Asynchronous Callback Handling
When a client interacts with the files and directories beneath the provider's virtualization root, these interactions typically result in the provider's callbacks being invoked. ProjFS invokes provider callbacks by sending a message from kernel mode to the ProjFS user-mode library, where a worker thread receives the message and calls the appropriate callback. Once the callback has returned, the worker thread waits for another message to arrive from kernel mode. If all the worker threads are busy executing provider callback code, any further client I/O that triggers a callback blocks until a worker thread becomes available to receive the message and invoke the relevant callback. When a provider starts up, it can specify the number of worker threads it wants ProjFS to create to service callbacks via the options parameter of PrjStartVirtualizing. A provider can improve the efficiency of the message-receiving worker threads by servicing its callbacks asynchronously.
If the provider does not specify the options parameter to PrjStartVirtualizing, or if it specifies 0 for the ConcurrentThreadCount member of the options parameter, ProjFS will use the number of logical processors in the system for the value of ConcurrentThreadCount.
If the provider does not specify the options parameter to PrjStartVirtualizing, or if it specifies 0 for the PoolThreadCount member of the options parameter, ProjFS will use twice the value of ConcurrentThreadCount for the value of PoolThreadCount.
A provider services callbacks asynchronously by returning HRESULT_FROM_WIN32(ERROR_IO_PENDING) from its callbacks and later completing them using PrjCompleteCommand. A provider that processes callbacks asynchronously should also support callback cancellation by implementing the PRJ_CANCEL_COMMAND_CB callback.
When ProjFS calls a provider's callback, it identifies the specific invocation of the callback using the CommandId member of the callback's callbackData parameter. If the provider decides to process that callback asynchronously, it must store the value of the CommandId member and return HRESULT_FROM_WIN32(ERROR_IO_PENDING) from the callback. Once the provider has finished processing the callback, it calls PrjCompleteCommand, passing the stored identifier in the commandId parameter. This tells ProjFS which callback invocation has been completed.
A provider that implements the PRJ_CANCEL_COMMAND_CB callback must keep track of which callbacks it has not yet completed. If the provider receives this callback, it indicates that the I/O that caused one of those callbacks to be invoked was canceled, either explicitly or because the thread it was issued on terminated. The provider should cancel processing the callback invocation identified by CommandId as soon as possible.
Although ProjFS will invoke PRJ_CANCEL_COMMAND_CB for a given CommandId only after the callback to be canceled is invoked, the cancellation and original invocation may run concurrently in a multithreaded provider. The provider must be able to gracefully handle this situation.
The following sample is a version of the sample given for the Enumerating Files and Directories topic, modified to illustrate asynchronous callback handling.
typedef struct MY_ENUM_ENTRY MY_ENUM_ENTRY;
typedef struct MY_ENUM_ENTRY {
MY_ENUM_ENTRY* Next;
PWSTR Name;
BOOLEAN IsDirectory;
INT64 FileSize;
} MY_ENUM_ENTRY;
typedef struct MY_ENUM_SESSION {
GUID EnumerationId;
PWSTR SearchExpression;
USHORT SearchExpressionMaxLen;
BOOLEAN SearchExpressionCaptured;
MY_ENUM_ENTRY* EnumHead;
MY_ENUM_ENTRY* LastEnumEntry;
BOOLEAN EnumCompleted;
} MY_ENUM_SESSION;
typedef struct MY_ENUM_PARAMS {
GUID EnumerationId;
PRJ_DIR_ENTRY_BUFFER_HANDLE DirEntryBufferHandle;
PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT NamespaceVirtualizationContext;
INT32 CommandId;
PRJ_CALLBACK_DATA_FLAGS Flags;
WCHAR SearchExpression[1];
} MY_ENUM_PARAMS;
// Prototype for the enumeration worker routine. In this example this is a
// ThreadProc. See https://msdn.microsoft.com/library/windows/desktop/ms686736.aspx
DWORD WINAPI MyGetEnumCallbackWorker(_In_ LPVOID parameter);
#define ROLLBACK_NONE 0
#define ROLLBACK_PARAM_ALLOC 1
#define ROLLBACK_TRACK_ENUM 2
HRESULT
MyGetEnumCallback(
_In_ const PRJ_CALLBACK_DATA* callbackData,
_In_ const GUID* enumerationId,
_In_opt_z_ PCWSTR searchExpression,
_In_ PRJ_DIR_ENTRY_BUFFER_HANDLE dirEntryBufferHandle
)
{
UINT rollback = ROLLBACK_NONE;
// This example creates a thread to run the enumeration in. The provider
// can access callbackData only while the callback is running, so we copy
// out the fields that the worker thread will need.
