config: default to home mount when mounts key is absent

config/configmanager/configmanager.go

@@ -44,6 +44,20 @@ func LoadFrom(file string) (config.Config, error) {
 		return c, fmt.Errorf("could not load config from file: %w", err)
 	}
 
+	// yaml.Unmarshal collapses both an absent `mounts` key and an explicit
+	// `mounts: null` into a nil slice, but they mean different things: an absent
+	// key falls back to the documented default (mount $HOME, i.e. an empty list),
+	// whereas an explicit null disables mounts. Restore the default for the
+	// absent case by inspecting the raw document for the key's presence.
+	if c.Mounts == nil {
+		var raw map[string]yaml.Node
+		if yaml.Unmarshal(b, &raw) == nil {
+			if _, present := raw["mounts"]; !present {
+				c.Mounts = []config.Mount{}
+			}
+		}
+	}
+
 	return c, nil
 }
 

config/configmanager/configmanager_test.go

@@ -0,0 +1,76 @@
+package configmanager
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// TestLoadFromMounts ensures the three mount states round-trip correctly when
+// loading a config file. In particular, an absent `mounts` key must fall back
+// to the documented default (mount $HOME) rather than being treated the same as
+// an explicit `mounts: null` (no mounts). See issue #1533.
+func TestLoadFromMounts(t *testing.T) {
+	tests := []struct {
+		name    string
+		content string
+		// wantHome is the expected result of MountsOrDefault containing the
+		// default home mount.
+		wantHome bool
+		wantLen  int
+	}{
+		{
+			name:     "absent key defaults to home",
+			content:  "cpu: 4\nmemory: 8\n",
+			wantHome: true,
+			wantLen:  1,
+		},
+		{
+			name:     "explicit empty list defaults to home",
+			content:  "cpu: 4\nmounts: []\n",
+			wantHome: true,
+			wantLen:  1,
+		},
+		{
+			name:     "explicit null disables mounts",
+			content:  "cpu: 4\nmounts: null\n",
+			wantHome: false,
+			wantLen:  0,
+		},
+		{
+			name:     "explicit mounts are preserved",
+			content:  "cpu: 4\nmounts:\n  - location: /x\n",
+			wantHome: false,
+			wantLen:  1,
+		},
+	}
+
+	dir := t.TempDir()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			f := filepath.Join(dir, tt.name+".yaml")
+			if err := os.WriteFile(f, []byte(tt.content), 0644); err != nil {
+				t.Fatal(err)
+			}
+
+			c, err := LoadFrom(f)
+			if err != nil {
+				t.Fatalf("LoadFrom() error = %v", err)
+			}
+
+			mounts := c.MountsOrDefault()
+			if len(mounts) != tt.wantLen {
+				t.Errorf("MountsOrDefault() len = %d, want %d", len(mounts), tt.wantLen)
+			}
+			if tt.wantHome {
+				if len(mounts) != 1 {
+					t.Fatalf("expected single default home mount, got %d", len(mounts))
+				}
+				home, err := os.UserHomeDir()
+				if err == nil && mounts[0].Location != home {
+					t.Errorf("default mount location = %q, want home %q", mounts[0].Location, home)
+				}
+			}
+		})
+	}
+}