Clean up .dll files on Windows (#493)

**ISSUE:** On Windows, the .dll file extracted to the temp directory was never being deleted. And repeated runs of the program were filling the disk with old .dll files.

**DESCRIPTION OF CHANGES:** On Windows only, during startup, scan the temp dir for old instances of `AWSCRT_*aws-crt-jni.dll` and try to delete them.

**DIAGNOSIS:** We were using [File.deleteOnExit()](https://docs.oracle.com/javase/8/docs/api/java/io/File.html#deleteOnExit--) to clean up the shared lib, which is working OK on other platforms, but not on Windows. On Windows, files cannot be deleted while they're in use, and it appears that Java doesn't try to "unload" the .dll, so it's apparently impossible for a Java process to delete a .dll that it's using.

I did an experiment with a dead-simple JNI library. I simply loaded it, and called `File.deleteOnExit()`. That .dll didn't get deleted either. So it's not just a problem with our .dll being very complex.

**SIDE-NOTE:**
I realized that on non-Windows platforms, the shared-lib won't be deleted on exit if Java dies suddenly (crash in C, OS terminated process, etc). But on non-Windows platforms you can delete the shared-lib from disk immediately after loading it, you don't need to wait until the process exits. I did experiments on Mac and this works. But I left it out of this PR because this is an emergency fix for windows and I don't want to introduce non-necessary changes.

Author: graebm

src/main/java/software/amazon/awssdk/crt/CRT.java

@@ -6,6 +6,7 @@
 
 import java.io.File;
 import java.io.FileOutputStream;
+import java.io.FilenameFilter;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.*;
@@ -148,17 +149,36 @@ private static void extractAndLoadLibrary(String path) {
                 throw new IOException(msg, ex);
             }
 
-            // Prefix the lib
-            String prefix = "AWSCRT_" + new Date().getTime();
             String libraryName = System.mapLibraryName(CRT_LIB_NAME);
-            String libraryPath = "/" + getOSIdentifier() + "/" + getArchIdentifier() + "/" + libraryName;
 
-            File tempSharedLib = File.createTempFile(prefix, libraryName, tmpdirFile);
+            // Prefix the lib we'll extract to disk
+            String tempSharedLibPrefix = "AWSCRT_";
+
+            File tempSharedLib = File.createTempFile(tempSharedLibPrefix, libraryName, tmpdirFile);
+
+			// The temp lib file should be deleted when we're done with it.
+			// Ask Java to try and delete it on exit. We call this immediately
+			// so that if anything goes wrong writing the file to disk, or
+			// loading it as a shared lib, it will still get cleaned up.
+			tempSharedLib.deleteOnExit();
+
+            // Unfortunately File.deleteOnExit() won't work on Windows, where
+            // files cannot be deleted while they're in use. On Windows, once
+            // our .dll is loaded, it can't be deleted by this process.
+            //
+            // The Windows-only solution to this problem is to scan on startup
+            // for old instances of the .dll and try to delete them. If another
+            // process is still using the .dll, the delete will fail, which is fine.
+            String os = getOSIdentifier();
+            if (os.equals("windows")) {
+                tryDeleteOldLibrariesFromTempDir(tmpdirFile, tempSharedLibPrefix, libraryName);
+            }
 
             // open a stream to read the shared lib contents from this JAR
-            try (InputStream in = CRT.class.getResourceAsStream(libraryPath)) {
+            String libResourcePath = "/" + os + "/" + getArchIdentifier() + "/" + libraryName;
+            try (InputStream in = CRT.class.getResourceAsStream(libResourcePath)) {
                 if (in == null) {
-                    throw new IOException("Unable to open library in jar for AWS CRT: " + libraryPath);
+                    throw new IOException("Unable to open library in jar for AWS CRT: " + libResourcePath);
                 }
 
                 if (tempSharedLib.exists()) {
@@ -185,9 +205,6 @@ private static void extractAndLoadLibrary(String path) {
                 throw new CrtRuntimeException("Unable to make shared library readable");
             }
 
-            // Ensure that the shared lib will be destroyed when java exits
-            tempSharedLib.deleteOnExit();
-
             // load the shared lib from the temp path
             System.load(tempSharedLib.getAbsolutePath());
         } catch (CrtRuntimeException crtex) {
@@ -236,6 +253,30 @@ private static void loadLibraryFromJar() {
         throw new CrtRuntimeException(failureMessage.toString());
     }
 
+    // Try to delete old CRT libraries that were extracted to the temp dir by previous runs.
+    private static void tryDeleteOldLibrariesFromTempDir(File tmpDir, String libNamePrefix, String libNameSuffix) {
+        try {
+            File[] oldLibs = tmpDir.listFiles(new FilenameFilter() {
+                public boolean accept(File dir, String name) {
+                    return name.startsWith(libNamePrefix) && name.endsWith(libNameSuffix);
+                }
+            });
+
+            // Don't delete files that are too new.
+            // We don't want to delete another process's lib in the
+            // millisecond between the file being written to disk,
+            // and the file being loaded as a shared lib.
+            long aFewMomentsAgo = System.currentTimeMillis() - 10_000; // 10sec
+            for (File oldLib : oldLibs) {
+                try {
+                    if (oldLib.lastModified() < aFewMomentsAgo) {
+                        oldLib.delete();
+                    }
+                } catch (Exception e) {}
+            }
+        } catch (Exception e) {}
+    }
+
     private static CrtPlatform findPlatformImpl() {
         ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
         String[] platforms = new String[] {