diff --git a/src/Composer/Util/ProcessExecutor.php b/src/Composer/Util/ProcessExecutor.php
index 03c5aa519dfb..6b6e84bee2b0 100644
--- a/src/Composer/Util/ProcessExecutor.php
+++ b/src/Composer/Util/ProcessExecutor.php
@@ -371,6 +371,9 @@ public function getErrorOutput()
         return $this->errorOutput;
     }
 
+    /**
+     * @private
+     */
     public function outputHandler($type, $buffer)
     {
         if ($this->captureOutput) {
diff --git a/tests/Composer/Test/Downloader/GitDownloaderTest.php b/tests/Composer/Test/Downloader/GitDownloaderTest.php
index bee7a09bd1f8..65b41a529969 100644
--- a/tests/Composer/Test/Downloader/GitDownloaderTest.php
+++ b/tests/Composer/Test/Downloader/GitDownloaderTest.php
@@ -17,6 +17,8 @@
 use Composer\Test\TestCase;
 use Composer\Util\Filesystem;
 use Composer\Util\Platform;
+use Composer\Test\Mock\ProcessExecutorMock;
+use DateTime;
 use Prophecy\Argument;
 
 class GitDownloaderTest extends TestCase
@@ -118,7 +120,11 @@ public function testDownload()
         $processExecutor->expects($this->at(1))
             ->method('execute')
             ->with($this->equalTo($this->winCompat("git branch -r")), $this->equalTo(null), $this->equalTo($this->winCompat('composerPath')))
-            ->will($this->returnValue(0));
+            ->will($this->returnCallback(function ($cmd, &$output) {
+                $output = '';
+
+                return 0;
+            }));
 
         $processExecutor->expects($this->at(2))
             ->method('execute')
@@ -189,7 +195,11 @@ public function testDownloadWithCache()
         $processExecutor->expects($this->at(4))
             ->method('execute')
             ->with($this->equalTo($this->winCompat("git branch -r")), $this->equalTo(null), $this->equalTo($this->winCompat('composerPath')))
-            ->will($this->returnValue(0));
+            ->will($this->returnCallback(function ($cmd, &$output, $path) {
+                $output = '';
+
+                return 0;
+            }));
 
         $processExecutor->expects($this->at(5))
             ->method('execute')
@@ -253,7 +263,11 @@ public function testDownloadUsesVariousProtocolsAndSetsPushUrlForGithub()
         $processExecutor->expects($this->at(5))
             ->method('execute')
             ->with($this->equalTo('git branch -r'))
-            ->will($this->returnValue(0));
+            ->will($this->returnCallback(function ($cmd, &$output, $path) {
+                $output = '';
+
+                return 0;
+            }));
 
         $processExecutor->expects($this->at(6))
             ->method('execute')
@@ -297,37 +311,29 @@ public function testDownloadAndSetPushUrlUseCustomVariousProtocolsForGithub($pro
         $packageMock->expects($this->any())
             ->method('getPrettyVersion')
             ->will($this->returnValue('1.0.0'));
-        $processExecutor = $this->getMockBuilder('Composer\Util\ProcessExecutor')->setMethods(array('execute'))->getMock();
-
-        $expectedGitCommand = $this->winCompat("git clone --no-checkout -- '{$url}' 'composerPath' && cd 'composerPath' && git remote add composer -- '{$url}' && git fetch composer && git remote set-url origin -- '{$url}' && git remote set-url composer -- '{$url}'");
-        $processExecutor->expects($this->at(0))
-            ->method('execute')
-            ->with($this->equalTo($expectedGitCommand))
-            ->will($this->returnValue(0));
-
-        $expectedGitCommand = $this->winCompat("git remote set-url --push origin -- '{$pushUrl}'");
-        $processExecutor->expects($this->at(1))
-            ->method('execute')
-            ->with($this->equalTo($expectedGitCommand), $this->equalTo(null), $this->equalTo($this->winCompat('composerPath')))
-            ->will($this->returnValue(0));
 
