Add getExecOutput function (#814)

* Add getExecOutput function

* Add tests for exec output

* Modify tests to not rely on buffer size, but only test larger output

* Handle split multi-byte characters + PR feedback

* Fix tests

* Lint

* Update how split byte are sent for tests

packages/exec/__tests__/exec.test.ts

@@ -636,6 +636,165 @@ describe('@actions/exec', () => {
     expect(output.trim()).toBe(`args[0]: "hello"${os.EOL}args[1]: "world"`)
   })
 
+  it('correctly outputs for getExecOutput', async () => {
+    const stdErrPath: string = path.join(
+      __dirname,
+      'scripts',
+      'stderroutput.js'
+    )
+    const stdOutPath: string = path.join(
+      __dirname,
+      'scripts',
+      'stdoutoutput.js'
+    )
+    const nodePath: string = await io.which('node', true)
+
+    const {exitCode: exitCodeOut, stdout} = await exec.getExecOutput(
+      `"${nodePath}"`,
+      [stdOutPath],
+      getExecOptions()
+    )
+
+    expect(exitCodeOut).toBe(0)
+    expect(stdout).toBe('this is output to stdout')
+
+    const {exitCode: exitCodeErr, stderr} = await exec.getExecOutput(
+      `"${nodePath}"`,
+      [stdErrPath],
+      getExecOptions()
+    )
+    expect(exitCodeErr).toBe(0)
+    expect(stderr).toBe('this is output to stderr')
+  })
+
+  it('correctly outputs for getExecOutput with additional listeners', async () => {
+    const stdErrPath: string = path.join(
+      __dirname,
+      'scripts',
+      'stderroutput.js'
+    )
+    const stdOutPath: string = path.join(
+      __dirname,
+      'scripts',
+      'stdoutoutput.js'
+    )
+
+    const nodePath: string = await io.which('node', true)
+    let listenerOut = ''
+
+    const {exitCode: exitCodeOut, stdout} = await exec.getExecOutput(
+      `"${nodePath}"`,
+      [stdOutPath],
+      {
+        ...getExecOptions(),
+        listeners: {
+          stdout: data => {
+            listenerOut = data.toString()
+          }
+        }
+      }
+    )
+
+    expect(exitCodeOut).toBe(0)
+    expect(stdout).toBe('this is output to stdout')
+    expect(listenerOut).toBe('this is output to stdout')
+
+    let listenerErr = ''
+    const {exitCode: exitCodeErr, stderr} = await exec.getExecOutput(
+      `"${nodePath}"`,
+      [stdErrPath],
+      {
+        ...getExecOptions(),
+        listeners: {
+          stderr: data => {
+            listenerErr = data.toString()
+          }
+        }
+      }
+    )
+    expect(exitCodeErr).toBe(0)
+    expect(stderr).toBe('this is output to stderr')
+    expect(listenerErr).toBe('this is output to stderr')
+  })
+
+  it('correctly outputs for getExecOutput when total size exceeds buffer size', async () => {
+    const stdErrPath: string = path.join(
+      __dirname,
+      'scripts',
+      'stderroutput.js'
+    )
+    const stdOutPath: string = path.join(
+      __dirname,
+      'scripts',
+      'stdoutoutputlarge.js'
+    )
+
+    const nodePath: string = await io.which('node', true)
+    let listenerOut = ''
+
+    const {exitCode: exitCodeOut, stdout} = await exec.getExecOutput(
+      `"${nodePath}"`,
+      [stdOutPath],
+      {
+        ...getExecOptions(),
+        listeners: {
+          stdout: data => {
+            listenerOut += data.toString()
+          }
+        }
+      }
+    )
+
+    expect(exitCodeOut).toBe(0)
+    expect(Buffer.byteLength(stdout || '', 'utf8')).toBe(2 ** 25)
+    expect(Buffer.byteLength(listenerOut, 'utf8')).toBe(2 ** 25)
+
+    let listenerErr = ''
+    const {exitCode: exitCodeErr, stderr} = await exec.getExecOutput(
+      `"${nodePath}"`,
+      [stdErrPath],
+      {
+        ...getExecOptions(),
+        listeners: {
+          stderr: data => {
+            listenerErr = data.toString()
+          }
+        }
+      }
+    )
+    expect(exitCodeErr).toBe(0)
+    expect(stderr).toBe('this is output to stderr')
+    expect(listenerErr).toBe('this is output to stderr')
+  })
+
+  it('correctly outputs for getExecOutput with multi-byte characters', async () => {
+    const stdOutPath: string = path.join(
+      __dirname,
+      'scripts',
+      'stdoutputspecial.js'
+    )
+
+    const nodePath: string = await io.which('node', true)
+    let numStdOutBufferCalls = 0
+    const {exitCode: exitCodeOut, stdout} = await exec.getExecOutput(
+      `"${nodePath}"`,
+      [stdOutPath],
+      {
+        ...getExecOptions(),
+        listeners: {
+          stdout: () => {
+            numStdOutBufferCalls += 1
+          }
+        }
+      }
+    )
+
+    expect(exitCodeOut).toBe(0)
+    //one call for each half of the © character, ensuring it was actually split and not sent together
+    expect(numStdOutBufferCalls).toBe(2)
+    expect(stdout).toBe('©')
+  })
+
   if (IS_WINDOWS) {
     it('Exec roots relative tool path using process.cwd (Windows path separator)', async () => {
       let exitCode: number