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.

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
-// the module side.
+// NewModuleTransactionParallelConv 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}
+}
+
+// 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 {