fix: detect mounts via /proc/self/mountinfo so stale FUSE works

mountinfo.Mounted lstats the path. When the sshfs link dies, every stat
on the mountpoint returns EIO, so -l filtered the dead mount out, mount
failed in MkdirAll, and -u failed before fusermount. Switch detection to
mountinfo.GetMounts (no stat) and add a fusermount -uz fallback so a
stale mount can actually be torn down.

DECISIONS.md

@@ -1,5 +1,45 @@
 # Decisions
 
+## 2026-05-04 – Detect mounts via `/proc/self/mountinfo` instead of `stat`
+- **Who**: Dom, ClaudeCode
+- **Why**: `mountinfo.Mounted(path)` from `github.com/moby/sys/mountinfo` works
+  by `lstat`-ing the path. When a sshfs connection dies, the FUSE endpoint
+  stays in `/proc/mounts` but every `stat` on the mountpoint returns EIO.
+  All three sshfsc paths broke as a result:
+  - `-l` silently filtered the dead mount out (`Mounted` returned an error,
+    code skipped on `merr != nil`)
+  - mount path failed with `mkdir … file exists` because `MkdirAll` stats
+    before creating
+  - `-u` failed with `lstat … input/output error` before reaching
+    `fusermount`
+  Net effect: a stale mount was a dead end — couldn't be listed, couldn't be
+  unmounted, blocked re-mount.
+- **Tradeoffs**:
+  - Switched mount detection to `mountinfo.GetMounts(SingleEntryFilter(path))`,
+    which parses `/proc/self/mountinfo` and never touches the path. Robust to
+    EIO on the mount target.
+  - `list_mounts` now uses `PrefixFilter(base)` and trims the base prefix.
+    Direct children only — nested mounts under an alias dir would be skipped
+    on purpose; sshfsc only manages one level.
+  - `verify_mount_dir` skips `MkdirAll` when the path is already a mountpoint.
+    Re-running `sshfsc <alias>` against a stale mount no longer errors on
+    mkdir; main's existing `is_mounted_at` check then prints "Already mounted"
+    and returns. The user can `sshfsc -u <alias>` to recover.
+  - `unmount_sshfs` falls back to `fusermount -uz` (lazy) when `-u` fails.
+    Lazy unmount detaches the FS from the tree immediately and lets dangling
+    refs settle later — exactly what stale FUSE needs. Plain `-u` first
+    keeps the clean path noise-free.
+- **How**:
+  - New helper `is_mounted_at(path)`; replaces `mountinfo.Mounted` at every
+    call site (`main`, `verify_mount_dir`, `unmount_sshfs`).
+  - `list_mounts` rewritten around `PrefixFilter`. Trims `base + "/"` and
+    skips entries containing further separators.
+  - `run_fusermount(flag, mount)` extracted to keep `-u` and `-uz` calls
+    tidy. `unmount_sshfs` retries with `-uz` on first-call error.
+  - New tests: `TestIsMountedAtFalseOnPlainDir`,
+    `TestIsMountedAtFalseOnMissingPath`. Existing tests stay green —
+    `mountinfo.GetMounts` returns empty under the test tmpdirs.
+
 ## 2026-05-04 – Add `-l` (list) and `-u` (unmount) flags
 - **Who**: Dom, ClaudeCode
 - **Why**: Mounts under `$XDG_RUNTIME_DIR/sshfs/<alias>` had to be listed and

main.go

@@ -167,9 +167,9 @@ func main() {
 		fmt.Println("Mount:	", mount)
 	}
 
-	chkmount, chkmount_err := mountinfo.Mounted(mount)
+	chkmount, chkmount_err := is_mounted_at(mount)
 	if chkmount_err != nil {
-		fmt.Fprintf(os.Stderr, "mountinfo.Mounted() failed with %s\n", chkmount_err)
+		fmt.Fprintf(os.Stderr, "is_mounted_at() failed with %s\n", chkmount_err)
 		os.Exit(5)
 	}
 	if !chkmount {
@@ -258,12 +258,28 @@ func verify_mount_dir(name string) (string, error) {
 	if err != nil {
 		return "", err
 	}
+	// If the path is already a mountpoint (possibly stale), skip MkdirAll —
+	// it would stat the target, which fails with EIO on stale FUSE mounts.
+	if ok, _ := is_mounted_at(mount); ok {
+		return mount, nil
+	}
 	if err := os.MkdirAll(mount, 0700); err != nil {
 		return "", fmt.Errorf("create mount dir %q: %w", mount, err)
 	}
 	return mount, nil
 }
 
