The value of fileCacheUtilized should use standard init value

In FsInfo.Path object the value of fileCacheUtilized is now initialized
like any other fields, that means it is -1, which means the value hasn't been
populated yet.

To make this possible it was necessary to fix FsProbeTest and use
a real FileCache object to enable internal logic to handle initialization
and safeguarding of the value properly.

Signed-off-by: Lukáš Vlček <lukas.vlcek@aiven.io>

server/src/main/java/org/opensearch/monitor/fs/FsInfo.java

@@ -75,7 +75,7 @@ public static class Path implements Writeable, ToXContentObject {
         long free = -1;
         long available = -1;
         long fileCacheReserved = -1;
-        long fileCacheUtilized = 0;
+        long fileCacheUtilized = -1;
 
         public Path() {}
 

server/src/main/java/org/opensearch/monitor/fs/FsProbe.java

@@ -78,7 +78,7 @@ public FsInfo stats(FsInfo previous) throws IOException {
         FsInfo.Path[] paths = new FsInfo.Path[dataLocations.length];
         for (int i = 0; i < dataLocations.length; i++) {
             paths[i] = getFSInfo(dataLocations[i]);
-            if (fileCache != null && dataLocations[i].fileCacheReservedSize != ByteSizeValue.ZERO) {
+            if (fileCache != null && dataLocations[i].fileCacheReservedSize.compareTo(ByteSizeValue.ZERO) >= 0) {
                 paths[i].fileCacheReserved = adjustForHugeFilesystems(dataLocations[i].fileCacheReservedSize.getBytes());
                 paths[i].fileCacheUtilized = adjustForHugeFilesystems(fileCache.usage().usage());
                 paths[i].available -= (paths[i].fileCacheReserved - paths[i].fileCacheUtilized);
@@ -206,6 +206,9 @@ public static FsInfo.Path getFSInfo(NodePath nodePath) throws IOException {
         fsPath.free = adjustForHugeFilesystems(nodePath.fileStore.getUnallocatedSpace());
         fsPath.available = adjustForHugeFilesystems(nodePath.fileStore.getUsableSpace());
         fsPath.fileCacheReserved = adjustForHugeFilesystems(nodePath.fileCacheReservedSize.getBytes());
+        // fsPath.fileCacheUtilized = adjustForHugeFilesystems(...);
+        // We can not do this ^^ here because information about utilization of file cache is hold by FileCache
+        // which is not accessible here and since this method is static we can not assume relevant context.
         fsPath.type = nodePath.fileStore.type();
         fsPath.mount = nodePath.fileStore.toString();
         return fsPath;

server/src/test/java/org/opensearch/monitor/fs/FsProbeTests.java

@@ -71,7 +71,17 @@ public class FsProbeTests extends OpenSearchTestCase {
     public void testFsInfo() throws IOException {
 
         try (NodeEnvironment env = newNodeEnvironment()) {
-            FsProbe probe = new FsProbe(env, null);
+            // Question: Shall we expose a public method in FileCacheTests to enable creation of FileCache
+            // so that it can be used by other testing classes?
+            int CONCURRENCY_LEVEL = 16; // not important
+            int CAPACITY = 1 * 1024; // not important
+            FileCache fileCache = FileCacheFactory.createConcurrentLRUFileCache(
+                CAPACITY,
+                CONCURRENCY_LEVEL,
+                new NoopCircuitBreaker(CircuitBreaker.REQUEST)
+            );
+            // We need to pass a real FileCache object to FsProbe ctor to have it safeguard "path.fileCacheUtilized" values properly!
+            FsProbe probe = new FsProbe(env, fileCache);
 
             FsInfo stats = probe.stats(null);
             assertNotNull(stats);
@@ -110,6 +120,16 @@ public void testFsInfo() throws IOException {
             assertThat(total.free, greaterThan(0L));
             assertThat(total.available, greaterThan(0L));
 
+            // The convention for "total" Path object is that some fields are not set
+            // which means they will not be included in output of toXContent method.
+            assertNull(total.path);
+            assertNull(total.mount);
+            assertNull(total.type);
+
+            // Total file cache (sum over all "paths"):
+            assertEquals(total.getFileCacheReserved().getBytes(), 0);
+            assertEquals(total.getFileCacheUtilized().getBytes(), 0);
+
             for (FsInfo.Path path : stats) {
                 assertNotNull(path);
                 assertThat(path.getPath(), is(not(emptyOrNullString())));