module-transaction: Do not allow parallel conversations by default

Pam conversations per se may also run in parallel, but this implies that
the application supports this.

Since this normally not the case, do not create modules that may invoke
the pam conversations in parallel by default, adding a mutex to protect
such calls.

cmd/pam-moduler/moduler.go

@@ -69,6 +69,7 @@ var (
 	moduleBuildFlags = flag.String("build-flags", "", "comma-separated list of go build flags to use when generating the module")
 	moduleBuildTags  = flag.String("build-tags", "", "comma-separated list of build tags to use when generating the module")
 	noMain           = flag.Bool("no-main", false, "whether to add an empty main to generated file")
+	parallelConv     = flag.Bool("parallel-conv", false, "whether to support performing PAM conversations in parallel")
 )
 
 // Usage is a replacement usage function for the flags package.
@@ -137,6 +138,7 @@ func main() {
 		generateTags: generateTags,
 		noMain:       *noMain,
 		typeName:     *typeName,
+		parallelConv: *parallelConv,
 	}
 
 	// Print the header and package clause.
@@ -169,6 +171,7 @@ type Generator struct {
 	generateTags []string
 	buildFlags   []string
 	noMain       bool
+	parallelConv bool
 }
 
 func (g *Generator) printf(format string, args ...interface{}) {
@@ -186,6 +189,11 @@ func (g *Generator) generate() {
 		buildTagsArg = fmt.Sprintf("-tags %s", strings.Join(g.generateTags, ","))
 	}
 
+	var transactionCreator = "NewModuleTransactionInvoker"
+	if g.parallelConv {
+		transactionCreator = "NewModuleTransactionInvokerParallelConv"
+	}
+
 	// We use a slice since we want to keep order, for reproducible builds.
 	vFuncs := []struct {
 		cName  string
@@ -257,8 +265,8 @@ func handlePamCall(pamh *C.pam_handle_t, flags C.int, argc C.int,
 		return C.int(pam.ErrIgnore)
 	}
 
-	mt := pam.NewModuleTransactionInvoker(pam.NativeHandle(pamh))
+	mt := pam.%s(pam.NativeHandle(pamh))
-	err := mt.InvokeHandler(moduleFunc, pam.Flags(flags),
+    err := mt.InvokeHandler(moduleFunc, pam.Flags(flags),
 		sliceFromArgv(argc, argv))
 	if err == nil {
 		return 0
@@ -275,7 +283,7 @@ func handlePamCall(pamh *C.pam_handle_t, flags C.int, argc C.int,
 
 	return C.int(pam.ErrSystem)
 }
-`)
+`, transactionCreator)
 
 	for _, f := range vFuncs {
 		g.printf(`

cmd/pam-moduler/tests/integration-tester-module/integration-tester-module.go

@@ -1,4 +1,4 @@
-//go:generate go run github.com/msteinert/pam/v2/cmd/pam-moduler -type integrationTesterModule
+//go:generate go run github.com/msteinert/pam/v2/cmd/pam-moduler -type integrationTesterModule -parallel-conv
 //go:generate go generate --skip="pam_module.go"
 
 // Package main is the package for the integration tester module PAM shared library.

cmd/pam-moduler/tests/integration-tester-module/pam_module.go

@@ -1,4 +1,4 @@
-// Code generated by "pam-moduler -type integrationTesterModule"; DO NOT EDIT.
+// Code generated by "pam-moduler -type integrationTesterModule -parallel-conv"; DO NOT EDIT.
 