-        $processExecutor->expects($this->exactly(4))
-            ->method('execute')
-            ->will($this->returnValue(0));
+        $process = new ProcessExecutorMock;
+        $process->expects(array(
+            $this->winCompat("git clone --no-checkout -- '{$url}' 'composerPath' && cd 'composerPath' && git remote add composer -- '{$url}' && git fetch composer && git remote set-url origin -- '{$url}' && git remote set-url composer -- '{$url}'"),
+            $this->winCompat("git remote set-url --push origin -- '{$pushUrl}'"),
+            'git branch -r',
+            $this->winCompat("git checkout 'ref' -- && git reset --hard 'ref' --"),
+        ), true);
 
         $config = new Config();
         $config->merge(array('config' => array('github-protocols' => $protocols)));
 
-        $downloader = $this->getDownloaderMock(null, $config, $processExecutor);
+        $downloader = $this->getDownloaderMock(null, $config, $process);
         $downloader->download($packageMock, 'composerPath');
         $downloader->prepare('install', $packageMock, 'composerPath');
         $downloader->install($packageMock, 'composerPath');
         $downloader->cleanup('install', $packageMock, 'composerPath');
+
+        $process->assertComplete($this);
     }
 
     public function testDownloadThrowsRuntimeExceptionIfGitCommandFails()
     {
-        $expectedGitCommand = $this->winCompat("git clone --no-checkout -- 'https://example.com/composer/composer' 'composerPath' && cd 'composerPath' && git remote add composer -- 'https://example.com/composer/composer' && git fetch composer && git remote set-url origin -- 'https://example.com/composer/composer' && git remote set-url composer -- 'https://example.com/composer/composer'");
         $packageMock = $this->getMockBuilder('Composer\Package\PackageInterface')->getMock();
         $packageMock->expects($this->any())
             ->method('getSourceReference')
@@ -335,19 +341,31 @@ public function testDownloadThrowsRuntimeExceptionIfGitCommandFails()
         $packageMock->expects($this->any())
             ->method('getSourceUrls')
             ->will($this->returnValue(array('https://example.com/composer/composer')));
-        $processExecutor = $this->getMockBuilder('Composer\Util\ProcessExecutor')->setMethods(array('execute'))->getMock();
-        $processExecutor->expects($this->at(0))
-            ->method('execute')
-            ->with($this->equalTo($expectedGitCommand))
-            ->will($this->returnValue(1));
+        $packageMock->expects($this->any())
+            ->method('getSourceUrl')
+            ->will($this->returnValue('https://example.com/composer/composer'));
+        $packageMock->expects($this->any())
+            ->method('getPrettyVersion')
+            ->will($this->returnValue('1.0.0'));
+
+        $process = new ProcessExecutorMock;
+        $process->expects(array(
+            array(
+                'cmd' => $this->winCompat("git clone --no-checkout -- 'https://example.com/composer/composer' 'composerPath' && cd 'composerPath' && git remote add composer -- 'https://example.com/composer/composer' && git fetch composer && git remote set-url origin -- 'https://example.com/composer/composer' && git remote set-url composer -- 'https://example.com/composer/composer'"),
+                'return' => 1,
+            ),
+        ));
 
         // not using PHPUnit's expected exception because Prophecy exceptions extend from RuntimeException too so it is not safe
         try {
-            $downloader = $this->getDownloaderMock(null, null, $processExecutor);
+            $downloader = $this->getDownloaderMock(null, null, $process);
             $downloader->download($packageMock, 'composerPath');
             $downloader->prepare('install', $packageMock, 'composerPath');
             $downloader->install($packageMock, 'composerPath');
             $downloader->cleanup('install', $packageMock, 'composerPath');
+
+            $process->assertComplete($this);
+
             $this->fail('This test should throw');
         } catch (\RuntimeException $e) {
             if ('RuntimeException' !== get_class($e)) {
@@ -388,21 +406,29 @@ public function testUpdate()
         $packageMock->expects($this->any())
             ->method('getVersion')
             ->will($this->returnValue('1.0.0.0'));
+        $packageMock->expects($this->any())
+            ->method('getPrettyVersion')
+            ->will($this->returnValue('1.0.0'));
 
