PowerShell · TravisEz13 · Mar 26, 2018 · Mar 21, 2018 · Mar 21, 2018 · Mar 21, 2018
@@ -1104,6 +1104,8 @@ v6.0.
 #region test/tools/WebListener/README.md Overrides
  - test/tools/WebListener/README.md
 Auth
+NoResume
 NTLM
+NumberBytes
 ResponseHeaders
 #endregion
@@ -354,6 +354,12 @@ public virtual string CustomMethod
         [Parameter]
         public virtual SwitchParameter PassThru { get; set; }
 
+        /// <summary>
+        /// Resumes downloading a partial or incomplete file. OutFile is required.
+        /// </summary>
+        [Parameter]
+        public virtual SwitchParameter Resume { get; set; }
+
         #endregion
 
         #endregion Virtual Properties
@@ -512,7 +518,15 @@ internal virtual void ValidateParameters()
             if (PassThru && (OutFile == null))
             {
                 ErrorRecord error = GetValidationError(WebCmdletStrings.OutFileMissing,
-                                                       "WebCmdletOutFileMissingException");
+                                                       "WebCmdletOutFileMissingException", nameof(PassThru));
+                ThrowTerminatingError(error);
+            }
+
+            // Resume requires OutFile.
+            if (Resume.IsPresent && OutFile == null)
+            {
+                ErrorRecord error = GetValidationError(WebCmdletStrings.OutFileMissing,
+                                                       "WebCmdletOutFileMissingException", nameof(Resume));
                 ThrowTerminatingError(error);
             }
         }
@@ -637,6 +651,14 @@ internal bool ShouldWriteToPipeline
             get { return (!ShouldSaveToOutFile || PassThru); }
         }
 
+        /// <summary>
+        /// Determines whether writing to a file should Resume and append rather than overwrite.
+        /// </summary>
+        internal bool ShouldResume
+        {
+            get { return (Resume.IsPresent && _resumeSuccess); }
+        }
+
         #endregion Helper Properties
 
         #region Helper Methods
@@ -860,6 +882,16 @@ public abstract partial class WebRequestPSCmdlet : PSCmdlet
         /// </summary>
         internal int _maximumFollowRelLink = Int32.MaxValue;
 
+        /// <summary>
+        /// The remote endpoint returned a 206 status code indicating successful resume.
+        /// </summary>
+        private bool _resumeSuccess = false;
+
+        /// <summary>
+        /// The current size of the local file being resumed.
+        /// </summary>
+        private long _resumeFileSize = 0;
+
         private HttpMethod GetHttpMethod(WebRequestMethod method)
         {
             switch (Method)
@@ -1062,6 +1094,22 @@ internal virtual HttpRequestMessage GetRequest(Uri uri, bool stripAuthorization)
                 }
             }
 
+            // If the file to resume downloading exists, create the Range request header using the file size.
+            // If not, create a Range to request the entire file.
+            if (Resume.IsPresent)
+            {
+                var fileInfo = new FileInfo(QualifiedOutFile);
+                if (fileInfo.Exists)
+                {
+                    request.Headers.Range = new RangeHeaderValue(fileInfo.Length, null);
+                    _resumeFileSize = fileInfo.Length;
+                }
+                else
+                {
+                    request.Headers.Range = new RangeHeaderValue(0, null);
+                }
+            }
+
             // Some web sites (e.g. Twitter) will return exception on POST when Expect100 is sent
             // Default behavior is continue to send body content anyway after a short period
             // Here it send the two part as a whole.
@@ -1233,6 +1281,9 @@ internal virtual HttpResponseMessage GetResponse(HttpClient client, HttpRequestM
             if (client == null) { throw new ArgumentNullException("client"); }
             if (request == null) { throw new ArgumentNullException("request"); }
 
+            // Track the current URI being used by various requests and re-requests.
+            var currentUri = request.RequestUri;
+
             _cancelToken = new CancellationTokenSource();
             HttpResponseMessage response = client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, _cancelToken.Token).GetAwaiter().GetResult();
 
@@ -1256,14 +1307,51 @@ internal virtual HttpResponseMessage GetResponse(HttpClient client, HttpRequestM
                 }
 
