Port 'Delay console CPR timer once protocol has worked' to 2.1 (#30759)

Author: wtgodbe

src/System.Console/src/System/ConsolePal.Unix.cs

@@ -332,6 +332,13 @@ public static int CursorTop
             }
         }
 
+        /// <summary>
+        /// Tracks whether we've ever successfully received a response to a cursor position request (CPR).
+        /// If we have, then we can be more aggressive about expecting a response to subsequent requests,
+        /// e.g. using a longer timeout.
+        /// </summary>
+        private static bool s_everReceivedCursorPositionResponse;
+
         /// <summary>Gets the current cursor position.  This involves both writing to stdout and reading stdin.</summary>
         private static unsafe void GetCursorPosition(out int left, out int top)
         {
@@ -358,7 +365,15 @@ private static unsafe void GetCursorPosition(out int left, out int top)
             // one thread's get_CursorLeft/Top from providing input to the other's Console.Read*.
             lock (StdInReader) 
             {
-                Interop.Sys.InitializeConsoleBeforeRead(minChars: 0, decisecondsTimeout: 10);
+                // Because the CPR request/response protocol involves blocking until we get a certain
+                // response from the terminal, we want to avoid doing so if we don't know the terminal
+                // will definitely response.  As such, we start with minChars == 0, which causes the
+                // terminal's read timer to start immediately.  Once we've received a response for
+                // a request such that we know the terminal supports the protocol, we then specify
+                // minChars == 1.  With that, the timer won't start until the first character is
+                // received.  This makes the mechanism more reliable when there are high latencies
+                // involved in reading/writing, such as when accessing a remote system.
+                Interop.Sys.InitializeConsoleBeforeRead(minChars: (byte)(s_everReceivedCursorPositionResponse ? 1 : 0), decisecondsTimeout: 10);
                 try
                 {
                     // Write out the cursor position report request.
@@ -425,6 +440,9 @@ private static unsafe void GetCursorPosition(out int left, out int top)
                     // else back into the StdInReader.
                     ReadRowOrCol(bracketPos, semiPos, r, readBytes, ref top);
                     ReadRowOrCol(semiPos, rPos, r, readBytes, ref left);
+
+                    // Mark that we've successfully received a CPR response at least once.
+                    s_everReceivedCursorPositionResponse = true;
                 }
                 finally
                 {