Handle EAGAIN in Console.Write (dotnet#23539)

If stdout/stderr is configured as a non-blocking file descriptor, Console.Write{Line} may fail if the descriptor is full and would block.  With this commit, we instead poll in that case waiting for the ability to write and then retry.

src/System.Console/src/System.Console.csproj

@@ -232,6 +232,9 @@
     <Compile Include="$(CommonPath)\Interop\Unix\System.Native\Interop.OpenFlags.cs">
       <Link>Common\Interop\Unix\Interop.OpenFlags.cs</Link>
     </Compile>
+    <Compile Include="$(CommonPath)\Interop\Unix\System.Native\Interop.Poll.cs">
+      <Link>Common\Interop\Unix\Interop.Poll.cs</Link>
+    </Compile>
     <Compile Include="$(CommonPath)\Interop\Unix\System.Native\Interop.GetEUid.cs">
       <Link>Common\Interop\Unix\Interop.GetEUid.cs</Link>
     </Compile>

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

@@ -977,6 +977,17 @@ private static unsafe void Write(SafeFileHandle fd, byte* bufPtr, int count)
                         // that ended, so simply pretend we were successful.
                         return;
                     }
+                    else if (errorInfo.Error == Interop.Error.EAGAIN) // aka EWOULDBLOCK
+                    {
+                        // May happen if the file handle is configured as non-blocking.
+                        // In that case, we need to wait to be able to write and then
+                        // try again. We poll, but don't actually care about the result,
+                        // only the blocking behavior, and thus ignore any poll errors
+                        // and loop around to do another write (which may correctly fail
+                        // if something else has gone wrong).
+                        Interop.Sys.Poll(fd, Interop.Sys.PollEvents.POLLOUT, Timeout.Infinite, out Interop.Sys.PollEvents triggered);
+                        continue;
+                    }
                     else
                     {
                         // Something else... fail.

src/System.Console/tests/NonStandardConfiguration.Unix.cs

@@ -0,0 +1,40 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Diagnostics;
+using System.Linq;
+using Xunit;
+
+namespace System.Tests
+{
+    public partial class NonStandardConfigurationTests : RemoteExecutorTestBase
+    {
+        [PlatformSpecific(TestPlatforms.AnyUnix)] // Uses P/Invokes
+        [Fact]
+        public void NonBlockingStdout_AllDataReceived()
+        {
+            RemoteInvokeHandle remote = RemoteInvoke(() =>
+            {
+                char[] data = Enumerable.Repeat('a', 1024).ToArray();
+
+                const int StdoutFd = 1;
+                Assert.Equal(0, Interop.Sys.Fcntl.DangerousSetIsNonBlocking((IntPtr)StdoutFd, 1));
+
+                for (int i = 0; i < 10_000; i++)
+                {
+                    Console.Write(data);
+                }
+
+                return SuccessExitCode;
+            }, new RemoteInvokeOptions { StartInfo = new ProcessStartInfo() { RedirectStandardOutput = true } });
+
+            using (remote)
+            {
+                Assert.Equal(
+                    new string('a', 1024 * 10_000),
+                    remote.Process.StandardOutput.ReadToEnd());
+            }
+        }
+    }
+}

src/System.Console/tests/System.Console.Tests.csproj

@@ -15,7 +15,6 @@
       <Link>Common\tests\System\PlatformDetection.cs</Link>
     </Compile>
     <Compile Include="CancelKeyPress.cs" />
-    <Compile Include="CancelKeyPress.Unix.cs" Condition="'$(TargetsWindows)' != 'true'" />
     <Compile Include="Helpers.cs" />
     <Compile Include="ReadAndWrite.cs" />
     <Compile Include="ConsoleKeyInfoTests.cs" />
@@ -25,7 +24,6 @@
     <Compile Include="SetOut.cs" />
     <Compile Include="NegativeTesting.cs" />
     <Compile Include="ConsoleEncoding.cs" />
-    <Compile Include="ConsoleEncoding.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
     <Compile Include="SyncTextReader.cs" />
     <Compile Include="SyncTextWriter.cs" />
     <Compile Include="Timeout.cs" />
@@ -51,11 +49,24 @@
     </Compile>
     <Compile Include="WindowAndCursorProps.cs" />
   </ItemGroup>
+  <ItemGroup Condition="'$(TargetsWindows)' == 'true'" >
+    <Compile Include="ConsoleEncoding.Windows.cs" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(TargetsWindows)' != 'true'" >
+    <Compile Include="CancelKeyPress.Unix.cs" />
+    <Compile Include="NonStandardConfiguration.Unix.cs" />
+    <Compile Include="$(CommonPath)\Interop\Unix\Interop.Libraries.cs">
+      <Link>Common\Interop\Windows\Interop.Libraries.cs</Link>
+    </Compile>
+    <Compile Include="$(CommonPath)\Interop\Unix\System.Native\Interop.Fcntl.cs">
+      <Link>Interop\Unix\System.Native\Interop.Fcntl.cs</Link>
+    </Compile>
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="$(CommonTestPath)\System\Diagnostics\RemoteExecutorConsoleApp\RemoteExecutorConsoleApp.csproj">
       <Project>{69e46a6f-9966-45a5-8945-2559fe337827}</Project>
       <Name>RemoteExecutorConsoleApp</Name>
     </ProjectReference>
   </ItemGroup>
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
-</Project>
+</Project>