target: Asyncify Target._prepare_xfer()

_prepare_xfer() deals with all the paths resulting from glob expansion,
so it can benefit from async capabilities in order to process multiple
files concurrently.

Convert the internals to async/await to enable useful map_concurrently()

devlib/target.py

@@ -663,10 +663,22 @@ class Target(object):
         transfering multiple sources.
         """
 
-        once = functools.lru_cache(maxsize=None)
+        def once(f):
+            cache = dict()
+
+            @functools.wraps(f)
+            async def wrapper(path):
+                try:
+                    return cache[path]
+                except KeyError:
+                    x = await f(path)
+                    cache[path] = x
+                    return x
+
+            return wrapper
 
         _target_cache = {}
-        def target_paths_kind(paths, as_root=False):
+        async def target_paths_kind(paths, as_root=False):
             def process(x):
                 x = x.strip()
                 if x == 'notexist':
@@ -686,7 +698,7 @@ class Target(object):
                     )
                     for path in _paths
                 )
-                res = self.execute(cmd, as_root=as_root)
+                res = await self.execute.asyn(cmd, as_root=as_root)
                 _target_cache.update(zip(_paths, map(process, res.split())))
 
             return [
@@ -695,7 +707,7 @@ class Target(object):
             ]
 
         _host_cache = {}
-        def host_paths_kind(paths, as_root=False):
+        async def host_paths_kind(paths, as_root=False):
             def path_kind(path):
                 if os.path.isdir(path):
                     return 'dir'
@@ -722,47 +734,55 @@ class Target(object):
             src_excep = HostError
             src_path_kind = host_paths_kind
 
-            _dst_mkdir = once(self.makedirs)
+            _dst_mkdir = once(self.makedirs.asyn)
             dst_path_join = self.path.join
             dst_paths_kind = target_paths_kind
-            dst_remove_file = once(functools.partial(self.remove, as_root=as_root))
+
+            @once
+            async def dst_remove_file(path):
+                return await self.remove.asyn(path, as_root=as_root)
         elif action == 'pull':
             src_excep = TargetStableError
             src_path_kind = target_paths_kind
 
-            _dst_mkdir = once(functools.partial(os.makedirs, exist_ok=True))
+            @once
+            async def _dst_mkdir(path):
+                return os.makedirs(path, exist_ok=True)
             dst_path_join = os.path.join
             dst_paths_kind = host_paths_kind
-            dst_remove_file = once(os.remove)
+
+            @once
+            async def dst_remove_file(path):
+                return os.remove(path)
         else:
             raise ValueError('Unknown action "{}"'.format(action))
 
         # Handle the case where path is None
-        def dst_mkdir(path):
+        async def dst_mkdir(path):
             if path:
-                _dst_mkdir(path)
+                await _dst_mkdir(path)
 
-        def rewrite_dst(src, dst):
+        async def rewrite_dst(src, dst):
             new_dst = dst_path_join(dst, os.path.basename(src))
 
-            src_kind, = src_path_kind([src], as_root)
+            src_kind, = await src_path_kind([src], as_root)
             # Batch both checks to avoid a costly extra execute()
-            dst_kind, new_dst_kind = dst_paths_kind([dst, new_dst], as_root)
+            dst_kind, new_dst_kind = await dst_paths_kind([dst, new_dst], as_root)
 
             if src_kind == 'file':
                 if dst_kind == 'dir':
                     if new_dst_kind == 'dir':
                         raise IsADirectoryError(new_dst)
                     if new_dst_kind == 'file':
-                        dst_remove_file(new_dst)
+                        await dst_remove_file(new_dst)
                         return new_dst
                     else:
                         return new_dst
                 elif dst_kind == 'file':
-                    dst_remove_file(dst)
+                    await dst_remove_file(dst)
                     return dst
                 else:
-                    dst_mkdir(os.path.dirname(dst))
+                    await dst_mkdir(os.path.dirname(dst))
                     return dst
             elif src_kind == 'dir':
                 if dst_kind == 'dir':
@@ -776,7 +796,7 @@ class Target(object):
                 elif dst_kind == 'file':
                     raise FileExistsError(dst_kind)
                 else:
-                    dst_mkdir(os.path.dirname(dst))
+                    await dst_mkdir(os.path.dirname(dst))
                     return dst
             else:
                 raise FileNotFoundError(src)
@@ -785,18 +805,19 @@ class Target(object):
             if not sources:
                 raise src_excep('No file matching source pattern: {}'.format(pattern))
 
-            if dst_paths_kind([dest]) != ['dir']:
+            if (await dst_paths_kind([dest])) != ['dir']:
                 raise NotADirectoryError('A folder dest is required for multiple matches but destination is a file: {}'.format(dest))
 
+        async def f(src):
+            return await rewrite_dst(src, dest)
+        mapping = await self.async_manager.map_concurrently(f, sources)
+
         # TODO: since rewrite_dst() will currently return a different path for
         # each source, it will not bring anything. In order to be useful,
         # connections need to be able to understand that if the destination is
         # an empty folder, the source is supposed to be transfered into it with
         # the same basename.
-        return groupby_value({
-            src: rewrite_dst(src, dest)
-            for src in sources
-        })
+        return groupby_value(mapping)
 
     @asyn.asyncf
     @call_conn