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

@@ -68,6 +68,7 @@ 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.
@@ -136,6 +137,7 @@ func main() {
 		generateTags: generateTags,
 		noMain:       *noMain,
 		typeName:     *typeName,
+		parallelConv: *parallelConv,
 	}
 
 	// Print the header and package clause.
@@ -168,6 +170,7 @@ type Generator struct {
 	generateTags []string
 	buildFlags   []string
 	noMain       bool
+	parallelConv bool
 }
 
 func (g *Generator) Printf(format string, args ...interface{}) {
@@ -185,6 +188,11 @@ func (g *Generator) generate() {
 		buildTagsArg = fmt.Sprintf("-tags %s", strings.Join(g.generateTags, ","))
 	}
 
+	var transactionCreator = "NewModuleTransaction"
+	if g.parallelConv {
+		transactionCreator = "NewModuleTransactionParallelConv"
+	}
+
 	vFuncs := map[string]string{
 		"authenticate":  "Authenticate",
 		"setcred":       "SetCred",
@@ -247,7 +255,7 @@ func handlePamCall(pamh *C.pam_handle_t, flags C.int, argc C.int,
 		return pam.Ignore
 	}
 
-	mt := pam.NewModuleTransaction(pam.NativeHandle(pamh))
+	mt := pam.%s(pam.NativeHandle(pamh))
 	ret, err := mt.InvokeHandler(moduleFunc, pam.Flags(flags),
 		sliceFromArgv(argc, argv))
 
@@ -257,7 +265,7 @@ func handlePamCall(pamh *C.pam_handle_t, flags C.int, argc C.int,
 
 	return ret
 }
-`)
+`, transactionCreator)
 
 	for cName, goName := 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/cmd/pam-moduler -type integrationTesterModule
+//go:generate go run github.com/msteinert/pam/cmd/pam-moduler -type integrationTesterModule -parallel-conv
 //go:generate go generate --skip="pam_module.go"
 
 package main

module-transaction.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"runtime"
 	"runtime/cgo"
+	"sync"
 	"sync/atomic"
 	"unsafe"
 )
@@ -32,6 +33,7 @@ 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
@@ -45,9 +47,19 @@ type ModuleHandler interface {
 	SetCred(*ModuleTransaction, Flags, []string) error
 }
 
-// NewModuleTransaction allows initializing a transaction from the module side
+// NewModuleTransaction allows initializing a transaction from the module side.
+// Using this transaction conversations can't be performed in parallel and a
+// mutex is used to ensure this is the case.
 func NewModuleTransaction(handle NativeHandle) *ModuleTransaction {
-	return &ModuleTransaction{transactionBase{handle: handle}}
+	return &ModuleTransaction{transactionBase{handle: handle}, &sync.Mutex{}}
+}
+
+// NewModuleTransaction allows initializing a transaction 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 NewModuleTransactionParallelConv(handle NativeHandle) *ModuleTransaction {
+	return &ModuleTransaction{transactionBase{handle: handle}, nil}
 }
 
 func (m *ModuleTransaction) InvokeHandler(handler ModuleHandlerFunc,
@@ -440,6 +452,10 @@ func (m *ModuleTransaction) startConvMultiImpl(iface moduleTransactionIface,
 		}
 	}
 
+	if m.convMutex != nil {
+		m.convMutex.Lock()
+		defer m.convMutex.Unlock()
+	}
 	var cResponses *C.struct_pam_response
 	if err := m.handlePamStatus(
 		iface.startConv(conv, C.int(len(requests)), cMessages, &cResponses)); err != nil {

module-transaction_test.go

@@ -296,11 +296,9 @@ func Test_ModuleTransaction_InvokeHandler(t *testing.T) {
 	}
 }
 
-func Test_MockModuleTransaction(t *testing.T) {
+func testMockModuleTransaction(t *testing.T, mt *ModuleTransaction) {
 	t.Parallel()
 
-	mt := NewModuleTransaction(nil)
-
 	tests := map[string]struct {
 		testFunc            func(mock *mockModuleTransaction) (any, error)
 		mockExpectations    mockModuleTransactionExpectations
@@ -857,3 +855,11 @@ func Test_MockModuleTransaction(t *testing.T) {
 		})
 	}
 }
+
+func Test_MockModuleTransaction(t *testing.T) {
+	testMockModuleTransaction(t, NewModuleTransaction(nil))
+}
+
+func Test_MockModuleTransactionParallelConv(t *testing.T) {
+	testMockModuleTransaction(t, NewModuleTransactionParallelConv(nil))
+}