Fix aligned buffer write

src/Compiler/Microsoft.AspNetCore.Razor.Language/test/CodeGeneration/CSharpCodeWriterTest.cs

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.AspNetCore.Razor.Language.Intermediate;
+using Roslyn.Test.Utilities;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration;
@@ -439,4 +440,46 @@ class C
 
             """, output);
     }
+
+    [Fact, WorkItem("https://github.com/dotnet/core/issues/9885")]
+    public void AlignedPages_WritesCorrectlyWhenPageAndBufferAreAligned()
+    {
+        var pages = new LinkedList<ReadOnlyMemory<char>[]>();
+
+        const string FirstLine = "First Line";
+        pages.AddLast([(FirstLine + FirstLine).AsMemory(), "Second".AsMemory()]);
+
+        var testWriter = CodeWriter.GetTestTextReader(pages);
+        var output = new char[FirstLine.Length];
+
+        testWriter.Read(output, 0, output.Length);
+        Assert.Equal(output, FirstLine.AsSpan());
+        Array.Clear(output);
+
+        testWriter.Read(output, 0, output.Length);
+        Assert.Equal(output, FirstLine.AsSpan());
+        Array.Clear(output);
+
+        testWriter.Read(output, 0, output.Length);
+        Assert.Equal(output, "Second\0\0\0\0".AsSpan());
+    }
+
+    [Fact]
+    public void ReaderOnlyReadsAsMuchAsRequested()
+    {
+        var pages = new LinkedList<ReadOnlyMemory<char>[]>();
+
+        const string FirstLine = "First Line";
+        pages.AddLast([FirstLine.AsMemory()]);
+
+        var testWriter = CodeWriter.GetTestTextReader(pages);
+        var output = new char[FirstLine.Length];
+
+        testWriter.Read(output, 0, 2);
+        Assert.Equal(output, "Fi\0\0\0\0\0\0\0\0".AsSpan());
+        Array.Clear(output);
+
+        testWriter.Read(output, 0, output.Length);
+        Assert.Equal(output, "rst Line\0\0".AsSpan());
+    }
 }

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/CodeGeneration/CodeWriter.cs

@@ -323,14 +323,20 @@ public CodeWriter WriteLine([InterpolatedStringHandlerArgument("")] ref WriteInt
 
     public SourceText GetText()
     {
-        using var reader = new Reader(this);
+        using var reader = new Reader(_pages, Length);
         return SourceText.From(reader, Length, Encoding.UTF8);
     }
 
-    private sealed class Reader(CodeWriter codeWriter) : TextReader
+    // Internal for testing
+    internal static TextReader GetTestTextReader(LinkedList<ReadOnlyMemory<char>[]> pages)
     {
-        private LinkedListNode<ReadOnlyMemory<char>[]>? _page = codeWriter._pages.First;
-        private int _remainingLength = codeWriter.Length;
+        return new Reader(pages, pages.Count);
+    }
+
+    private sealed class Reader(LinkedList<ReadOnlyMemory<char>[]> pages, int length) : TextReader
+    {
+        private LinkedListNode<ReadOnlyMemory<char>[]>? _page = pages.First;
+        private int _remainingLength = length;
         private int _chunkIndex;
         private int _charIndex;
 
@@ -413,7 +419,7 @@ public override int Read(char[] buffer, int index, int count)
                 return -1;
             }
 
-            var destination = buffer.AsSpan(index);
+            var destination = buffer.AsSpan(index, count);
             var charsWritten = 0;
 
             var page = _page;
@@ -466,15 +472,14 @@ public override int Read(char[] buffer, int index, int count)
                     if (endOfChunkWritten)
                     {
                         chunkIndex++;
+                        charIndex = 0;
                     }
 
                     // Break if we are done writing. chunkIndex and charIndex should have their correct values at this point.
                     if (destination.IsEmpty)
                     {
                         break;
                     }
-
-                    charIndex = 0;
                 }
 
                 if (destination.IsEmpty)