C#: Add DependabotProxy class

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/DependabotProxy.cs

@@ -0,0 +1,53 @@
+using System;
+using System.IO;
+using Semmle.Util;
+
+namespace Semmle.Extraction.CSharp.DependencyFetching
+{
+    internal class DependabotProxy
+    {
+        private readonly string? host;
+        private readonly string? port;
+        private readonly FileInfo? certFile;
+
+        /// <summary>
+        /// The full address of the Dependabot proxy, if available.
+        /// </summary>
+        internal readonly string? Address;
+
+        /// <summary>
+        /// Gets a value indicating whether a Dependabot proxy is configured.
+        /// </summary>
+        internal bool IsConfigured => !string.IsNullOrEmpty(this.Address);
+
+        internal DependabotProxy(TemporaryDirectory tempWorkingDirectory)
+        {
+            // Obtain and store the address of the Dependabot proxy, if available.
+            this.host = Environment.GetEnvironmentVariable(EnvironmentVariableNames.ProxyHost);
+            this.port = Environment.GetEnvironmentVariable(EnvironmentVariableNames.ProxyPort);
+
+            if (string.IsNullOrWhiteSpace(host) || string.IsNullOrWhiteSpace(port))
+            {
+                return;
+            }
+
+            this.Address = $"http://{this.host}:{this.port}";
+
+            // Obtain and store the proxy's certificate, if available.
+            var cert = Environment.GetEnvironmentVariable(EnvironmentVariableNames.ProxyCertificate);
+
+            if (string.IsNullOrWhiteSpace(cert))
+            {
+                return;
+            }
+
+            var certDirPath = new DirectoryInfo(Path.Join(tempWorkingDirectory.DirInfo.FullName, ".dependabot-proxy"));
+            Directory.CreateDirectory(certDirPath.FullName);
+
+            this.certFile = new FileInfo(Path.Join(certDirPath.FullName, "proxy.crt"));
+
+            using var writer = this.certFile.CreateText();
+            writer.Write(cert);
+        }
+    }
+}