Xcode generation error for setting product type to the same type

[markfinal]

This came up during a test suite run

* FAILURE *: Package 'Test14 in /Users/mark/dev/BuildAMation/tests' with extra arguments '--C.bitdepth=32 --Xcode.generateSchemes'
Command was: bam -o=build -b=Xcode --config=debug --config=profile -j=0 -v=0 --clean --C.bitdepth=32 --Xcode.generateSchemes
Executed in: /Users/mark/dev/BuildAMation/tests/Test14
Messages:
BuildAMation (Bam) v1.0.2 (c) Mark Final, 2010-2016. Licensed under BSD 3-clause. See License.md.
Parts of this software are licensed under the Microsoft Limited Public License (MS-PL). See MS-PL.md.
Compiling package assembly (C# compiler v4.0, targetting .NETFramework,Version=v4.5), because no previously compiled package assembly exists.
Creating modules
Analysing module dependencies
Apply settings to modules
Parsing strings
Running build

Build Failed
Errors:
ERROR: (Bam.Core.Exception) Error during threaded build
ERROR: (Bam.Core.Exception) Product type has already been set to DynamicLibrary. Cannot change it to DynamicLibrary
ERROR:
  at XcodeBuilder.Target.SetProductType (EProductType type) <0x3345e20 + 0x00147> in <filename unknown>:0
  at XcodeBuilder.Target.EnsureOutputFileReferenceExists (Bam.Core.TokenizedString path, EFileType type, EProductType productType) <0x3345cd8 + 0x0001b> in <filename unknown>:0
  at C.XcodeLinker.C.ILinkingPolicy.Link (C.ConsoleApplication sender, Bam.Core.ExecutionContext context, Bam.Core.TokenizedString executablePath, System.Collections.ObjectModel.ReadOnlyCollection`1 objectFiles, System.Collections.ObjectModel.ReadOnlyCollection`1 headers, System.Collections.ObjectModel.ReadOnlyCollection`1 libraries) <0x33423a0 + 0x0034b> in <filename unknown>:0
  at C.ConsoleApplication.ExecuteInternal (Bam.Core.ExecutionContext context) <0x32bf248 + 0x000e9> in <filename unknown>:0
  at C.DynamicLibrary.ExecuteInternal (Bam.Core.ExecutionContext context) <0x32bea30 + 0x0005f> in <filename unknown>:0
  at Bam.Core.Module.Bam.Core.IModuleExecution.Execute (Bam.Core.ExecutionContext context) <0x32b6618 + 0x00143> in <filename unknown>:0
  at Bam.Core.Executor+<Run>c__AnonStorey2.<>m__0 () <0x32b5d00 + 0x0027e> in <filename unknown>:0

[markfinal]

Although there is a guard in SetProductType, this function can be run from multiple threads. So lock synchronization is needed.