Change the getCacheKey implentation to not return null on executable …

…source

src/CredentialSource/ExecutableSource.php

@@ -108,7 +108,7 @@ public function __construct(
      */
     public function getCacheKey(): ?string
     {
-        return null;
+        return $this->command . $this->outputFile;
     }
 
     /**

src/Credentials/ExternalAccountCredentials.php

@@ -288,18 +288,12 @@ public function fetchAuthToken(callable $httpHandler = null)
      */
     public function getCacheKey(): ?string
     {
-        $cacheKey = $this->auth->getSubjectTokenFetcher()->getCacheKey();
-
-        if ($cacheKey === null) {
-            return null;
-        }
-
         $scopeOrAudience = $this->auth->getAudience();
         if (!$scopeOrAudience) {
             $scopeOrAudience = $this->auth->getScope();
         }
 
-        return $cacheKey .
+        return $this->auth->getSubjectTokenFetcher()->getCacheKey() .
             $scopeOrAudience .
             $this->serviceAccountImpersonationUrl .
             $this->auth->getSubjectTokenType() .

tests/Credentials/ExternalAccountCredentialsTest.php

@@ -566,13 +566,16 @@ public function testExecutableSourceCacheKey()
     {
         $this->baseCreds['credential_source'] = [
             'executable' => [
-                'command' => 'ls -al'
+                'command' => 'ls -al',
+                'output_file' => './output.txt'
             ]
         ];
 
         $credentials = new ExternalAccountCredentials('scope1', $this->baseCreds);
         $cacheKey = $credentials->getCacheKey();
-        $this->assertNull($cacheKey);
+
+        $expectedCacheKey = 'ls -al./output.txtscope1';
+        $this->assertEquals($cacheKey, $expectedCacheKey);
     }
 
     /**