private async Task <HttpFetchResult> TryFetchWithHttp2Client(Uri url, params string[] acceptHeaders)
{
try
{
using var http2Request = new HttpRequestMessage(HttpMethod.Get, url)
{
Version = new Version(2, 0)
};
http2Request.AddAcceptHeaders(acceptHeaders);
using var result = await http.SendAsync(http2Request);
result.EnsureSuccessStatusCode();
var contentString = await result.Content.ReadAsStringAsync();
logger.LogInformation("Successfully fetched {url} via HTTP/2 fallback", url);
return(new HttpFetchResult
{
Result = contentString
});
}
catch (Exception http2Error)
{
logger.LogWarning(http2Error, "Unable to fetch {url} using HTTP/2 fallback.", url);
return(new HttpFetchResult
{
Error = http2Error
});
}
}
/// <summary>
/// Sets the user agent to exclude CURL, which some sites (e.g. Facebook) require for programmatic fetching to work.
/// </summary>
/// <param name="url">The URL to fetch</param>
/// <param name="acceptHeaders">Additional headers to set on the fetch request.</param>
/// <returns>A fetch result containing the fetch response.</returns>
private async Task <HttpFetchResult> TryFetchWithCurlUserAgent(Uri url, string[] acceptHeaders)
{
try
{
// Append CURL to the user agent.
http.SetUserAgent(userAgent + " curl/7.64.1");
using var httpRequest = new HttpRequestMessage(HttpMethod.Get, url);
httpRequest.AddAcceptHeaders(acceptHeaders);
var httpResponse = await http.SendAsync(httpRequest);
httpResponse.EnsureSuccessStatusCode();
var content = await httpResponse.Content.ReadAsStringAsync();
return(new HttpFetchResult
{
Result = content
});
}
catch (Exception fetchError)
{
logger.LogWarning(fetchError, "Unable to fetch {url} using CURL user agent fallback.", url);
return(new HttpFetchResult
{
Error = fetchError
});
}
finally
{
// Reset the user agent back to the default user agent.
http.SetUserAgent(userAgent);
}
}
/// <summary>
/// Attempts to fetch a resource at the specified URL.
/// If the fetch fails, it will attempt to fetch using HTTP/2.
/// Failures due to encoding errors will also attempt fetch using UTF-8 encoding as a fallback.
/// If all fetches fail, the result will contain the exception.
/// </summary>
/// <param name="url"></param>
/// <param name="acceptHeaders"></param>
/// <returns></returns>
private async Task <HttpFetchResult> TryFetch(Uri url, params string[] acceptHeaders)
{
try
{
using var httpRequest = new HttpRequestMessage(HttpMethod.Get, url);
httpRequest.AddAcceptHeaders(acceptHeaders);
var httpResponse = await http.SendAsync(httpRequest);
// If it's a 403, we have special handling for this.
if (httpResponse.StatusCode == System.Net.HttpStatusCode.Forbidden)
{
var errorMessage = !string.IsNullOrWhiteSpace(httpResponse.ReasonPhrase) ?
httpResponse.ReasonPhrase :
"Web server's response was 403 Forbidden.";
throw new HttpForbiddenException(errorMessage);
}
httpResponse.EnsureSuccessStatusCode();
var content = await httpResponse.Content.ReadAsStringAsync();
return(new HttpFetchResult
{
Result = content
});
}
catch (InvalidOperationException invalidOpError) when(invalidOpError.Message.Contains("The character set provided in ContentType is invalid."))
{
// Invalid encoding? Sometimes webpages have incorrectly set their charset / content type.
// See if we can just parse the thing using UTF-8.
logger.LogWarning(invalidOpError, "Unable to parse using HTTP client due to invalid ContentType. Attempting to parse using UTF-8.");
return(await TryFetchWithForcedUtf8(url, acceptHeaders));
}
catch (HttpForbiddenException forbiddenError)
{
logger.LogWarning(forbiddenError, "Received 403 Forbidden when fetching {url}. Attempting fetch with CURL user agent fallback.");
return(await TryFetchWithCurlUserAgent(url, acceptHeaders)); // TODO: should we always try this when we encounter exception, not just forbidden exception?
}
catch (Exception httpException)
{
logger.LogWarning(httpException, "Failed to fetch {url}. Falling back to HTTP/2 fetch.", url);
return(await TryFetchWithHttp2Client(url, acceptHeaders));
}
}