StaticFiles extension serving truncated file for symbolic link

[colindembovsky]

(Moving from this issue) I have a SPA app that uses an config.json file with the following contents:

{
   "api": {
     "baseUri": "http://192.168.1.27:30081/api"
   }
}

In the Startup.cs file I configure StaticFiles:

    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
      ...

      app.UseDefaultFiles();
      app.UseStaticFiles();
    }

I'm deploying the app in a docker container (FROM microsoft/aspnetcore:1.1) using Kubernetes. In the Kubernetes Pod I map a configMap volume to override the config file. Kubernetes creates a symlink in the contianer and the file contents are correct within the container:

host ~> kubectl exec -it $podname -c frontend /bin/bash
container:/app# ls -l wwwroot/config/config.json
lrwxrwxrwx 1 root root 18 Jun 13 15:30 wwwroot/config/config.json -> ..data/config.json
container:/app# cat wwwroot/config/config.json
{
    "api": {
        "baseUri": "http://kubernetes-value:30081/api"
    }
}

However, doing a curl only returns 18 bytes:

host ~> curl http://192.168.1.160:30080/config/config.json
{
    "api": {
host ~>

I think that the StaticFile is seeing the file length as 18 (the length for the symlink, which is the length of the path to the physical file: in this case, ../data/config.json which is 18 chars) and only serving that length even though the file proper is longer.

I don't think this is expected behavior - but perhaps there is some other setting that I should use for StaticFiles?

[Tratcher]

I doubt StaticFiles is doing any special here. I'd bet you could repro the issue using IFileProvider directly. See IHostingEnvironment.WebRootFileProvider and see if you can read your file as expected.

[colindembovsky]

Thanks @Tratcher - I'm busy digging in now. The FileProvider is providing the length field and StaticFiles is returning 0 - length bytes in the response. I suspect that the length property is being set incorrectly for symlinks. Tracking down the point in the clr where the length is read from the OS to see if there's a workaround.

[colindembovsky]

Yep @Tratcher , I've confirmed it. If you do

var fi = new FileInfo("path/to/symLink");

then fi.Length is the length of the symlink (as described above), not the lenght of the file! I think this is a bug on the FileInfo object for Unix.

[BrennanConroy]

Here is the change that made unix FileInfo not follow symlinks and a comment about why https://github.com/dotnet/corefx/pull/5020#r47822345

TLDR; FileInfo behavior on Unix matches Windows behavior for symlinks.

[colindembovsky]

Thanks @brennanconroy. So is there an IFileProvider that uses Stat for symlinks so that I get the expected length? I could roll my own if there isn't one, but was hoping you might know of one before I start hacking ☺️!

[colindembovsky]

Turns out it was pretty easy to write my own IFileProvider to work around the symlink length issue.

Once that was in place, I just changed Startup.cs:

      app.UseStaticFiles(new StaticFileOptions()
      {
          FileProvider = new SymlinkFileProvider(env.WebRootPath)
      });

Now all is well!