efficient build

- postpone building until really needed
- rebuild only if artifacts change
- no build on source update
- prune takes current resolver state instead of global var

lektor_groupby/pruner.py

@@ -2,29 +2,36 @@
 Static collector for build-artifact urls.
 All non-tracked VPATH-urls will be pruned after build.
 '''
-from lektor.builder import Builder  # typing
 from lektor.reporter import reporter  # report_pruned_artifact
 from lektor.utils import prune_file_and_folder
-
-_cache = set()
-# Note: this var is static or otherwise two instances of
-#       this module would prune each others artifacts.
+from typing import TYPE_CHECKING, Set, Iterable
+if TYPE_CHECKING:
+    from lektor.builder import Builder
 
 
-def track_not_prune(url: str) -> None:
-    ''' Add url to build cache to prevent pruning. '''
-    _cache.add(url.lstrip('/'))
+def _normalize_url_cache(url_cache: Iterable[str]) -> Set[str]:
+    cache = set()
+    for url in url_cache:
+        if url.endswith('/'):
+            url += 'index.html'
+        cache.add(url.lstrip('/'))
+    return cache
 
 
-def prune(builder: Builder, vpath: str) -> None:
-    ''' Remove previously generated, unreferenced Artifacts. '''
+def prune(builder: 'Builder', vpath: str, url_cache: Iterable[str]) -> None:
+    '''
+    Remove previously generated, unreferenced Artifacts.
+    All urls in url_cache must have a trailing "/index.html" (instead of "/")
+    and also, no leading slash, "blog/index.html" instead of "/blog/index.html"
+    '''
     vpath = '@' + vpath.lstrip('@')  # just in case of user error
     dest_path = builder.destination_path
+    url_cache = _normalize_url_cache(url_cache)
     con = builder.connect_to_database()
     try:
         with builder.new_build_state() as build_state:
             for url, file in build_state.iter_artifacts():
-                if url.lstrip('/') in _cache:
+                if url.lstrip('/') in url_cache:
                     continue  # generated in this build-run
                 infos = build_state.get_artifact_dependency_infos(url, [])
                 for artifact_name, _ in infos:
@@ -36,4 +43,3 @@ def prune(builder: Builder, vpath: str) -> None:
                     break  # there is only one VPATH-entry per source
     finally:
         con.close()
-    _cache.clear()