fuse integration: verify TEST_MOUNT_POINT is a real mount

test/fuse_integration/framework.go

@@ -103,6 +103,12 @@ func (f *FuseTestFramework) Setup(config *TestConfig) error {
 				return fmt.Errorf("existing mount point not accessible: %v", err)
 			}
 
+			// A plain directory passes every check below; refuse to run the suite
+			// against the local disk
+			if !mountedAt(existingMount) {
+				return fmt.Errorf("mount point %s is not a mounted filesystem", existingMount)
+			}
+
 			// Run inside a private work dir so test files do not pile up in
 			// the shared mount root and cleanup can remove only what it made
 			workDir, err := os.MkdirTemp(existingMount, ".fuse_test_")

test/fuse_integration/mounted_unix.go

@@ -2,7 +2,10 @@
 
 package fuse
 
-import "syscall"
+import (
+	"path/filepath"
+	"syscall"
+)
 
 // deviceID returns the device ID of the filesystem containing path.
 func deviceID(path string) (uint64, error) {
@@ -12,3 +15,17 @@ func deviceID(path string) (uint64, error) {
 	}
 	return uint64(st.Dev), nil
 }
+
+// mountedAt reports whether something is actually mounted at path: a plain
+// directory shares its parent's device, a mount point does not.
+func mountedAt(path string) bool {
+	dev, err := deviceID(path)
+	if err != nil {
+		return false
+	}
+	parentDev, err := deviceID(filepath.Dir(path))
+	if err != nil {
+		return false
+	}
+	return dev != parentDev
+}

test/fuse_integration/mounted_unsupported.go

@@ -8,3 +8,9 @@ package fuse
 func deviceID(path string) (uint64, error) {
 	return 0, nil
 }
+
+// mountedAt cannot be detected off unix; report mounted so an external
+// TEST_MOUNT_POINT keeps its best-effort write verification.
+func mountedAt(path string) bool {
+	return true
+}