Concurrent transforms with the same source and/or transform file can fail

When using slow cheetah to transform json files we discovered that the transforms would fail intermittently. This was particularly frequent when building a project targeting multiple frameworks (net461 and netcore2.1 in this instance). The parallel build behaviour of msbuild meant it would regularly attempt to perform the same transforms at the same time (once for each target framework).

This would result in msbuild output similar to:

[17:52:50] [Step 6/12]      2>UpdateLocalAppSettings:
[17:52:50] [Step 6/12]          Beginning transformation.
[17:52:50] [Step 6/12]     15>UpdateLocalAppSettings:
[17:52:50] [Step 6/12]          Beginning transformation.
[17:52:50] [Step 6/12]          Transformation failed.

This seems to be caused by the source file being opened with exclusive access for the duration of the transform. The files need to be opened with a FileShare mode of Read to allow concurrent transformations.

Locate the code that opens the source and transform files during JSON transformation, then inspect how parallel MSBuild executions share those handles. Reproduce the issue with a multi-target parallel build and verify that concurrent transformations complete without intermittent failures when files permit shared reads.