Automatically detect GCF enviroments and enable unix socket. (#141)

Author: kurtisvg

core/src/main/java/com/google/cloud/sql/core/CoreSocketFactory.java

@@ -167,8 +167,8 @@ public static Socket connect(Properties props, String socketPathFormat) throws I
         "cloudSqlInstance property not set. Please specify this property in the JDBC URL or the "
             + "connection Properties with value in form \"project:region:instance\"");
 
-    // GAE Standard runtimes provide a connection path at "/cloudsql/<CONNECTION_NAME>"
-    if (forceUnixSocket || runningOnGaeStandard()) {
+    // GAE Standard + GCF provide a connection path at "/cloudsql/<CONNECTION_NAME>"
+    if (forceUnixSocket || runningOnGaeStandard() || runningOnGoogleCloudFunctions()) {
       logger.info(
           String.format(
               "Connecting to Cloud SQL instance [%s] via unix socket.", csqlInstanceName));
@@ -195,6 +195,13 @@ private static boolean runningOnGaeStandard() {
         && ("Production".equals(runEnv) || "java11".equals(gaeRuntime));
   }
 
+  /** Returns {@code true} if running in a Google Cloud Functions runtime. */
+  private static boolean runningOnGoogleCloudFunctions() {
+    // Functions automatically sets a few variables we can use to guess the env:
+    // See https://cloud.google.com/functions/docs/env-var#nodejs_10_and_subsequent_runtimes
+    return System.getenv("K_SERVICE") != null && System.getenv("K_REVISION") != null;
+  }
+
   /**
    * Creates a secure socket representing a connection to a Cloud SQL instance.
    *