mountutil: give an actionable error for unshared bind-propagation source

pkg/mountutil/mountutil_linux.go

@@ -247,7 +247,7 @@ func parseVolumeOptionsWithMountInfo(vType, src, optsRaw, ociRuntime string, get
 			if err != nil {
 				return nil, nil, err
 			}
-			if err := ensureMountOptionalValue(mi, "shared:"); err != nil {
+			if err := ensureMountOptionalValue(mi, got, "shared:"); err != nil {
 				return nil, nil, err
 			}
 
@@ -276,7 +276,7 @@ func parseVolumeOptionsWithMountInfo(vType, src, optsRaw, ociRuntime string, get
 			if err != nil {
 				return nil, nil, err
 			}
-			if err := ensureMountOptionalValue(mi, "shared:", "master:"); err != nil {
+			if err := ensureMountOptionalValue(mi, got, "shared:", "master:"); err != nil {
 				return nil, nil, err
 			}
 
@@ -320,7 +320,7 @@ func parseVolumeOptionsWithMountInfo(vType, src, optsRaw, ociRuntime string, get
 //
 // For more details about "optional" field:
 // - https://github.com/moby/sys/blob/mountinfo/v0.4.1/mountinfo/mountinfo.go#L52-L56
-func ensureMountOptionalValue(mi mount.Info, vals ...string) error {
+func ensureMountOptionalValue(mi mount.Info, propagation string, vals ...string) error {
 	var hasValue bool
 	for _, opt := range strings.Split(mi.Optional, " ") {
 		for _, mark := range vals {
@@ -330,7 +330,7 @@ func ensureMountOptionalValue(mi mount.Info, vals ...string) error {
 		}
 	}
 	if !hasValue {
-		return fmt.Errorf("mountpoint %q doesn't have optional field neither of %+v", mi.Mountpoint, vals)
+		return fmt.Errorf("mountpoint %q is not a shared or slave mount, so it cannot be bind-mounted with propagation %q; try running `mount --make-rshared %q` on the host", mi.Mountpoint, propagation, mi.Mountpoint)
 	}
 	return nil
 }

pkg/mountutil/mountutil_linux_test.go

@@ -59,6 +59,7 @@ func TestParseVolumeOptions(t *testing.T) {
 		wants                    []string
 		wantRootfsPropagation    string
 		wantFail                 bool
+		wantErrContains          string
 	}{
 		{
 			name:    "unknown option is ignored (with warning)",
@@ -176,12 +177,13 @@ func TestParseVolumeOptions(t *testing.T) {
 			wants:                    []string{"ro", "rshared"},
 		},
 		{
-			name:        "shared propagation is not allowed if the src is not shared",
-			vType:       "bind",
-			src:         "dummy",
-			optsRaw:     "ro,shared",
-			srcOptional: nil,
-			wantFail:    true,
+			name:            "shared propagation is not allowed if the src is not shared",
+			vType:           "bind",
+			src:             "dummy",
+			optsRaw:         "ro,shared",
+			srcOptional:     nil,
+			wantFail:        true,
+			wantErrContains: `mount --make-rshared "dummy"`,
 		},
 		{
 			name:                  "make bind slave",
@@ -203,12 +205,13 @@ func TestParseVolumeOptions(t *testing.T) {
 			wants:                    []string{"ro", "slave"},
 		},
 		{
-			name:        "slave propagation is not allowed if the src is not slave",
-			vType:       "bind",
-			src:         "dummy",
-			optsRaw:     "ro,slave",
-			srcOptional: nil,
-			wantFail:    true,
+			name:            "slave propagation is not allowed if the src is not slave",
+			vType:           "bind",
+			src:             "dummy",
+			optsRaw:         "ro,slave",
+			srcOptional:     nil,
+			wantFail:        true,
+			wantErrContains: `mount --make-rshared "dummy"`,
 		},
 	}
 	for _, tt := range tests {
@@ -222,6 +225,9 @@ func TestParseVolumeOptions(t *testing.T) {
 			})
 			if err != nil {
 				if tt.wantFail {
+					if tt.wantErrContains != "" {
+						assert.ErrorContains(t, err, tt.wantErrContains)
+					}
 					return
 				}
 				t.Errorf("failed to parse option %q: %v", tt.optsRaw, err)