 //go:generate go build "-ldflags=-extldflags -Wl,-soname,pam_go.so" -buildmode=c-shared -o pam_go.so -tags go_pam_module
 
@@ -43,7 +43,7 @@ func handlePamCall(pamh *C.pam_handle_t, flags C.int, argc C.int,
 		return C.int(pam.ErrIgnore)
 	}
 
-	mt := pam.NewModuleTransactionInvoker(pam.NativeHandle(pamh))
+	mt := pam.NewModuleTransactionInvokerParallelConv(pam.NativeHandle(pamh))
 	err := mt.InvokeHandler(moduleFunc, pam.Flags(flags),
 		sliceFromArgv(argc, argv))
 	if err == nil {

module-transaction.go

@@ -43,6 +43,7 @@ type ModuleHandlerFunc func(ModuleTransaction, Flags, []string) error
 // ModuleTransaction is the module-side handle for a PAM transaction.
 type moduleTransaction struct {
 	transactionBase
+	convMutex *sync.Mutex
 }
 
 // ModuleHandler is an interface for objects that can be used to create
@@ -63,10 +64,27 @@ type ModuleTransactionInvoker interface {
 	InvokeHandler(handler ModuleHandlerFunc, flags Flags, args []string) error
 }
 
-// NewModuleTransactionInvoker allows initializing a transaction invoker from
+// NewModuleTransactionParallelConv allows initializing a transaction from the
-// the module side.
+// module side. Conversations using this transaction can be multi-thread, but
+// this requires the application loading the module to support this, otherwise
+// we may just break their assumptions.
+func NewModuleTransactionParallelConv(handle NativeHandle) ModuleTransaction {
+	return &moduleTransaction{transactionBase{handle: handle}, nil}
+}
+
+// NewModuleTransactionInvoker allows initializing a transaction invoker from the
+// module side.
 func NewModuleTransactionInvoker(handle NativeHandle) ModuleTransactionInvoker {
-	return &moduleTransaction{transactionBase{handle: handle}}
+	return &moduleTransaction{transactionBase{handle: handle}, &sync.Mutex{}}
+}
+
+// NewModuleTransactionInvokerParallelConv allows initializing a transaction invoker
+// from the module side.
+// Conversations using this transaction can be multi-thread, but this requires
+// the application loading the module to support this, otherwise we may just
+// break their assumptions.
+func NewModuleTransactionInvokerParallelConv(handle NativeHandle) ModuleTransactionInvoker {
+	return &moduleTransaction{transactionBase{handle: handle}, nil}
 }
 
 func (m *moduleTransaction) InvokeHandler(handler ModuleHandlerFunc,
@@ -542,6 +560,10 @@ func (m *moduleTransaction) startConvMultiImpl(iface moduleTransactionIface,
 		goMsgs[i] = cMessage
 	}
 
+	if m.convMutex != nil {
+		m.convMutex.Lock()
+		defer m.convMutex.Unlock()
+	}
 	var cResponses *C.struct_pam_response
 	ret := iface.startConv(conv, C.int(len(requests)), cMessages, &cResponses)
 	if ret != success {

module-transaction_test.go

@@ -305,11 +305,10 @@ func Test_ModuleTransaction_InvokeHandler(t *testing.T) {
 	}
 }
 
-func Test_MockModuleTransaction(t *testing.T) {
+func testMockModuleTransaction(t *testing.T, mt *moduleTransaction) {
+	t.Helper()
 	t.Parallel()
 
-	mt, _ := NewModuleTransactionInvoker(nil).(*moduleTransaction)
-
 	tests := map[string]struct {
 		testFunc            func(mock *mockModuleTransaction) (any, error)
 		mockExpectations    mockModuleTransactionExpectations
@@ -914,3 +913,13 @@ func Test_MockModuleTransaction(t *testing.T) {
 		})
 	}
 }
+
+func Test_MockModuleTransaction(t *testing.T) {
+	mt, _ := NewModuleTransactionInvoker(nil).(*moduleTransaction)
+	testMockModuleTransaction(t, mt)
+}
+
+func Test_MockModuleTransactionParallelConv(t *testing.T) {
+	mt, _ := NewModuleTransactionInvokerParallelConv(nil).(*moduleTransaction)
+	testMockModuleTransaction(t, mt)
+}