Avoid opening PDB writer on existing PDB file

src/Compilers/Core/Desktop/CommandLine/CommonCompiler.CompilerEmitStreamProvider.cs

@@ -20,15 +20,13 @@ private sealed class CompilerEmitStreamProvider : Compilation.EmitStreamProvider
 
             private readonly CommonCompiler _compiler;
             private readonly string _filePath;
-            private readonly bool _streamCreatedByNativePdbWriter;
 
             private Stream _lazyStream;
 
-            internal CompilerEmitStreamProvider(CommonCompiler compiler, string filePath, bool streamCreatedByNativePdbWriter)
+            internal CompilerEmitStreamProvider(CommonCompiler compiler, string filePath)
             {
                 _compiler = compiler;
                 _filePath = filePath;
-                _streamCreatedByNativePdbWriter = streamCreatedByNativePdbWriter;
                 _lazyStream = s_uninitialized;
             }
 
@@ -45,7 +43,7 @@ public override Stream GetStream(DiagnosticBag diagnostics)
             {
                 if (_lazyStream == s_uninitialized)
                 {
-                    _lazyStream = _streamCreatedByNativePdbWriter ? null : OpenFile(_filePath, diagnostics);
+                    _lazyStream = OpenFile(_filePath, diagnostics);
                 }
 
                 return _lazyStream;

src/Compilers/Core/Desktop/CommandLine/CommonCompiler.cs

@@ -416,8 +416,8 @@ private int RunCore(TextWriter consoleOutput, ErrorLogger errorLogger, Cancellat
                         WithOutputNameOverride(outputName).
                         WithPdbFilePath(finalPdbFilePath);
 
-                    using (var peStreamProvider = new CompilerEmitStreamProvider(this, finalOutputPath, streamCreatedByNativePdbWriter: false))
-                    using (var pdbStreamProviderOpt = Arguments.EmitPdb ? new CompilerEmitStreamProvider(this, finalOutputPath, streamCreatedByNativePdbWriter: true) : null)
+                    using (var peStreamProvider = new CompilerEmitStreamProvider(this, finalOutputPath))
+                    using (var pdbStreamProviderOpt = Arguments.EmitPdb ? new CompilerEmitStreamProvider(this, finalPdbFilePath) : null)
                     {
                         emitResult = compilation.Emit(
                             peStreamProvider,

src/Compilers/Core/Portable/Compilation/Compilation.cs

@@ -1641,6 +1641,7 @@ private static EmitResult ToEmitResultAndFree(DiagnosticBag diagnostics, bool su
                         pdbStream = pdbStreamProvider.GetStream(diagnostics);
                         if (pdbStream == null)
                         {
+                            Debug.Assert(diagnostics.HasAnyErrors());
                             return null;
                         }
 
@@ -1648,8 +1649,8 @@ private static EmitResult ToEmitResultAndFree(DiagnosticBag diagnostics, bool su
 
                         // Native PDB writer is able to update an existing stream.
                         // It checks for length to determine whether the given stream has existing data to be updated,
-                        // or whether it should start writing PDB data from scratch. Thus if not writing to a seekable empty stream ,
-                        // let's create an in-memory temp stream for the PDB writer and copy all data to the actual stream at once at the end.
+                        // or whether it should start writing PDB data from scratch. Thus if not writing to a seekable empty stream,
+                        // we have to create an in-memory temp stream for the PDB writer and copy all data to the actual stream at once at the end.
                         if (!retStream.CanSeek || retStream.Length != 0)
                         {
                             retStream = pdbTempStream = new MemoryStream();
@@ -1725,7 +1726,7 @@ private static EmitResult ToEmitResultAndFree(DiagnosticBag diagnostics, bool su
                     {
                         // Note: Native PDB writer may operate on the underlying stream during disposal.
                         // So close it here before we read data from the underlying stream.
-                        nativePdbWriter.WritePdbToOutput();
+                        nativePdbWriter?.WritePdbToOutput();
 
                         pdbTempStream.Position = 0;
                         pdbTempStream.CopyTo(pdbStream);

src/Compilers/Core/Portable/PEWriter/PdbWriter.cs

@@ -75,8 +75,7 @@ public void Dispose()
         }
 
         /// <summary>
-        /// Close the PDB writer and write the contents to the stream provided by <see cref="_streamProvider"/> 
-        /// or file name specified by <see cref="_fileName"/> value if no stream has been provided. 
+        /// Close the PDB writer and write the PDB data to the stream provided by <see cref="_streamProvider"/>.
         /// </summary>
         public void WritePdbToOutput()
         {
@@ -559,14 +558,17 @@ private static Type GetCorSymWriterSxSType()
 
         public void SetMetadataEmitter(MetadataWriter metadataWriter)
         {
-            Stream streamOpt = _streamProvider();
+            Stream stream = _streamProvider() ?? new System.IO.MemoryStream();
 
             try
             {
                 var instance = (ISymUnmanagedWriter2)(_symWriterFactory != null ? _symWriterFactory() : Activator.CreateInstance(GetCorSymWriterSxSType()));
-                var comStream = (streamOpt != null) ? new ComStreamWrapper(streamOpt) : null;
 
-                instance.Initialize(new PdbMetadataWrapper(metadataWriter), _fileName, comStream, fullBuild: true);
+                // Important: If the stream is not specified or if it is non-empty the SymWriter appends data to it (provided it contains valid PDB)
+                // and the resulting PDB has Age = existing_age + 1.
+                Debug.Assert(stream.Length == 0);
+
+                instance.Initialize(new PdbMetadataWrapper(metadataWriter), _fileName, new ComStreamWrapper(stream), fullBuild: true);
 
                 _metadataWriter = metadataWriter;
                 _symWriter = instance;