fix: clarify JavaProfiler init API behavior

Author: kaahos

ddprof-lib/src/main/cpp/vmEntry.cpp

@@ -574,6 +574,8 @@ bool VM::initializeRequestStackTrace() {
 
 bool VM::initProfilerBridge(JavaVM *vm, bool attach,
                             bool delegateMonitorEvents, bool wallPrecheck) {
+  // wallPrecheck is kept for Java API compatibility; the active wall-precheck
+  // mode is read from Arguments when a profiling session starts.
   (void)wallPrecheck;
   TEST_LOG("VM::initProfilerBridge");
   if (!initShared(vm)) {

ddprof-lib/src/main/java/com/datadoghq/profiler/JavaProfiler.java

@@ -94,7 +94,7 @@ public static JavaProfiler getInstance(String scratchDir) throws IOException {
      * @param libLocation the path to the native library to be used instead of the bundled one
      * @param scratchDir directory where the bundled library will be exploded before linking; ignored when 'libLocation' is {@literal null}
      */
-    public static JavaProfiler getInstance(String libLocation, String scratchDir) throws IOException {
+    public static synchronized JavaProfiler getInstance(String libLocation, String scratchDir) throws IOException {
         return getInstance(libLocation, scratchDir, false, false);
     }