fuse integration: require an external mount to be a FUSE filesystem

A device-ID boundary alone does not prove FUSE: a tmpfs mount would pass mountedAt and report its results as FUSE compliance. Check the filesystem type (statfs magic on linux, fstypename on the BSDs); platforms without a check keep the mount and write verification.

test/fuse_integration/framework.go

@@ -108,6 +108,11 @@ func (f *FuseTestFramework) Setup(config *TestConfig) error {
 			if !mountedAt(existingMount) {
 				return fmt.Errorf("mount point %s is not a mounted filesystem", existingMount)
 			}
+			// So can a non-FUSE mount like tmpfs: a device boundary alone does
+			// not make the results FUSE compliance
+			if !fuseFilesystem(existingMount) {
+				return fmt.Errorf("mount point %s is not a FUSE 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

test/fuse_integration/fuse_mount_bsd.go

@@ -0,0 +1,25 @@
+//go:build darwin || freebsd
+
+package fuse
+
+import (
+	"strings"
+	"syscall"
+)
+
+// fuseFilesystem reports whether path sits on a FUSE mount; macFUSE, osxfuse
+// and fusefs all carry "fuse" in the filesystem type name.
+func fuseFilesystem(path string) bool {
+	var st syscall.Statfs_t
+	if err := syscall.Statfs(path, &st); err != nil {
+		return false
+	}
+	name := make([]byte, 0, len(st.Fstypename))
+	for _, c := range st.Fstypename {
+		if c == 0 {
+			break
+		}
+		name = append(name, byte(c))
+	}
+	return strings.Contains(string(name), "fuse")
+}

test/fuse_integration/fuse_mount_linux.go

@@ -0,0 +1,17 @@
+//go:build linux
+
+package fuse
+
+import "syscall"
+
+// fuseSuperMagic is statfs f_type for fuse and fuseblk mounts.
+const fuseSuperMagic = 0x65735546
+
+// fuseFilesystem reports whether path sits on a FUSE mount.
+func fuseFilesystem(path string) bool {
+	var st syscall.Statfs_t
+	if err := syscall.Statfs(path, &st); err != nil {
+		return false
+	}
+	return st.Type == fuseSuperMagic
+}

test/fuse_integration/fuse_mount_other.go

@@ -0,0 +1,9 @@
+//go:build (unix && !linux && !darwin && !freebsd) || (!unix && !windows)
+
+package fuse
+
+// fuseFilesystem has no portable check on these platforms; the mount check
+// and write verification in Setup stay the only gates.
+func fuseFilesystem(path string) bool {
+	return true
+}

test/fuse_integration/mounted_linux_test.go

@@ -1,4 +1,4 @@
-//go:build unix
+//go:build linux
 
 package fuse
 
@@ -17,3 +17,11 @@ func TestMountedAt(t *testing.T) {
 		}
 	}
 }
+
+func TestFuseFilesystem(t *testing.T) {
+	for _, p := range []string{"/proc", t.TempDir()} {
+		if fuseFilesystem(p) {
+			t.Errorf("fuseFilesystem(%q) = true, want false", p)
+		}
+	}
+}

test/fuse_integration/mounted_windows.go

@@ -32,3 +32,9 @@ func mountedAt(path string) bool {
 	data, ok := info.Sys().(*syscall.Win32FileAttributeData)
 	return ok && data.FileAttributes&syscall.FILE_ATTRIBUTE_REPARSE_POINT != 0
 }
+
+// fuseFilesystem is covered by the WinFsp checks in mountedAt: WinFsp is the
+// only FUSE provider on the platform.
+func fuseFilesystem(path string) bool {
+	return true
+}