Fix implementation of PosixFileAttributes.permissions() to return a modifiable set as specified in the Javadoc for that method.

Fixes #413

RELNOTES=Fixed implementation of `PosixFileAttributes.permissions()` to return a modifiable set as specified in the Javadoc for that method.
PiperOrigin-RevId: 762097962

Author: cgdecker

jimfs/src/main/java/com/google/common/jimfs/PosixAttributeProvider.java

@@ -33,6 +33,7 @@
 import java.nio.file.attribute.PosixFilePermission;
 import java.nio.file.attribute.PosixFilePermissions;
 import java.nio.file.attribute.UserPrincipal;
+import java.util.LinkedHashSet;
 import java.util.Map;
 import java.util.Set;
 import org.jspecify.annotations.Nullable;
@@ -68,7 +69,6 @@ public ImmutableSet<String> fixedAttributes() {
     return ATTRIBUTES;
   }
 
-  @SuppressWarnings("unchecked")
   @Override
   public ImmutableMap<String, ?> defaultValues(Map<String, ?> userProvidedDefaults) {
     Object userProvidedGroup = userProvidedDefaults.get("posix:group");
@@ -262,8 +262,8 @@ public GroupPrincipal group() {
     }
 
     @Override
-    public ImmutableSet<PosixFilePermission> permissions() {
-      return permissions;
+    public Set<PosixFilePermission> permissions() {
+      return new LinkedHashSet<>(permissions);
     }
   }
 }

jimfs/src/test/java/com/google/common/jimfs/PosixAttributeProviderTest.java

@@ -121,4 +121,11 @@ public void testAttributes() {
     assertThat(attrs.group()).isEqualTo(createGroupPrincipal("group"));
     assertThat(attrs.fileKey()).isEqualTo(0);
   }
+
+  @Test
+  public void testAttributes_permissionsAreModifiable() {
+    Set<PosixFilePermission> permissions = provider.readAttributes(file).permissions();
+    permissions.add(PosixFilePermission.OWNER_EXECUTE);
+    assertThat(permissions).isEqualTo(PosixFilePermissions.fromString("rwxr--r--"));
+  }
 }