Added fallback logic for imap.strato.de if LIST doesn't return the INBOX

Fixes issue #1957

Author: jstedfast

MailKit/Net/Imap/ImapEngine.cs

@@ -3686,6 +3686,32 @@ public void QuerySpecialFolders (CancellationToken cancellationToken)
 
 			ProcessListInboxResponse (ic, command, list);
 
+			if (Inbox == null) {
+				if ((Capabilities & ImapCapabilities.ListExtended) != 0) {
+					// Note: This is a work-around for IMAP servers such as imap.strato.de which do not return a list of folders
+					// for the `LIST "" "INBOX" RETURN (SUBSCRIBED CHILDREN)` command. Disable the LIST-EXTENDED (and dependent)
+					// capabilities since they are clearly broken.
+					//
+					// See https://github.com/jstedfast/MailKit/issues/1957 for details.
+					Capabilities &= ~(ImapCapabilities.ListExtended | ImapCapabilities.ListStatus | ImapCapabilities.SpecialUse);
+
+					// Send a vanilla `LIST "" "INBOX"` command to get the INBOX folder.
+					ic = QueueListInboxCommand (cancellationToken, out command, out list);
+
+					Run (ic);
+
+					ProcessListInboxResponse (ic, command, list);
+				}
+
+				if (Inbox == null) {
+					// If we still don't have the INBOX folder, just create a placeholder for it.
+					char delim = PersonalNamespaces.Count > 0 ? PersonalNamespaces[0].DirectorySeparator : '/';
+					var inbox = CreateImapFolder ("INBOX", FolderAttributes.Inbox, delim);
+					CacheFolder (inbox);
+					Inbox = inbox;
+				}
+			}
+
 			if ((Capabilities & ImapCapabilities.SpecialUse) != 0) {
 				ic = QueueListSpecialUseCommand (command, list, cancellationToken);
 
@@ -3717,6 +3743,32 @@ public async Task QuerySpecialFoldersAsync (CancellationToken cancellationToken)
 
 			ProcessListInboxResponse (ic, command, list);
 
+			if (Inbox == null) {
+				if ((Capabilities & ImapCapabilities.ListExtended) != 0) {
+					// Note: This is a work-around for IMAP servers such as imap.strato.de which do not return a list of folders
+					// for the `LIST "" "INBOX" RETURN (SUBSCRIBED CHILDREN)` command. Disable the LIST-EXTENDED (and dependent)
+					// capabilities since they are clearly broken.
+					//
+					// See https://github.com/jstedfast/MailKit/issues/1957 for details.
+					Capabilities &= ~(ImapCapabilities.ListExtended | ImapCapabilities.ListStatus | ImapCapabilities.SpecialUse);
+
+					// Send a vanilla `LIST "" "INBOX"` command to get the INBOX folder.
+					ic = QueueListInboxCommand (cancellationToken, out command, out list);
+
+					await RunAsync (ic).ConfigureAwait (false);
+
+					ProcessListInboxResponse (ic, command, list);
+				}
+
+				if (Inbox == null) {
+					// If we still don't have the INBOX folder, just create a placeholder for it.
+					char delim = PersonalNamespaces.Count > 0 ? PersonalNamespaces[0].DirectorySeparator : '/';
+					var inbox = CreateImapFolder ("INBOX", FolderAttributes.Inbox, delim);
+					CacheFolder (inbox);
+					Inbox = inbox;
+				}
+			}
+
 			if ((Capabilities & ImapCapabilities.SpecialUse) != 0) {
 				ic = QueueListSpecialUseCommand (command, list, cancellationToken);
 

UnitTests/Net/Imap/ImapClientTests.cs

@@ -1987,6 +1987,8 @@ public void TestInvalidUntaggedBadResponse ()
 
 				Assert.That (alerts, Is.EqualTo (5), $"Unexpected number of alerts: {alerts}");
 
+				Assert.That (client.Inbox, Is.Not.Null, "Inbox");
+
 				client.Disconnect (false);
 			}
 		}
@@ -2021,6 +2023,8 @@ public async Task TestInvalidUntaggedBadResponseAsync ()
 
