_workspaces.py: Use file mtime for workspace cache keys instead of checksumming

The rationale for this is basically:

  o A workspace is completely local

  o A cache key for a workspace is used to determine if the
    workspace has changed or not

  o Within reason, one should not expect buildstream to detect
    a change without a file's mtime having been modified

On my machine, it takes about 1 minute to checksum 11GB of WebKit checkout,
that is annoyingly a long time. With this change, it takes only 5 seconds.

Not perfect, but it's a start.

This is related to issues #294, #295 and #392

1 files changed, 6 insertions, 9 deletions

diff --git a/buildstream/_workspaces.py b/buildstream/_workspaces.py
index 828343538..f6cdeb88b 100644
--- a/buildstream/_workspaces.py
+++ b/buildstream/_workspaces.py
@@ -173,17 +173,14 @@ class Workspace():
     #
     def get_key(self, recalculate=False):
         def unique_key(filename):
-            if os.path.isdir(filename):
-                return "0"
-            elif os.path.islink(filename):
-                return "1"
-
             try:
-                return utils.sha256sum(filename)
-            except FileNotFoundError as e:
+                stat = os.lstat(filename)
+            except OSError as e:
                 raise LoadError(LoadErrorReason.MISSING_FILE,
-                                "Failed loading workspace. Did you remove the "
-                                "workspace directory? {}".format(e))
+                                "Failed to stat file in workspace: {}".format(e))
+
+            # Use the mtime of any file with sub second precision
+            return stat.st_mtime_ns
 
         if recalculate or self._key is None:
             fullpath = self.get_absolute_path()