Better catch when vcs programs not installed.

Check if vcs commands exist before running commands. Cache the result so
that the check doesn't happen for every file (if the user installs git
or hg, they will need to restart runmanager or the compiler
subprocess for saving of vcs info to start working).

Should address labscript-suite/labscript-suite#66

Author: chrisjbillington

labscript/labscript.py

@@ -18,7 +18,7 @@
 import keyword
 import threading
 from inspect import getcallargs
-from functools import wraps
+from functools import wraps, lru_cache
 
 # Notes for v3
 #
@@ -3194,6 +3194,18 @@ def _file_watcher_callback(name, info, event):
 
 _file_watcher = FileWatcher(_file_watcher_callback)
 
+@lru_cache(None)
+def _have_vcs(vcs):
+    try:
+        subprocess.check_output([vcs, '--version'])
+        return True
+    except FileNotFoundError:
+        msg = f"""Warning: Cannot run {vcs} commands, {vcs} info for labscriptlib files
+        will not be saved. You can disable this warning by setting
+        [labscript]/save_{vcs}_info = False in labconfig."""
+        sys.stderr.write(dedent(msg) + '\n')
+        return False
+
 def _run_vcs_commands(path):
     """Run some VCS commands on a file and return their output.
     
@@ -3221,7 +3233,7 @@ def _run_vcs_commands(path):
     # Gather together a list of commands to run.
     module_directory, module_filename = os.path.split(path)
     vcs_commands = []
-    if compiler.save_hg_info:
+    if compiler.save_hg_info and _have_vcs('hg'):
         hg_commands = [
             ['log', '--limit', '1'],
             ['status'],
@@ -3230,7 +3242,7 @@ def _run_vcs_commands(path):
         for command in hg_commands:
             command = tuple(['hg'] + command + [module_filename])
             vcs_commands.append((command, module_directory))
-    if compiler.save_git_info:
+    if compiler.save_git_info and _have_vcs('git'):
         git_commands = [
             ['branch', '--show-current'],
             ['describe', '--tags', '--always', 'HEAD'],
@@ -3280,34 +3292,33 @@ def save_labscripts(hdf5_file):
     script.attrs['path'] = os.path.dirname(compiler.labscript_file).encode() if compiler.labscript_file is not None else sys.path[0]
     try:
         import labscriptlib
-        prefix = os.path.dirname(labscriptlib.__file__)
-        for module in sys.modules.values():
-            if getattr(module, '__file__', None) is not None:
-                path = os.path.abspath(module.__file__)
-                if path.startswith(prefix) and (path.endswith('.pyc') or path.endswith('.py')):
-                    path = path.replace('.pyc', '.py')
-                    save_path = 'labscriptlib/' + path.replace(prefix, '').replace('\\', '/').replace('//', '/')
-                    if save_path in hdf5_file:
-                        # Don't try to save the same module script twice! 
-                        # (seems to at least double count __init__.py when you import an entire module as in from labscriptlib.stages import * where stages is a folder with an __init__.py file.
-                        # Doesn't seem to want to double count files if you just import the contents of a file within a module
-                        continue
-                    hdf5_file.create_dataset(save_path, data=open(path).read())
-                    with _vcs_cache_rlock:
-                        already_cached = path in _vcs_cache
-                    if not already_cached:
+    except ImportError:
+        return
+    prefix = os.path.dirname(labscriptlib.__file__)
+    for module in sys.modules.values():
+        if getattr(module, '__file__', None) is not None:
+            path = os.path.abspath(module.__file__)
+            if path.startswith(prefix) and (path.endswith('.pyc') or path.endswith('.py')):
+                path = path.replace('.pyc', '.py')
+                save_path = 'labscriptlib/' + path.replace(prefix, '').replace('\\', '/').replace('//', '/')
+                if save_path in hdf5_file:
+                    # Don't try to save the same module script twice! (seems to at least
+                    # double count __init__.py when you import an entire module as in
+                    # from labscriptlib.stages import * where stages is a folder with an
+                    # __init__.py file. Doesn't seem to want to double count files if
+                    # you just import the contents of a file within a module
+                    continue
+                hdf5_file.create_dataset(save_path, data=open(path).read())
+                with _vcs_cache_rlock:
+                    if not path in _vcs_cache:
                         # Add file to watch list and create its entry in the cache.
                         _file_watcher.add_file(path)
                         _file_watcher_callback(path, None, None)
-                    with _vcs_cache_rlock:
-                        # Save the cached vcs output to the file.
-                        for command, info, err in _vcs_cache[path]:
-                            attribute_str = command[0] + ' ' + command[1]
-                            hdf5_file[save_path].attrs[attribute_str] = (info + '\n' + err)
-    except ImportError:
-        pass
-    except WindowsError if os.name == 'nt' else None:
-        sys.stderr.write('Warning: Cannot save version control data for imported scripts. Check that the hg and/or git command can be run from the command line.\n')
+                    # Save the cached vcs output to the file.
+                    for command, info, err in _vcs_cache[path]:
+                        attribute_str = command[0] + ' ' + command[1]
+                        hdf5_file[save_path].attrs[attribute_str] = (info + '\n' + err)
+
 
 
 def write_device_properties(hdf5_file):