bug in aspnet_compiler?

I have a solution with several web projects.

I have built each project into its own folder (there's about 10 projects):

/build/projA
/build/projB
/build/projC

If I call

aspnet_compiler -v / -p \"build/#{proj}\" merged/#{proj}

on each one in turn, everything is fine.

If I make all 10 calls to aspnet_compiler in parallel, I get assembly loading errors, saying the referenced assemblies are in use.

Is this a bug? Why would it be in use? It shouldn't be.

The assemblies that cant be loaded are in multiple projects, but each one has its own copy in its bin folder.

Update: it seems to occur (for some) of the assemblies that are references of references. Those loaded with complete binding information seem to work.

Here's a dump from one of the failed loadings from fuslogvw

The operation failed.
Bind result: hr = 0x80070020. The process cannot access the file because it is being used by another process.

Assembly manager loaded from:  C:\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll
Running under executable  c:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\aspnet_compiler.exe
--- A detailed error log follows. 

=== Pre-bind state information ===
LOG: User = andrew.bullock
LOG: DisplayName = Yahoo.Yui.Compressor
 (Partial)
WRN: Partial binding information was supplied for an assembly:
WRN: Assembly Name: Yahoo.Yui.Compressor | Domain ID: 2
WRN: A partial bind occurs when only part of the assembly display name is provided.
WRN: This might result in the binder loading an incorrect assembly.
WRN: It is recommended to provide a fully specified textual identity for the assembly,
WRN: that consists of the simple name, version, culture, and public key token.
WRN: See whitepaper http://go.microsoft.com/fwlink/?LinkId=109270 for more information and common solutions to this issue.
LOG: Appbase = file:///E:/build/ProjA/
LOG: Initial PrivatePath = E:\build\ProjA\bin
LOG: Dynamic Base = C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\src_projects_proja\061ad2e7
LOG: Cache Base = C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\src_projects_proja\061ad2e7
LOG: AppName = 627cc9a9
Calling assembly : (Unknown).
===
LOG: This bind starts in default load context.
LOG: Using application configuration file: E:\build\ProjA\web.config
LOG: Using host configuration file: 
LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework\v4.0.30319\config\machine.config.
LOG: Policy not being applied to reference at this time (private, custom, partial, or location-based assembly bind).
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/src_projects_proja/061ad2e7/627cc9a9/Yahoo.Yui.Compressor.DLL.
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/src_projects_proja/061ad2e7/627cc9a9/Yahoo.Yui.Compressor/Yahoo.Yui.Compressor.DLL.
LOG: Attempting download of new URL file:///E:/build/ProjA/bin/Yahoo.Yui.Compressor.DLL.
LOG: Assembly download was successful. Attempting setup of file: E:\build\ProjA\bin\Yahoo.Yui.Compressor.dll
LOG: Entering download cache setup phase.
ERR: Error extracting manifest import from file (hr = 0x80070020).
ERR: Setup failed with hr = 0x80070020.
ERR: Failed to complete setup of assembly (hr = 0x80070020). Probing terminated.

If you call the ClientBuildManager directly, outside of aspnet_compiler there doesnt seem to be a problem, no idea why.

var parameter = new ClientBuildManagerParameter
    {
        PrecompilationFlags = PrecompilationFlags.ForceDebug
    };
var client = new ClientBuildManager("/", src, dest, parameter);
client.PrecompileApplication();