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// Copyright 2020, Google LLC
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using System;
using System.Reflection;
using System.Threading.Tasks;
namespace Google.Cloud.Functions.Hosting
/// <summary>
/// The entry point for the hosting package. This is used automatically by the entry point generated by MS Build
/// targets within the Google.Cloud.Functions.Hosting NuGet package.
/// </summary>
public static class EntryPoint
/// <summary>
/// The environment variable used to detect the function target name, when not otherwise provided.
/// </summary>
public const string FunctionTargetEnvironmentVariable = "FUNCTION_TARGET";
/// <summary>
/// The environment variable used to detect the port to listen on.
/// </summary>
public const string PortEnvironmentVariable = "PORT";
/// <summary>
/// Starts a web server to serve the function in the specified assembly. This method is called
/// automatically be the generated entry point.
/// </summary>
/// <param name="functionAssembly">The assembly containing the function to execute.</param>
/// <param name="args">Arguments to parse </param>
/// <returns>A task representing the asynchronous operation.
/// The result of the task is an exit code for the process, which is 0 for success or non-zero
/// for any failures.
/// </returns>
public static async Task<int> StartAsync(Assembly functionAssembly, string[] args)
// Clear out the ASPNETCORE_URLS environment variable in order to avoid a warning when we start the server.
// An alternative would be to *use* the environment variable, but as it's populated (with a non-ideal value) by
// default, I suspect that would be tricky.
Environment.SetEnvironmentVariable("ASPNETCORE_URLS", null);
// Guess the function type by creating a configuration with just the environment variables and command line
// arguments in it. We do this so we can work out the function startup classes to use - and then validate that
// when we've used those functions startups and worked out the actual function target, the set of
// function startups is still valid. Note that it's possible for this to return null, if an assembly-specified
// function startup determines the actual function target. That's valid, so long as the function target doesn't
// later require any specific startups. (It would be very, very rare for a startup to affect which function is
// used, but I can imagine some scenarios where it's useful.)
var expectedFunctionTarget = GuessFunctionTarget();
// TODO: Catch exceptions and return 1, or just let the exception propagate? It probably
// doesn't matter much. Potentially catch exceptions during configuration, but let any
// during web server execution propagate.
var host = Host.CreateDefaultBuilder()
.ConfigureWebHostDefaults(webHostBuilder => webHostBuilder
.ConfigureAppConfiguration(builder => builder.AddFunctionsEnvironment().AddFunctionsCommandLine(args))
.ConfigureLogging((context, logging) => logging.ClearProviders().AddFunctionsConsoleLogging(context))
.ConfigureServices((context, services) => services.AddFunctionTarget(context, functionAssembly))
.UseFunctionsStartups(functionAssembly, expectedFunctionTarget)
.Configure((context, app) => app.UseFunctionsFramework(context, validateStartups: true)))
await host.RunAsync();
return 0;
Type? GuessFunctionTarget()
var configuration = new ConfigurationBuilder()
return HostingInternals.TryGetFunctionTarget(configuration, functionAssembly);