 				Assert.That (alerts, Is.EqualTo (5), $"Unexpected number of alerts: {alerts}");
 
+				Assert.That (client.Inbox, Is.Not.Null, "Inbox");
+
 				await client.DisconnectAsync (false);
 			}
 		}
@@ -7643,6 +7647,121 @@ public async Task TestNamespaceExtensionsAsync ()
 			}
 		}
 
+		static List<ImapReplayCommand> CreateListInboxFallbackAfterEmptyListExtendedCommands ()
+		{
+			return new List<ImapReplayCommand> {
+				new ImapReplayCommand ("", "strato.de.greeting.txt"),
+				new ImapReplayCommand ("A00000000 AUTHENTICATE PLAIN\r\n", ImapReplayCommandResponse.Plus),
+				new ImapReplayCommand ("A00000000", "AHVzZXJuYW1lAHBhc3N3b3Jk\r\n", "strato.de.authenticate.txt"),
+				new ImapReplayCommand ("A00000001 CAPABILITY\r\n", "strato.de.capability.txt"),
+				new ImapReplayCommand ("A00000002 NAMESPACE\r\n", "strato.de.namespace.txt"),
+				new ImapReplayCommand ("A00000003 LIST \"\" \"INBOX\" RETURN (SUBSCRIBED CHILDREN)\r\n", ImapReplayCommandResponse.OK),
+				new ImapReplayCommand ("A00000004 LIST \"\" \"INBOX\"\r\n", "strato.de.list-inbox.txt"),
+				new ImapReplayCommand ("A00000005 XLIST \"\" \"*\"\r\n", "strato.de.xlist.txt"),
+				new ImapReplayCommand ("A00000006 LOGOUT\r\n", ImapReplayCommandResponse.OK)
+			};
+		}
+
+		[Test]
+		public void TestListInboxFallbackAfterEmptyListExtended ()
+		{
+			const ImapCapabilities InitialCapabilities = ImapCapabilities.IMAP4 | ImapCapabilities.IMAP4rev1 | ImapCapabilities.Status |
+				ImapCapabilities.AppendLimit | ImapCapabilities.Enable | ImapCapabilities.Id | ImapCapabilities.Idle | ImapCapabilities.Move |
+				ImapCapabilities.ListExtended | ImapCapabilities.Namespace | ImapCapabilities.Quota | ImapCapabilities.Sort |
+				ImapCapabilities.SpecialUse | ImapCapabilities.UidPlus;
+			const ImapCapabilities AuthenticatedCapabilities = ImapCapabilities.IMAP4 | ImapCapabilities.IMAP4rev1 | ImapCapabilities.Status |
+				ImapCapabilities.AppendLimit | ImapCapabilities.CreateSpecialUse | ImapCapabilities.Quota | ImapCapabilities.Children |
+				ImapCapabilities.CondStore | ImapCapabilities.Enable | ImapCapabilities.ESort | ImapCapabilities.ESearch | ImapCapabilities.I18NLevel |
+				ImapCapabilities.Id | ImapCapabilities.Idle | ImapCapabilities.Move | /*ImapCapabilities.ListStatus | ImapCapabilities.ListExtended |*/
+				ImapCapabilities.LiteralPlus | ImapCapabilities.Namespace | /*ImapCapabilities.Preview |*/ ImapCapabilities.FuzzySearch |
+				ImapCapabilities.Sort | ImapCapabilities.SearchResults | /*ImapCapabilities.SpecialUse |*/ ImapCapabilities.StatusSize |
+				ImapCapabilities.UidPlus | ImapCapabilities.Unselect | ImapCapabilities.Within | ImapCapabilities.XList;
+			var commands = CreateListInboxFallbackAfterEmptyListExtendedCommands ();
+
+			using (var client = new ImapClient () { TagPrefix = 'A' }) {
+				try {
+					client.Connect (new ImapReplayStream (commands, false), "localhost", 143, SecureSocketOptions.None);
+				} catch (Exception ex) {
+					Assert.Fail ($"Did not expect an exception in Connect: {ex}");
+				}
+
+				Assert.That (client.IsConnected, Is.True, "Client failed to connect.");
+
+				Assert.That (client.Capabilities, Is.EqualTo (InitialCapabilities));
+				Assert.That (client.AppendLimit, Is.EqualTo (104857600), "AppendLimit");
+				Assert.That (client.AuthenticationMechanisms, Has.Count.EqualTo (2));
+				Assert.That (client.AuthenticationMechanisms, Does.Contain ("PLAIN"), "Expected SASL PLAIN auth mechanism");
+				Assert.That (client.AuthenticationMechanisms, Does.Contain ("LOGIN"), "Expected SASL LOGIN auth mechanism");
+
+				try {
+					client.Authenticate ("username", "password");
+				} catch (Exception ex) {
+					Assert.Fail ($"Did not expect an exception in Authenticate: {ex}");
+				}
+
+				Assert.That (client.Capabilities, Is.EqualTo (AuthenticatedCapabilities));
+				Assert.That (client.PersonalNamespaces, Has.Count.EqualTo (1), "PersonalNamespaces.Count");
