grgit.getRepository().getRootDir());
+      grgit.close();
+    });
+  }
+
+  private Grgit makeGrgit() {
     if (getParameters().getCurrentDirectory().isPresent()) {
-      this.grgit = Grgit.open(op -> {
+      return Grgit.open(op -> {
         op.setCurrentDir(getParameters().getCurrentDirectory().get().getAsFile());
       });
-      return;
     }
 
     var dir = getParameters().getDirectory().get().getAsFile();
     var gitDir = new File(dir, ".git");
     if (gitDir.exists()) {
-      this.grgit = Grgit.open(op -> {
+      return Grgit.open(op -> {
         op.setDir(dir);
       });
     } else if (getParameters().getInitIfNotExists().getOrElse(false)) {
-      this.grgit = Grgit.init(op -> {
+      return Grgit.init(op -> {
         op.setDir(dir);
       });
     } else {
       throw new IllegalStateException("No Git repo exists at " + dir + " and initIfNotExists is false. Cannot proceed.");
     }
   }
-
-  public Grgit getGrgit() {
-    return grgit;
-  }
-
-  @Override
-  public void close() throws Exception {
-    logger.info("Closing Git repo: {}", grgit.getRepository().getRootDir());
-    grgit.close();
-  }
 }