IOptionsMonitor<T> does not work with rooted file paths, but PhysicalFileProvider itself does

[Measurity]

Issue

It's possible to have any file be used with PhysicalFileProvider, but they can't be watched for changes if the path isn't relative.

No errors are thrown with a rooted path, simply no watcher is listening for changes. Which means IOptionsMonitor<T> does not work as expected.

Code in question

runtime/src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFileProvider.cs

Lines 362 to 363 in 07f3e33

	// Relative paths starting with leading slashes are okay
	filter = filter.TrimStart(_pathSeparators);

runtime/src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFilesWatcher.cs

Lines 134 to 138 in 07f3e33

	// Absolute paths and paths traversing above root not permitted.
	if (Path.IsPathRooted(filter) || PathUtils.PathNavigatesAboveRoot(filter))
	{
	return NullChangeToken.Singleton;
	}

Workaround

The following extension method is what I use to add a custom config file, while having change detection working. See first comment for workaround.

public static IConfigurationBuilder AddMyConfigFile<TOptions>(this IConfigurationBuilder configurationBuilder, string filePath, string configSectionPath = "", bool optional = true) where TOptions : class, new()
{
    ArgumentException.ThrowIfNullOrWhiteSpace(filePath);
    string dirPath = Path.GetDirectoryName(filePath);
    Directory.CreateDirectory(dirPath ?? throw new ArgumentException(nameof(filePath)));
    if (!File.Exists(filePath))
    {
        MyConfig.CreateFile<TOptions>(filePath);
    }

    // Link the config to a relative path within the working directory so that IOptionsMonitor<T> works. See https://github.com/dotnet/runtime/issues/114833
    try
    {
        FileInfo configFile = new(Path.GetFileName(filePath));
        if (configFile.Exists && configFile.LinkTarget != null)
        {
            configFile.Delete();
        }
        configFile.CreateAsSymbolicLink(filePath);
        // Fix targets to point to symbolic link instead.
        dirPath = AppContext.BaseDirectory;
        filePath = configFile.Name; // Now a relative path.
    }
    catch (IOException)
    {
        // ignored - config change detection isn't critical for server.
    }

    return configurationBuilder.Add(new MyConfigurationSource(filePath, configSectionPath, optional, new PhysicalFileProvider(dirPath)
    {
        UsePollingFileWatcher = true,
        UseActivePolling = true
    }));
}

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