+				Assert.That (client.PersonalNamespaces[0].Path, Is.EqualTo (string.Empty), "PersonalNamespaces[0].Path");
+				Assert.That (client.PersonalNamespaces[0].DirectorySeparator, Is.EqualTo ('.'), "PersonalNamespaces[0].DirectorySeparator");
+				Assert.That (client.OtherNamespaces, Has.Count.EqualTo (0), "OtherNamespaces.Count");
+				Assert.That (client.SharedNamespaces, Has.Count.EqualTo (0), "SharedNamespaces.Count");
+
+				Assert.That (client.Inbox, Is.Not.Null, "Inbox");
+
+				client.Disconnect (true);
+			}
+		}
+
+		[Test]
+		public async Task TestListInboxFallbackAfterEmptyListExtendedAsync ()
+		{
+			const ImapCapabilities InitialCapabilities = ImapCapabilities.IMAP4 | ImapCapabilities.IMAP4rev1 | ImapCapabilities.Status |
+				ImapCapabilities.AppendLimit | ImapCapabilities.Enable | ImapCapabilities.Id | ImapCapabilities.Idle | ImapCapabilities.Move |
+				ImapCapabilities.ListExtended | ImapCapabilities.Namespace | ImapCapabilities.Quota | ImapCapabilities.Sort |
+				ImapCapabilities.SpecialUse | ImapCapabilities.UidPlus;
+			const ImapCapabilities AuthenticatedCapabilities = ImapCapabilities.IMAP4 | ImapCapabilities.IMAP4rev1 | ImapCapabilities.Status |
+				ImapCapabilities.AppendLimit | ImapCapabilities.CreateSpecialUse | ImapCapabilities.Quota | ImapCapabilities.Children |
+				ImapCapabilities.CondStore | ImapCapabilities.Enable | ImapCapabilities.ESort | ImapCapabilities.ESearch | ImapCapabilities.I18NLevel |
+				ImapCapabilities.Id | ImapCapabilities.Idle | ImapCapabilities.Move | /*ImapCapabilities.ListStatus | ImapCapabilities.ListExtended |*/
+				ImapCapabilities.LiteralPlus | ImapCapabilities.Namespace | /*ImapCapabilities.Preview |*/ ImapCapabilities.FuzzySearch |
+				ImapCapabilities.Sort | ImapCapabilities.SearchResults | /*ImapCapabilities.SpecialUse |*/ ImapCapabilities.StatusSize |
+				ImapCapabilities.UidPlus | ImapCapabilities.Unselect | ImapCapabilities.Within | ImapCapabilities.XList;
+			var commands = CreateListInboxFallbackAfterEmptyListExtendedCommands ();
+
+			using (var client = new ImapClient () { TagPrefix = 'A' }) {
+				try {
+					await client.ConnectAsync (new ImapReplayStream (commands, true), "localhost", 143, SecureSocketOptions.None);
+				} catch (Exception ex) {
+					Assert.Fail ($"Did not expect an exception in Connect: {ex}");
+				}
+
+				Assert.That (client.IsConnected, Is.True, "Client failed to connect.");
+
+				Assert.That (client.Capabilities, Is.EqualTo (InitialCapabilities));
+				Assert.That (client.AppendLimit, Is.EqualTo (104857600), "AppendLimit");
+				Assert.That (client.AuthenticationMechanisms, Has.Count.EqualTo (2));
+				Assert.That (client.AuthenticationMechanisms, Does.Contain ("PLAIN"), "Expected SASL PLAIN auth mechanism");
+				Assert.That (client.AuthenticationMechanisms, Does.Contain ("LOGIN"), "Expected SASL LOGIN auth mechanism");
+
+				try {
+					await client.AuthenticateAsync ("username", "password");
+				} catch (Exception ex) {
+					Assert.Fail ($"Did not expect an exception in Authenticate: {ex}");
+				}
+
+				Assert.That (client.Capabilities, Is.EqualTo (AuthenticatedCapabilities));
+				Assert.That (client.PersonalNamespaces, Has.Count.EqualTo (1), "PersonalNamespaces.Count");
+				Assert.That (client.PersonalNamespaces[0].Path, Is.EqualTo (string.Empty), "PersonalNamespaces[0].Path");
+				Assert.That (client.PersonalNamespaces[0].DirectorySeparator, Is.EqualTo ('.'), "PersonalNamespaces[0].DirectorySeparator");
+				Assert.That (client.OtherNamespaces, Has.Count.EqualTo (0), "OtherNamespaces.Count");
+				Assert.That (client.SharedNamespaces, Has.Count.EqualTo (0), "SharedNamespaces.Count");
+
+				Assert.That (client.Inbox, Is.Not.Null, "Inbox");
+
+				await client.DisconnectAsync (true);
+			}
+		}
+
 		[Test]
 		public void TestLowercaseImapResponses ()
 		{

