Add Windows Caveats to README, add bash.exe check at startup

README

@@ -30,8 +30,9 @@ BTest has the following prerequisites:
 - Python version >= 3.7 (older versions may work, but are not
   well-tested).
 
-- Bash (note that on FreeBSD and Alpine Linux, bash is not installed
-  by default).
+- Bash. Note that on FreeBSD and Alpine Linux, bash is not installed by
+  default. This is also required on Windows, in the form of Git's msys2, Cygwin,
+  etc.
 
 BTest has the following optional prerequisites to enable additional
 functionality:
@@ -41,6 +42,20 @@ functionality:
 - perf (Linux only).  Note that on Debian/Ubuntu, you also need to install
   the "linux-tools" package.
 
+Windows Caveats
+---------------
+
+When running BTest on Windows, you must have a bash shell installed of some
+sort. This can be from WSL, Cygwin, msys2, Git, or any number of other methods,
+but ``bash.exe`` must be available. BTest will check for its existence at
+startup and exit if it is not available.
+
+A minor change must be made to any configuration value that is a path list. For
+example, if you are setting the ``PATH`` environment variable from your
+btest.cfg. In these cases, you should use ``$(pathsep)s`` in the configuration
+instead of bare ``:`` or ``;`` values to separate the paths. This ensures that
+both POSIX and Windows systems handle the path lists correctly.
+
 Download and Installation
 =========================
 

btest

@@ -2788,7 +2788,22 @@ if __name__ == '__main__':
     pyver_min = sys.version_info[1]
 
     if sys.platform == 'win32':
+        # The "fork" method doesn't exist at all on Windows, so force over to
+        # "spawn" instead.
         mp.set_start_method('spawn')
+
+        # Double-check that `bash.exe` exists and is executable, since it's
+        # required for pretty much anything here to work on Windows. Note we're
+        # doing this prior to parsing the config file because it's required for
+        # backtick-expansion there as well.
+        try:
+            subprocess.call(['bash.exe', '--version'],
+                            stdout=subprocess.DEVNULL,
+                            stderr=subprocess.DEVNULL)
+        except FileNotFoundError:
+            print("error: bash.exe is required to be in your PATH to run BTest.", file=sys.stderr)
+            sys.exit(1)
+
     elif (pyver_maj == 3 and pyver_min >= 8) or pyver_maj > 3:
         mp.set_start_method('fork')