Intermittent 404 on GetFolderByServerRelativeUrl followed by 400 “already exists” when using EnsureFolderAsync

[aramB]

We’re intermittently seeing 404 Not Found responses when EnsureFolderAsync internally calls GetFolderByServerRelativeUrl. Immediately after, the operation continues and fails with a 400 stating the folder already exists. This behavior is not consistent and is difficult to reliably reproduce.

Observed sequence (from logs):

1- GetFolderByServerRelativeUrl → 404
2- Operation continues
3- Exception:

HttpResponseCode: 400
Code: Microsoft.SharePoint.SPException
Message: A file or folder with the name sites/C7/SE/C7/#3705-1/4379 already exists.
ClientRequestId: 90a5e1a1-801c-e000-887f-4fc30128cff1
SPClientServiceRequestDuration: 157
X-SharePointHealthScore: 3
X-SP-SERVERSTATE: ReadOnly=0

public async Task<string?> EnsureFolderAsync(string siteUrl, string listTitle, string folderPath)
{
    using PnPContext pnpContext = await _pnpContextFactory.CreateAsync(siteUrl);

    // Get a root folder via a list
    IList list = await GetList(pnpContext, listTitle);

    IFolder? folder = await list.RootFolder.EnsureFolderAsync(folderPath);

    return folder?.ServerRelativeUrl;
}

Is this a known issue with EnsureFolderAsync and/or GetFolderByServerRelativeUrl under certain conditions?

PnP Core SDK version: [1.14]
.NET runtime: [.NET 8]

NOTE! I didn't report this as an issue since there is no clear instruction for how to reproduce this. I appreciate any help we can get here.

Thanks in advance!

[martynnorman]

I'm also seeing this.

[Adam-it]

Sorry for the late reply.
This is some good find and IMO we should totally create an issue for it. From what I quickly checked the problem is most likely here

pnpcore/src/sdk/PnP.Core/Model/SharePoint/Core/Internal/Folder.cs

Lines 407 to 416 in be4b565

	try
	{
	currentFolder = await currentFolder.PnPContext.Web.GetFolderByServerRelativeUrlAsync(currentUrl, expressions).ConfigureAwait(false);
	}
	catch (SharePointRestServiceException)
	{
	currentFolder = await currentFolder.AddFolderAsync(folderName).ConfigureAwait(false);
	currentFolderWasCreated = true;
	}
	}

It seems like we have a race condition between the GET and the POST like

Thread A:  GET  /sites/C7/SE/.../4379  →  404 (doesn't exist yet)
Thread B:  GET  /sites/C7/SE/.../4379  →  404 (doesn't exist yet)
Thread A:  POST AddUsingPath(.../4379) →  200 OK, folder created
Thread B:  POST AddUsingPath(.../4379) →  400 "A file or folder with the name ... already exists"

I think the easiest way would be to add to AddFolderAsync a second try/catch that handles the 400 "already exists" case.

What do you think? May I kindly ask you to just rise up a follow up issue for that and lets try to get this fixed 👍

[aramB]

Thanks for looking into this! That explanation makes perfect sense.

I agree this looks like a classic race condition between the existence check and the folder creation call inside EnsureFolderAsync. The GET -> POST flow can definitely fail under concurrent requests exactly as you described.

Your suggestion also sounds reasonable to me: handle the specific “already exists” error from AddFolderAsync, then re-fetch the folder instead of failing the operation.

I’ll open a follow-up issue with the details and include the concurrency scenario + proposed handling approach so we can track a proper fix.

Or do you prefer a pull request instead? What ever that is easiest for you.

Thanks again 👍

[aramB]

Issue created: #1765