-        $process = $this->prophesize('Composer\Util\ProcessExecutor');
-        $process->execute($this->winCompat('git show-ref --head -d'), Argument::cetera())->willReturn(0);
-        $process->execute($this->winCompat('git status --porcelain --untracked-files=no'), Argument::cetera())->willReturn(0);
-        $process->execute($this->winCompat('git remote -v'), Argument::cetera())->willReturn(0);
-        $process->execute($this->winCompat('git branch -r'), Argument::cetera())->willReturn(0);
-        $process->execute($expectedGitUpdateCommand, null, $this->winCompat($this->workingDir))->willReturn(0)->shouldBeCalled();
-        $process->execute($this->winCompat("git checkout 'ref' -- && git reset --hard 'ref' --"), null, $this->winCompat($this->workingDir))->willReturn(0)->shouldBeCalled();
+        $process = new ProcessExecutorMock;
+        $process->expects(array(
+            $this->winCompat('git show-ref --head -d'),
+            $this->winCompat('git status --porcelain --untracked-files=no'),
+            $this->winCompat('git remote -v'),
+            $expectedGitUpdateCommand,
+            $this->winCompat('git branch -r'),
+            $this->winCompat("git checkout 'ref' -- && git reset --hard 'ref' --"),
+            $this->winCompat('git remote -v'),
+        ), true);
 
         $this->fs->ensureDirectoryExists($this->workingDir.'/.git');
-        $downloader = $this->getDownloaderMock(null, new Config(), $process->reveal());
+        $downloader = $this->getDownloaderMock(null, new Config(), $process);
         $downloader->download($packageMock, $this->workingDir, $packageMock);
         $downloader->prepare('update', $packageMock, $this->workingDir, $packageMock);
         $downloader->update($packageMock, $packageMock, $this->workingDir);
         $downloader->cleanup('update', $packageMock, $this->workingDir, $packageMock);
+
+        $process->assertComplete($this);
     }
 
     public function testUpdateWithNewRepoUrl()
@@ -422,59 +448,38 @@ public function testUpdateWithNewRepoUrl()
         $packageMock->expects($this->any())
             ->method('getVersion')
             ->will($this->returnValue('1.0.0.0'));
+        $packageMock->expects($this->any())
+            ->method('getPrettyVersion')
+            ->will($this->returnValue('1.0.0'));
 
