fix: race condition for file writes (GoogleCloudPlatform#1461)

Author: bshaffer

functions/tips_scopes/index.php

@@ -42,14 +42,19 @@ function scopeDemo(ServerRequestInterface $request): string
 
     $response = '';
     if (file_exists($cachePath)) {
-        // Read cached value from file
+        // Read cached value from file, using file locking to prevent race
+        // conditions between function executions.
         $response .= "Reading cached value." . PHP_EOL;
-        $instanceVar = file_get_contents($cachePath);
+        $fh = fopen($cachePath, 'r');
+        flock($fh, LOCK_EX);
+        $instanceVar = stream_get_contents($fh);
+        flock($fh, LOCK_UN);
     } else {
-        // Compute cached value + write to file
+        // Compute cached value + write to file, using file locking to prevent
+        // race conditions between function executions.
         $response .= "Cache empty, computing value." . PHP_EOL;
         $instanceVar = _heavyComputation();
-        file_put_contents($cachePath, $instanceVar);
+        file_put_contents($cachePath, $instanceVar, LOCK_EX);
     }
 
     // Lighter computations can re-run on each function invocation.