[release/6.0] Fixes #62167. WriteAsync may truncate data if called after .Advance(int)

src/libraries/System.IO.Pipelines/src/System.IO.Pipelines.csproj

@@ -9,7 +9,8 @@ Commonly Used Types:
 System.IO.Pipelines.Pipe
 System.IO.Pipelines.PipeWriter
 System.IO.Pipelines.PipeReader</PackageDescription>
-    <ServicingVersion>1</ServicingVersion>
+    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
+    <ServicingVersion>2</ServicingVersion>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonPath)System\Threading\Tasks\TaskToApm.cs"

src/libraries/System.IO.Pipelines/src/System/IO/Pipelines/Pipe.cs

@@ -1082,7 +1082,7 @@ private void WriteMultiSegment(ReadOnlySpan<byte> source)
                 }
 
                 // We filled the segment
-                _writingHead.End += writable;
+                _writingHead.End += _writingHeadBytesBuffered;
                 _writingHeadBytesBuffered = 0;
 
                 // This is optimized to use pooled memory. That's why we pass 0 instead of

src/libraries/System.IO.Pipelines/tests/PipeLengthTests.cs

@@ -277,5 +277,19 @@ public async Task NullExaminedAndConsumedNoops()
             ReadResult result = await _pipe.Reader.ReadAsync();
             _pipe.Reader.AdvanceTo(default, default);
         }
+
+        [Fact]
+        public async Task AdvanceFollowedByWriteAsyncTest()
+        {
+            Memory<byte> buffer = new byte[26];
+            Pipe pipe = new(new PipeOptions(minimumSegmentSize: 1));
+
+            var mem = pipe.Writer.GetMemory(14)[..14];
+            buffer[..14].CopyTo(mem);
+            pipe.Writer.Advance(14);
+            await pipe.Writer.WriteAsync(buffer[14..]);
+            ReadResult res = await pipe.Reader.ReadAsync();
+            Assert.Equal(res.Buffer.Length, buffer.Length);
+        }
     }
 }