-        $processExecutor = $this->getMockBuilder('Composer\Util\ProcessExecutor')->setMethods(array('execute'))->getMock();
-        $processExecutor->expects($this->at(0))
-            ->method('execute')
-            ->with($this->equalTo($this->winCompat("git show-ref --head -d")))
-            ->will($this->returnValue(0));
-        $processExecutor->expects($this->at(1))
-            ->method('execute')
-            ->with($this->equalTo($this->winCompat("git status --porcelain --untracked-files=no")))
-            ->will($this->returnValue(0));
-        $processExecutor->expects($this->at(2))
-            ->method('execute')
-            ->with($this->equalTo($this->winCompat("git remote -v")))
-            ->will($this->returnValue(0));
-        $processExecutor->expects($this->at(3))
-            ->method('execute')
-            ->with($this->equalTo($this->winCompat($expectedGitUpdateCommand)), $this->equalTo(null), $this->equalTo($this->winCompat($this->workingDir)))
-            ->will($this->returnValue(0));
-        $processExecutor->expects($this->at(4))
-            ->method('execute')
-            ->with($this->equalTo('git branch -r'))
-            ->will($this->returnValue(0));
-        $processExecutor->expects($this->at(5))
-            ->method('execute')
-            ->with($this->equalTo($this->winCompat("git checkout 'ref' -- && git reset --hard 'ref' --")), $this->equalTo(null), $this->equalTo($this->winCompat($this->workingDir)))
-            ->will($this->returnValue(0));
-        $processExecutor->expects($this->at(6))
-            ->method('execute')
-            ->with($this->equalTo($this->winCompat("git remote -v")))
-            ->will($this->returnCallback(function ($cmd, &$output, $cwd) {
-                $output = 'origin https://github.com/old/url (fetch)
+        $process = new ProcessExecutorMock;
+        $process->expects(array(
+            $this->winCompat("git show-ref --head -d"),
+            $this->winCompat("git status --porcelain --untracked-files=no"),
+            $this->winCompat("git remote -v"),
+            $this->winCompat($expectedGitUpdateCommand),
+            'git branch -r',
+            $this->winCompat("git checkout 'ref' -- && git reset --hard 'ref' --"),
+            array(
+                'cmd' => $this->winCompat("git remote -v"),
+                'stdout' => 'origin https://github.com/old/url (fetch)
 origin https://github.com/old/url (push)
 composer https://github.com/old/url (fetch)
 composer https://github.com/old/url (push)
-';
-
-                return 0;
-            }));
-        $processExecutor->expects($this->at(7))
-            ->method('execute')
-            ->with($this->equalTo($this->winCompat("git remote set-url origin -- 'https://github.com/composer/composer'")), $this->equalTo(null), $this->equalTo($this->winCompat($this->workingDir)))
-            ->will($this->returnValue(0));
-        $processExecutor->expects($this->at(8))
-            ->method('execute')
-            ->with($this->equalTo($this->winCompat("git remote set-url --push origin -- 'git@github.com:composer/composer.git'")), $this->equalTo(null), $this->equalTo($this->winCompat($this->workingDir)))
-            ->will($this->returnValue(0));
+',
+            ),
+            $this->winCompat("git remote set-url origin -- 'https://github.com/composer/composer'"),
+            $this->winCompat("git remote set-url --push origin -- 'git@github.com:composer/composer.git'"),
+        ), true);
 
         $this->fs->ensureDirectoryExists($this->workingDir.'/.git');
-        $downloader = $this->getDownloaderMock(null, new Config(), $processExecutor);
+        $downloader = $this->getDownloaderMock(null, new Config(), $process);
         $downloader->download($packageMock, $this->workingDir, $packageMock);
         $downloader->prepare('update', $packageMock, $this->workingDir, $packageMock);
         $downloader->update($packageMock, $packageMock, $this->workingDir);
         $downloader->cleanup('update', $packageMock, $this->workingDir, $packageMock);
+
+        $process->assertComplete($this);
     }
 
