make sure that SymbolReader.Dispose closes all opened pdb files

[adamsitnik]

I've recently profiled an app using PerfView, opened the trace file, used Goto Source (Def), closed the Source Window, modified the app source code and tried to rebuild the app.

The rebuild has failed with file in use exception:

C:\Program Files\dotnet\sdk\3.0.100-preview9-013617\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Publish.targets(153,5): warning MSB3026: Could not copy "obj\Release\netcoreapp3.0\win-x64\ReportedTime.pdb" to "bin\Release\netcoreapp3.0\win-x64\publish\ReportedTime.pdb". Beginning retry 1 in 1000ms. The process cannot access the file 'C:\Projects\PerfViewDemo\ReportedTime\bin\Release\netcoreapp3.0\win-x64\publish\ReportedTime.pdb' because it is being used by another process.  [C:\Projects\PerfViewDemo\ReportedTime\0_ReportedTime.csproj]

This PR helps with that by making sure that when the SymbolReader.Dispose is called, it closes all the opened pdb files.

It's still not a perfect solution because SymbolReader.Dispose is called on the static symbol reader instance only when a new trace file is opened:

perfview/src/PerfView/PerfViewData.cs

Line 3887 in 96321f1

using (var reader = etlDataFile.GetSymbolReader(Viewer.StatusBar.LogWriter,

perfview/src/PerfView/App.cs

Line 821 in 1a14779

s_symbolReader.Dispose();

So closing the Source Code window still keeps the .pdf file opened but opening another trace file closes it. A better solution would require much more changes related to symbol reader cache lifetime management but I don't think that it's worth it as of today.

[adamsitnik]

this is a minor bug fix: this method opens a stream, seeks to the begining of it but when it creates and instance of PortableSymbolModule it does not reuse the stream but instead calls a ctor that opens the file one more time:

perfview/src/TraceEvent/Symbols/PortableSymbolModule.cs

Line 11 in ebc8046

public PortableSymbolModule(SymbolReader reader, string pdbFileName) : this(reader, File.Open(pdbFileName, FileMode.Open, FileAccess.Read, FileShare.Read), pdbFileName) { }

so far we always had the file opened twice