MY_ENUM_PARAMS* enumParams = NULL;
size_t bufSize = sizeof(MY_ENUM_PARAMS);
// Allocate enough space for the parameter buffer and the search expression
// string plus a terminating NULL character.
if (searchExpression != NULL)
{
bufSize += (wcslen(searchExpression) + 1) * sizeof(WCHAR);
}
else
{
// We'll store "*" as the search expression in this case.
bufSize += 2 * sizeof(WCHAR);
}
enumParams = (MY_ENUM_PARAMS*) calloc(1, bufSize);
if (enumParams == NULL)
{
hr = E_OUTOFMEMORY;
goto RollbackOnError;
}
// enumParams allocated.
rollback = ROLLBACK_PARAM_ALLOC;
// Copy the search expression into our parameter buffer.
if (searchExpression != NULL)
{
wcsncpy_s(enumParams->SearchExpression,
wcslen(searchExpression),
searchExpression,
wcslen(searchExpression));
}
else
{
wcsncpy_s(enumParams->SearchExpression, 1, "*", 1);
}
// Copy the other parameters into our parameter buffer.
enumParams->EnumerationId = enumerationId;
enumParams->DirEntryBufferHandle = dirEntryBufferHandle;
enumParams->NamespaceVirtualizationContext = callbackData->NamespaceVirtualizationContext
enumParams->CommandId = callbackData->CommandId;
enumParams->Flags = callbackData->Flags;
// MyTrackEnumForCancellation is a routine the provider might implement to
// track active enumeration callbacks in case they are cancelled. It stores
// the CommandId for the callback in some structure.
hr = MyTrackEnumForCancellation(callbackData->CommandId);
if (FAILED(hr))
{
goto RollbackOnError;
}
// Enum callback tracked.
rollback = ROLLBACK_TRACK_ENUM;
// Create the worker thread.
HANDLE workerThread = NULL;
workerThread = CreateThread(NULL,
0,
MyGetEnumCallbackWorker,
enumParams,
0
NULL);
// We'll treat failure to create the thread as a low-resources error.
if (workerThread == NULL)
{
hr = E_OUTOFMEMORY;
goto RollbackOnError;
}
// Return pending to signal that we'll complete the enumeration later.
return HRESULT_FROM_WIN32(ERROR_IO_PENDING);
RollbackOnError:
// Clean up manually. Note the fall-through structure of the switch, and that
// we clean up in reverse order.
switch(rollback)
{
case ROLLBACK_TRACK_ENUM:
// MyUntrackEnumForCancellation removes the CommandId for the callback
// from the structure that MyTrackEnumForCancellation put it in.
MyUntrackEnumForCancellation(callbackData->CommandId);
case ROLLBACK_PARAM_ALLOC:
free(enumParams);
default:
break;
}
return hr;
}
DWORD
WINAPI
MyGetEnumCallbackWorker(
_In_ LPVOID parameter
)
{
HRESULT hr = S_OK;
MY_ENUM_PARAMS* enumParams = (MY_ENUM_PARAMS*) parameter;
// MyCheckEnumForCancellation is a routine the provider might implement to
// check whether a pended enumeration callback has been cancelled. Even
// though the enumeration has been cancelled, ProjFS will still call the
// provider's end-enumeration callback. This allows general cleanup, such
// as tearing down the enumeration session, to always happen in that callback.
if (MyCheckEnumForCancellation(enumParams->CommandId))
{
// The operation has been cancelled. Clean up and get out.
free(enumParams);
return 0;
}
// MyGetEnumSession is a routine the provider might implement to find
// information about the enumeration session that it first stored
// when processing its PRJ_START_DIRECTORY_ENUMERATION_CB callback.
//
// In this example the PRJ_START_DIRECTORY_ENUMERATION_CB callback has
// already retrieved a list of the items in the backing store located at
// callbackData->FilePathName, sorted them using PrjFileNameCompare()
// to determine collation order, and stored the list in session->EnumHead.
