Commits on Nov 6, 2019

1. fix: TOCTOU race when creating directories …

   When create_directories() is called to create a chain of directories,
   path components are created one by one. For each path component, ghc
   checks if the directory exists. If not it attempts to create it. In
   case of failure to create this directory, the function fails.
   
   This behavior exhibits a TOCTOU (Time Of Check/Time Of Use) race when
   different threads of execution (e.g. different processes) attempt to
   create the same paths (or paths that contain a common hierarchy).
   The following sequence of events between threads T1 and T2 illustrates
   the issue:
   
     T1: checks path P exists: no. Will attempt to create P...
     T2: checks path P exists: no. Will attempt to create P...
     T1: creates P, no error
     T2: fails to create P: raises an error
   
   It is not desirable for create_directories() to fail in this case. This
   commit mirrors the GNU libstdc++ implementation of the c++ filesystem
   library: if the creation of a directory fails, we will inspect the path
   that should have been created as a directory. If it is indeed a
   directory (another thread of execution created it for us), the error is
   now reset and the function continues to iterate to the next path
   component.

   

   jeanguyomarch committed Nov 6, 2019
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