     /**
diff --git a/tests/Composer/Test/Mock/ProcessExecutorMock.php b/tests/Composer/Test/Mock/ProcessExecutorMock.php
new file mode 100644
index 000000000000..267bc086c686
--- /dev/null
+++ b/tests/Composer/Test/Mock/ProcessExecutorMock.php
@@ -0,0 +1,145 @@
+<?php
+
+/*
+ * This file is part of Composer.
+ *
+ * (c) Nils Adermann <naderman@naderman.de>
+ *     Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Composer\Test\Mock;
+
+use Composer\Util\ProcessExecutor;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Process\Process;
+
+/**
+ * @author Jordi Boggiano <j.boggiano@seld.be>
+ */
+class ProcessExecutorMock extends ProcessExecutor
+{
+    private $expectations = array();
+    private $strict = false;
+    private $defaultHandler = array('return' => 0, 'stdout' => '', 'stderr' => '');
+    private $log = array();
+
+    /**
+     * @param array<string|array{cmd: string, return: int, stdout?: string, stderr?: string}> $expectations
+     * @param bool                                                                     $strict       set to true if you want to provide *all* expected commands, and not just a subset you are interested in testing
+     * @param array{return: int, stdout?: string, stderr?: string}                     $default      default command handler for undefined commands if not in strict mode
+     */
+    public function expects(array $expectations, $strict = false, array $defaultHandler = array('return' => 0, 'stdout' => '', 'stderr' => ''))
+    {
+        $default = array('return' => 0, 'stdout' => '', 'stderr' => '');
+        $this->expectations = array_map(function ($expect) use ($default) {
+            if (is_string($expect)) {
+                $expect = array('cmd' => $expect);
+            }
+
+            return array_merge($default, $expect);
+        }, $expectations);
+        $this->strict = $strict;
+        $this->defaultHandler = array_merge($default, $defaultHandler);
+    }
+
+    public function assertComplete(TestCase $testCase)
+    {
+        if ($this->expectations) {
+            $expectations = array_map(function ($expect) {
+                return $expect['cmd'];
+            }, $this->expectations);
+            throw new \LogicException(
+                'There are still '.count($this->expectations).' expected process calls which have not been consumed:'.PHP_EOL.
+                implode(PHP_EOL, $expectations).PHP_EOL.PHP_EOL.
+                'Received calls:'.PHP_EOL.implode(PHP_EOL, $this->log)
+            );
+        }
+
+        $testCase->assertTrue(true);
+    }
+
+    public function execute($command, &$output = null, $cwd = null)
+    {
+        if (func_num_args() > 1) {
+            return $this->doExecute($command, $cwd, false, $output);
+        }
+
+        return $this->doExecute($command, $cwd, false);
+    }
+
+    public function executeTty($command, $cwd = null)
+    {
+        if (Platform::isTty()) {
+            return $this->doExecute($command, $cwd, true);
+        }
+
+        return $this->doExecute($command, $cwd, false);
+    }
+
+    private function doExecute($command, $cwd, $tty, &$output = null)
+    {
+        $this->captureOutput = func_num_args() > 3;
+        $this->errorOutput = '';
+
+        $callback = is_callable($output) ? $output : array($this, 'outputHandler');
+
+        if ($this->expectations && $command === $this->expectations[0]['cmd']) {
+            $expect = array_shift($this->expectations);
+            $stdout = $expect['stdout'];
+            $stderr = $expect['stderr'];
+            $return = $expect['return'];
+        } elseif (!$this->strict) {
+            $stdout = $this->defaultHandler['stdout'];
+            $stderr = $this->defaultHandler['stderr'];
+            $return = $this->defaultHandler['return'];
+        } else {
+            throw new \LogicException(
+                'Received unexpected command "'.$command.'" in "'.$cwd.'"'.PHP_EOL.
+                ($this->expectations ? 'Expected "'.$this->expectations[0]['cmd'].'" at this point.' : 'Expected no more calls at this point.')
+            );
+        }
+
+        if ($stdout) {
+            $callback(Process::STDOUT, $stdout);
+        }
+        if ($stderr) {
+            $callback(Process::ERR, $stderr);
+        }
+
+        if ($this->captureOutput && !is_callable($output)) {
+            $output = $stdout;
+        }
+
+        $this->log[] = $command;
+
+        $this->errorOutput = $stderr;
+
+        return $return;
+    }
+
+    public function executeAsync($command, $cwd = null)
+    {
+        if (!$this->allowAsync) {
+            throw new \LogicException('You must use the ProcessExecutor instance which is part of a Composer\Loop instance to be able to run async processes');
+        }
+
+        $resolver = function ($resolve, $reject) {
+            // TODO strictly speaking this should resolve with a mock Process instance here
+            $resolve();
+        };
+
+        $canceler = function () {
+            throw new \RuntimeException('Aborted process');
+        };
+
+        return new Promise($resolver, $canceler);
+    }
+
+    public function getErrorOutput()
+    {
+        return $this->errorOutput;
+    }
+}