MY_ENUM_SESSION* session = NULL;
session = MyGetEnumSession(enumParams->EnumerationId);
if (session == NULL)
{
hr = HRESULT_FROM_WIN32(ERROR_INVALID_PARAMETER);
goto CompleteCallback;
}
if (!session->SearchExpressionCaptured ||
(enumParams->Flags & PRJ_CB_DATA_FLAG_ENUM_RESTART_SCAN))
{
if (wcsncpy_s(session->SearchExpression,
session->SearchExpressionMaxLen,
enumParams->SearchExpression,
wcslen(enumParams->SearchExpression)))
{
// Failed to copy the search expression; perhaps the provider
// could try reallocating session->SearchExpression.
}
session->SearchExpressionCaptured = TRUE;
}
MY_ENUM_ENTRY* enumHead = NULL;
// We have to start the enumeration from the beginning if we aren't
// continuing an existing session or if the caller is requesting a restart.
if (((session->LastEnumEntry == NULL) &&
!session->EnumCompleted) ||
(enumParams->Flags & PRJ_CB_DATA_FLAG_ENUM_RESTART_SCAN))
{
// Ensure that if the caller is requesting a restart we reset our
// bookmark of how far we got in the enumeration.
session->LastEnumEntry = NULL;
// In case we're restarting ensure we don't think we're done.
session->EnumCompleted = FALSE;
// We need to start considering items from the beginning of the list
// retrieved from the backing store.
enumHead = session->EnumHead;
}
else
{
// We are resuming an existing enumeration session. Note that
// session->LastEnumEntry may be NULL. That is okay; it means
// we got all the entries the last time this callback was invoked.
// Returning S_OK without adding any new entries to the
// enumParams->DirEntryBufferHandle buffer signals ProjFS that the
// enumeration has returned everything it can.
enumHead = session->LastEnumEntry;
}
if (enumHead == NULL)
{
// There are no items to return. Remember that we've returned everything
// we can.
session->EnumCompleted = TRUE;
}
else
{
MY_ENUM_ENTRY* thisEntry = enumHead;
while (thisEntry != NULL)
{
// We'll insert the entry into the return buffer if it matches
// the search expression captured for this enumeration session.
if (PrjFileNameMatch(thisEntry->Name, session->SearchExpression))
{
PRJ_FILE_BASIC_INFO fileBasicInfo = {};
fileBasicInfo.IsDirectory = thisEntry->IsDirectory;
fileBasicInfo.FileSize = thisEntry->FileSize;
// Format the entry for return to ProjFS.
if (S_OK != PrjFillDirEntryBuffer(thisEntry->Name,
&fileBasicInfo,
enumParams->DirEntryBufferHandle))
{
// We couldn't add this entry to the buffer; remember where we left
// off for the next time we're called for this enumeration session.
session->LastEnumEntry = thisEntry;
hr = S_OK;
goto CompleteCallback;
}
}
thisEntry = thisEntry->Next;
}
// We reached the end of the list of entries; remember that we've returned
// everything we can.
session->EnumCompleted = TRUE;
}
CompleteCallback:
// Signal ProjFS that we've completed this callback. Since this is an enumeration
// we use the extendedParameters parameter to PrjCompleteCommand.
PRJ_COMPLETE_COMMAND_EXTENDED_PARAMETERS extendedParams = {};
extendedParams.CommandType = PRJ_COMPLETE_COMMAND_TYPE_ENUMERATION;
extendedParams.enumeration.DirEntryBufferHandle = enumParams->DirEntryBufferHandle;
PrjCompleteCommand(enumParams->NamespaceVirtualizationContext,
enumParams->CommandId,
hr,
&extendedParams);
MyUntrackEnumForCancellation(enumParams->CommandId);
free(enumParams);
// ThreadProc returns 0 on successful completion.
return 0;
}
And here is a brief example PRJ_CANCEL_COMMAND_CB that works with the preceding sample code.
void
MyCancelCommandCallback(
_In_ const PRJ_CALLBACK_DATA* callbackData
)
{
// MyMarkEnumForCancellation is a routine the provider might implement to
// mark a pended enumeration callback as cancelled. If the given CommandId
// is not found, it is not an error. It may not yet have been tracked or
// it may already have completed and been untracked.
MyMarkEnumForCancellation(callbackData->CommandId);
}