                 // recreate the HttpClient with redirection enabled since the first call suppressed redirection
+                currentUri = new Uri(request.RequestUri, response.Headers.Location);
                 using (client = GetHttpClient(false))
-                using (HttpRequestMessage redirectRequest = GetRequest(new Uri(request.RequestUri, response.Headers.Location), stripAuthorization:true))
+                using (HttpRequestMessage redirectRequest = GetRequest(currentUri, stripAuthorization:true))
                 {
                     FillRequestStream(redirectRequest);
                     _cancelToken = new CancellationTokenSource();
                     response = client.SendAsync(redirectRequest, HttpCompletionOption.ResponseHeadersRead, _cancelToken.Token).GetAwaiter().GetResult();
                 }
             }
+
+            // Request again without the Range header because the server indicated the range was not satisfiable.
+            // This happens when the local file is larger than the remote file.
+            // If the size of the remote file is the same as the local file, there is nothing to resume.
+            if (Resume.IsPresent && 
+                response.StatusCode == HttpStatusCode.RequestedRangeNotSatisfiable && 
+                (response.Content.Headers.ContentRange.HasLength &&
+                response.Content.Headers.ContentRange.Length != _resumeFileSize))
+            {
+                _cancelToken.Cancel();
+
+                WriteVerbose(WebCmdletStrings.WebMethodResumeFailedVerboseMsg);
+
+                // Disable the Resume switch so the subsequent calls to GetResponse() and FillRequestStream()
+                // are treated as a standard -OutFile request. This also disables appending local file.
+                Resume = new SwitchParameter(false);
+
+                using (HttpRequestMessage requestWithoutRange = GetRequest(currentUri, stripAuthorization:false))
+                {
+                    FillRequestStream(requestWithoutRange);
+                    long requestContentLength = 0;
+                    if (requestWithoutRange.Content != null)
+                        requestContentLength = requestWithoutRange.Content.Headers.ContentLength.Value;
+
+                    string reqVerboseMsg = String.Format(CultureInfo.CurrentCulture,
+                        WebCmdletStrings.WebMethodInvocationVerboseMsg,
+                        requestWithoutRange.Method,
+                        requestWithoutRange.RequestUri,
+                        requestContentLength);
+                    WriteVerbose(reqVerboseMsg);
+
+                    return GetResponse(client, requestWithoutRange, stripAuthorization);
+                }
+            }
+
+            _resumeSuccess = response.StatusCode == HttpStatusCode.PartialContent;
             return response;
         }
 
@@ -1336,7 +1424,22 @@ protected override void ProcessRecord()
                                     contentType);
                                 WriteVerbose(respVerboseMsg);
 
-                                if (!response.IsSuccessStatusCode)
+                                bool _isSuccess = response.IsSuccessStatusCode;
+
+                                // Check if the Resume range was not satisfiable because the file already completed downloading.
+                                // This happens when the local file is the same size as the remote file.
+                                if (Resume.IsPresent &&
+                                    response.StatusCode == HttpStatusCode.RequestedRangeNotSatisfiable && 
+                                    response.Content.Headers.ContentRange.HasLength && 
+                                    response.Content.Headers.ContentRange.Length == _resumeFileSize)
+                                {
+                                    _isSuccess = true;
+                                    WriteVerbose(String.Format(CultureInfo.CurrentCulture, WebCmdletStrings.OutFileWritingSkipped, OutFile));
+                                    // Disable writing to the OutFile.
+                                    OutFile = null;
+                                }
+
+                                if (!_isSuccess)
                                 {
                                     string message = String.Format(CultureInfo.CurrentCulture, WebCmdletStrings.ResponseStatusCodeFailure,
                                         (int)response.StatusCode, response.ReasonPhrase);

@@ -342,9 +342,20 @@ internal static void WriteToStream(byte[] input, Stream output)
         /// <param name="cmdlet"></param>
         internal static void SaveStreamToFile(Stream stream, string filePath, PSCmdlet cmdlet)
         {
-            using (FileStream output = File.Create(filePath))
+            // If the web cmdlet should resume, append the file instead of overwriting.
+            if(cmdlet is WebRequestPSCmdlet webCmdlet && webCmdlet.ShouldResume)
             {
-                WriteToStream(stream, output, cmdlet);
+                using (FileStream output = new FileStream(filePath, FileMode.Append, FileAccess.Write, FileShare.Read))
+                {
+                    WriteToStream(stream, output, cmdlet);
+                }
+            }
+            else
+            {
+                using (FileStream output = File.Create(filePath))
+                {
+                    WriteToStream(stream, output, cmdlet);
+                }
             }
         }
 

@@ -187,7 +187,10 @@
     <value>Path '{0}' is not a file system path. Please specify the path to a file in the file system.</value>
   </data>
   <data name="OutFileMissing" xml:space="preserve">
-    <value>The cmdlet cannot run because the following parameter is missing: OutFile. Provide a valid OutFile parameter value when using the PassThru parameter, then retry.</value>
+    <value>The cmdlet cannot run because the following parameter is missing: OutFile. Provide a valid OutFile parameter value when using the {0} parameter, then retry.</value>
+  </data>
+  <data name="OutFileWritingSkipped" xml:space="preserve">
+    <value>The file will not be re-downloaded because the remote file is the same size as the OutFile: {0}</value>
   </data>
   <data name="ProxyCredentialConflict" xml:space="preserve">
     <value>The cmdlet cannot run because the following conflicting parameters are specified: ProxyCredential and ProxyUseDefaultCredentials. Specify either ProxyCredential or ProxyUseDefaultCredentials, then retry.</value>
@@ -249,6 +252,9 @@
   <data name="WebMethodInvocationVerboseMsg" xml:space="preserve">
     <value>{0} {1} with {2}-byte payload</value>
   </data>
+  <data name="WebMethodResumeFailedVerboseMsg" xml:space="preserve">
+    <value>The remote server indicated it could not resume downloading. The local file will be overwritten.</value>
+  </data>
   <data name="WebResponseVerboseMsg" xml:space="preserve">
     <value>received {0}-byte response of content type {1}</value>
   </data>