UnitTests/Net/Imap/Resources/strato.de/authenticate.txt

@@ -0,0 +1 @@
+A######## OK User logged in (234)

UnitTests/Net/Imap/Resources/strato.de/capability.txt

@@ -0,0 +1,2 @@
+* CAPABILITY IMAP4 IMAP4rev1 APPENDLIMIT=104857600 AUTH=PLAIN AUTH=LOGIN CREATE-SPECIAL-USE QUOTA=RES-MAILBOX CHILDREN CONDSTORE ENABLE ESORT ESEARCH I18NLEVEL=2 ID IDLE MOVE LIST-STATUS LIST-EXTENDED LITERAL+ NAMESPACE PREVIEW=FUZZY QUOTA SEARCH=FUZZY SORT SEARCHRES SNIPPET=FUZZY SPECIAL-USE STATUS=SIZE UIDPLUS UNSELECT WITHIN XLIST
+A######## OK CAPABILITY completed

UnitTests/Net/Imap/Resources/strato.de/greeting.txt

@@ -0,0 +1 @@
+* OK [CAPABILITY IMAP4 IMAP4rev1 APPENDLIMIT=104857600 AUTH=PLAIN AUTH=LOGIN ENABLE ID IDLE MOVE LIST-EXTENDED NAMESPACE QUOTA SORT SPECIAL-USE UIDPLUS] IMAP server ready (P2 TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384)

UnitTests/Net/Imap/Resources/strato.de/list-inbox.txt

@@ -0,0 +1,2 @@
+* LIST (\HasChildren) "." "INBOX"
+A######## OK LIST completed

UnitTests/Net/Imap/Resources/strato.de/namespace.txt

@@ -0,0 +1,2 @@
+* NAMESPACE (("" ".")) NIL NIL
+A######## OK NAMESPACE completed

UnitTests/Net/Imap/Resources/strato.de/xlist.txt

@@ -0,0 +1 @@
+A######## OK XLIST completed