[Backport 2.x] Fix fs info reporting negative available size

CHANGELOG.md

@@ -38,6 +38,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fixed rest-high-level client searchTemplate & mtermVectors endpoints to have a leading slash ([#14465](https://github.com/opensearch-project/OpenSearch/pull/14465))
 - Write shard level metadata blob when snapshotting searchable snapshot indexes ([#13190](https://github.com/opensearch-project/OpenSearch/pull/13190))
 - Fix aggs result of NestedAggregator with sub NestedAggregator ([#13324](https://github.com/opensearch-project/OpenSearch/pull/13324))
+- Fix fs info reporting negative available size ([#11573](https://github.com/opensearch-project/OpenSearch/pull/11573))
 - Add ListPitInfo::getKeepAlive() getter ([#14495](https://github.com/opensearch-project/OpenSearch/pull/14495))
 
 ### Security

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

@@ -82,6 +82,10 @@ public FsInfo stats(FsInfo previous) throws IOException {
                 paths[i].fileCacheReserved = adjustForHugeFilesystems(dataLocations[i].fileCacheReservedSize.getBytes());
                 paths[i].fileCacheUtilized = adjustForHugeFilesystems(fileCache.usage().usage());
                 paths[i].available -= (paths[i].fileCacheReserved - paths[i].fileCacheUtilized);
+                // occurs if reserved file cache space is occupied by other files, like local indices
+                if (paths[i].available < 0) {
+                    paths[i].available = 0;
+                }
             }
         }
         FsInfo.IoStats ioStats = null;

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

@@ -58,6 +58,7 @@
 import java.util.function.Function;
 import java.util.function.Supplier;
 
+import static org.opensearch.monitor.fs.FsProbe.adjustForHugeFilesystems;
 import static org.hamcrest.CoreMatchers.equalTo;
 import static org.hamcrest.Matchers.emptyOrNullString;
 import static org.hamcrest.Matchers.greaterThan;
@@ -162,6 +163,46 @@ public void testFsCacheInfo() throws IOException {
         }
     }
 
+    public void testFsInfoWhenFileCacheOccupied() throws IOException {
+        Settings settings = Settings.builder().putList("node.roles", "search", "data").build();
+        try (NodeEnvironment env = newNodeEnvironment(settings)) {
+            // Use the total space as reserved space to simulate the situation where the cache space is occupied
+            final long totalSpace = adjustForHugeFilesystems(env.fileCacheNodePath().fileStore.getTotalSpace());
+            ByteSizeValue gbByteSizeValue = new ByteSizeValue(totalSpace, ByteSizeUnit.BYTES);
+            env.fileCacheNodePath().fileCacheReservedSize = gbByteSizeValue;
+            FileCache fileCache = FileCacheFactory.createConcurrentLRUFileCache(
+                gbByteSizeValue.getBytes(),
+                16,
+                new NoopCircuitBreaker(CircuitBreaker.REQUEST)
+            );
+
+            FsProbe probe = new FsProbe(env, fileCache);
+            FsInfo stats = probe.stats(null);
+            assertNotNull(stats);
+            assertTrue(stats.getTimestamp() > 0L);
+            FsInfo.Path total = stats.getTotal();
+            assertNotNull(total);
+            assertTrue(total.total > 0L);
+            assertTrue(total.free > 0L);
+            assertTrue(total.fileCacheReserved > 0L);
+
+            for (FsInfo.Path path : stats) {
+                assertNotNull(path);
+                assertFalse(path.getPath().isEmpty());
+                assertFalse(path.getMount().isEmpty());
+                assertFalse(path.getType().isEmpty());
+                assertTrue(path.total > 0L);
+                assertTrue(path.free > 0L);
+
+                if (path.fileCacheReserved > 0L) {
+                    assertEquals(0L, path.available);
+                } else {
+                    assertTrue(path.available > 0L);
+                }
+            }
+        }
+    }
+
     public void testFsInfoOverflow() throws Exception {
         final FsInfo.Path pathStats = new FsInfo.Path(
             "/foo/bar",