Allow action to be used without Gradle invocation

If the user supplies no 'arguments' parameter, the action will function as a
'setup-gradle' action, adding Gradle to the PATH and enabling other features
without actually running a Gradle build.

Any subsequent Gradle invocations in the workflow will benefit from:
- Save/restore of Gradle User Home
- Save/restore of configuration-cache data
- Capture of build-scan URLs

These features are enabled via Gradle User Home, so any Gradle invocation that
uses the same Gradle User Home will be included.

src/execution.ts

@@ -1,8 +1,10 @@
+import * as core from '@actions/core'
 import * as exec from '@actions/exec'
 import fs from 'fs'
 import path from 'path'
+import * as gradlew from './gradlew'
 
-export async function execute(executable: string, root: string, args: string[]): Promise<BuildResult> {
+export async function executeGradleBuild(executable: string | undefined, root: string, args: string[]): Promise<void> {
     let buildScanUrl: string | undefined
 
     const buildScanFile = path.resolve(root, 'gradle-build-scan.txt')
@@ -10,7 +12,9 @@ export async function execute(executable: string, root: string, args: string[]):
         fs.unlinkSync(buildScanFile)
     }
 
-    const status: number = await exec.exec(executable, args, {
+    // Use the provided executable, or look for a Gradle wrapper script to run
+    const toExecute = executable ?? gradlew.locateGradleWrapperScript(root)
+    const status: number = await exec.exec(toExecute, args, {
         cwd: root,
         ignoreReturnCode: true
     })
@@ -19,14 +23,11 @@ export async function execute(executable: string, root: string, args: string[]):
         buildScanUrl = fs.readFileSync(buildScanFile, 'utf-8')
     }
 
-    return new BuildResultImpl(status, buildScanUrl)
-}
-
-export interface BuildResult {
-    readonly status: number
-    readonly buildScanUrl?: string
-}
-
-class BuildResultImpl implements BuildResult {
-    constructor(readonly status: number, readonly buildScanUrl?: string) {}
+    if (status !== 0) {
+        if (buildScanUrl) {
+            core.setFailed(`Gradle build failed: ${buildScanUrl}`)
+        } else {
+            core.setFailed(`Gradle build failed: process exited with status ${status}`)
+        }
+    }
 }

src/main.ts

@@ -5,7 +5,6 @@ import {parseArgsStringToArgv} from 'string-argv'
 
 import * as caches from './caches'
 import * as execution from './execution'
-import * as gradlew from './gradlew'
 import * as provision from './provision'
 
 /**
@@ -19,20 +18,16 @@ export async function run(): Promise<void> {
 
         await caches.restore(gradleUserHome)
 
-        const args: string[] = parseCommandLineArguments()
-
-        const result = await execution.execute(
-            await resolveGradleExecutable(workspaceDirectory, buildRootDirectory),
-            buildRootDirectory,
-            args
-        )
-
-        if (result.status !== 0) {
-            if (result.buildScanUrl) {
-                core.setFailed(`Gradle build failed: ${result.buildScanUrl}`)
-            } else {
-                core.setFailed(`Gradle build failed: process exited with status ${result.status}`)
+        const executable = await provisionGradle(workspaceDirectory)
+        // executable will be undefined if using Gradle wrapper
+        if (executable !== undefined) {
+            core.addPath(path.dirname(executable))
         }
+
+        // Only execute if arguments have been provided
+        const args: string[] = parseCommandLineArguments()
+        if (args.length > 0) {
+            await execution.executeGradleBuild(executable, buildRootDirectory, args)
         }
     } catch (error) {
         core.setFailed(String(error))
@@ -44,7 +39,7 @@ export async function run(): Promise<void> {
 
 run()
 
-async function resolveGradleExecutable(workspaceDirectory: string, buildRootDirectory: string): Promise<string> {
+async function provisionGradle(workspaceDirectory: string): Promise<string | undefined> {
     const gradleVersion = core.getInput('gradle-version')
     if (gradleVersion !== '' && gradleVersion !== 'wrapper') {
         return path.resolve(await provision.gradleVersion(gradleVersion))
@@ -55,7 +50,7 @@ async function resolveGradleExecutable(workspaceDirectory: string, buildRootDire
         return path.resolve(workspaceDirectory, gradleExecutable)
     }
 
-    return gradlew.locateGradleWrapperScript(buildRootDirectory)
+    return undefined
 }
 
 function resolveBuildRootDirectory(baseDirectory: string): string {