+// is_mounted_at reports whether path is a mountpoint per /proc/self/mountinfo.
+// Unlike mountinfo.Mounted, this does not stat the path, so stale FUSE mounts
+// (whose targets return EIO on stat) are still detected.
+func is_mounted_at(path string) (bool, error) {
+	mounts, err := mountinfo.GetMounts(mountinfo.SingleEntryFilter(path))
+	if err != nil {
+		return false, err
+	}
+	return len(mounts) > 0, nil
+}
+
 func list_mounts(out io.Writer) error {
 	base, err := mount_base(false)
 	if err != nil {
@@ -272,24 +288,28 @@ func list_mounts(out io.Writer) error {
 		}
 		return err
 	}
-	entries, err := os.ReadDir(base)
+	mounts, err := mountinfo.GetMounts(mountinfo.PrefixFilter(base))
 	if err != nil {
-		return fmt.Errorf("read base %q: %w", base, err)
+		return fmt.Errorf("read mountinfo: %w", err)
 	}
-	for _, entry := range entries {
-		if !entry.IsDir() {
+	prefix := base + string(os.PathSeparator)
+	for _, m := range mounts {
+		rel := strings.TrimPrefix(m.Mountpoint, prefix)
+		if rel == "" || strings.Contains(rel, string(os.PathSeparator)) {
 			continue
 		}
-		path := filepath.Join(base, entry.Name())
-		ok, merr := mountinfo.Mounted(path)
-		if merr != nil || !ok {
-			continue
-		}
-		fmt.Fprintln(out, entry.Name())
+		fmt.Fprintln(out, rel)
 	}
 	return nil
 }
 
+func run_fusermount(flag, mount string) error {
+	cmd := exec.Command("fusermount", flag, mount)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
+}
+
 func unmount_sshfs(alias string) error {
 	if err := validate_ssh_field("alias", alias, rxHostUser); err != nil {
 		return err
@@ -305,18 +325,19 @@ func unmount_sshfs(alias string) error {
 	if err != nil {
 		return err
 	}
-	ok, err := mountinfo.Mounted(mount)
+	ok, err := is_mounted_at(mount)
 	if err != nil {
-		return fmt.Errorf("mountinfo.Mounted(%q): %w", mount, err)
+		return fmt.Errorf("check mount %q: %w", mount, err)
 	}
 	if !ok {
 		return fmt.Errorf("not mounted: %s", alias)
 	}
-	cmd := exec.Command("fusermount", "-u", mount)
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-	if err := cmd.Run(); err != nil {
-		return fmt.Errorf("fusermount -u %q: %w", mount, err)
+	if err := run_fusermount("-u", mount); err != nil {
+		// stale FUSE mounts often need lazy unmount; -u fails with
+		// "Transport endpoint is not connected" on those.
+		if err2 := run_fusermount("-uz", mount); err2 != nil {
+			return fmt.Errorf("fusermount -u %q: %w (lazy retry: %v)", mount, err, err2)
+		}
 	}
 	if err := os.Remove(mount); err != nil {
 		fmt.Fprintf(os.Stderr, "warning: remove empty mount dir %q: %v\n", mount, err)

main_test.go

@@ -200,3 +200,25 @@ func TestUnmountNotMountedExistingDir(t *testing.T) {
 		t.Fatalf("want not-mounted error, got %v", err)
 	}
 }
+
+func TestIsMountedAtFalseOnPlainDir(t *testing.T) {
+	dir := t.TempDir()
+	ok, err := is_mounted_at(dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if ok {
+		t.Fatalf("plain tempdir should not be a mountpoint")
+	}
+}
+
+func TestIsMountedAtFalseOnMissingPath(t *testing.T) {
+	dir := t.TempDir()
+	ok, err := is_mounted_at(filepath.Join(dir, "nope"))
+	if err != nil {
+		t.Fatalf("unexpected error (no stat should happen): %v", err)
+	}
+	if ok {
+		t.Fatalf("missing path should not be a mountpoint")
+	}
+}