[Buildsystem] Improve cache handling

Prevents cache issues by not purging cache before starting a build.
Splits cache purge related code from progress code and delays the purge
until after final build is done.

SConstruct

@@ -1092,7 +1092,6 @@ if "check_c_headers" in env:
             env.AppendUnique(CPPDEFINES=[headers[header]])
 
 
-# FIXME: This method mixes both cosmetic progress stuff and cache handling...
 methods.show_progress(env)
 # TODO: replace this with `env.Dump(format="json")`
 # once we start requiring SCons 4.0 as min version.
@@ -1124,3 +1123,5 @@ def purge_flaky_files():
 
 
 atexit.register(purge_flaky_files)
+
+methods.clean_cache(env)

methods.py

@@ -906,21 +906,18 @@ def show_progress(env):
     node_count_fname = str(env.Dir("#")) + "/.scons_node_count"
 
     import math
-    import time
 
     class cache_progress:
-        # The default is 1 GB cache and 12 hours half life
-        def __init__(self, path=None, limit=1073741824, half_life=43200):
+        # The default is 1 GB cache
+        def __init__(self, path=None, limit=pow(1024, 3)):
             self.path = path
             self.limit = limit
-            self.exponent_scale = math.log(2) / half_life
             if env["verbose"] and path is not None:
                 screen.write(
                     "Current cache limit is {} (used: {})\n".format(
                         self.convert_size(limit), self.convert_size(self.get_size(path))
                     )
                 )
-            self.delete(self.file_list())
 
         def __call__(self, node, *args, **kw):
             nonlocal node_count, node_count_max, node_count_interval, node_count_fname, show_progress
@@ -937,12 +934,66 @@ def show_progress(env):
                     screen.write("\r[Initial build] ")
                     screen.flush()
 
+        def convert_size(self, size_bytes):
+            if size_bytes == 0:
+                return "0 bytes"
+            size_name = ("bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB")
+            i = int(math.floor(math.log(size_bytes, 1024)))
+            p = math.pow(1024, i)
+            s = round(size_bytes / p, 2)
+            return "%s %s" % (int(s) if i == 0 else s, size_name[i])
+
+        def get_size(self, start_path="."):
+            total_size = 0
+            for dirpath, dirnames, filenames in os.walk(start_path):
+                for f in filenames:
+                    fp = os.path.join(dirpath, f)
+                    total_size += os.path.getsize(fp)
+            return total_size
+
+    def progress_finish(target, source, env):
+        nonlocal node_count, progressor
+        try:
+            with open(node_count_fname, "w", encoding="utf-8", newline="\n") as f:
+                f.write("%d\n" % node_count)
+        except Exception:
+            pass
+
+    try:
+        with open(node_count_fname, "r", encoding="utf-8") as f:
+            node_count_max = int(f.readline())
+    except Exception:
+        pass
+
+    cache_directory = os.environ.get("SCONS_CACHE")
+    # Simple cache pruning, attached to SCons' progress callback. Trim the
+    # cache directory to a size not larger than cache_limit.
+    cache_limit = float(os.getenv("SCONS_CACHE_LIMIT", 1024)) * 1024 * 1024
+    progressor = cache_progress(cache_directory, cache_limit)
+    Progress(progressor, interval=node_count_interval)
+
+    progress_finish_command = Command("progress_finish", [], progress_finish)
+    AlwaysBuild(progress_finish_command)
+
+
+def clean_cache(env):
+    import atexit
+    import time
+
+    class cache_clean:
+        def __init__(self, path=None, limit=pow(1024, 3)):
+            self.path = path
+            self.limit = limit
+
+        def clean(self):
+            self.delete(self.file_list())
+
         def delete(self, files):
             if len(files) == 0:
                 return
             if env["verbose"]:
                 # Utter something
-                screen.write("\rPurging %d %s from cache...\n" % (len(files), len(files) > 1 and "files" or "file"))
+                print("Purging %d %s from cache..." % (len(files), "files" if len(files) > 1 else "file"))
             [os.remove(f) for f in files]
 
         def file_list(self):
@@ -976,35 +1027,10 @@ def show_progress(env):
             else:
                 return [x[0] for x in file_stat[mark:]]
 
-        def convert_size(self, size_bytes):
-            if size_bytes == 0:
-                return "0 bytes"
-            size_name = ("bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB")
-            i = int(math.floor(math.log(size_bytes, 1024)))
-            p = math.pow(1024, i)
-            s = round(size_bytes / p, 2)
-            return "%s %s" % (int(s) if i == 0 else s, size_name[i])
-
-        def get_size(self, start_path="."):
-            total_size = 0
-            for dirpath, dirnames, filenames in os.walk(start_path):
-                for f in filenames:
-                    fp = os.path.join(dirpath, f)
-                    total_size += os.path.getsize(fp)
-            return total_size
-
-    def progress_finish(target, source, env):
-        nonlocal node_count, progressor
+    def cache_finally():
+        nonlocal cleaner
         try:
-            with open(node_count_fname, "w", encoding="utf-8", newline="\n") as f:
-                f.write("%d\n" % node_count)
-            progressor.delete(progressor.file_list())
-        except Exception:
-            pass
-
-    try:
-        with open(node_count_fname, "r", encoding="utf-8") as f:
-            node_count_max = int(f.readline())
+            cleaner.clean()
         except Exception:
             pass
 
@@ -1012,11 +1038,9 @@ def show_progress(env):
     # Simple cache pruning, attached to SCons' progress callback. Trim the
     # cache directory to a size not larger than cache_limit.
     cache_limit = float(os.getenv("SCONS_CACHE_LIMIT", 1024)) * 1024 * 1024
-    progressor = cache_progress(cache_directory, cache_limit)
-    Progress(progressor, interval=node_count_interval)
+    cleaner = cache_clean(cache_directory, cache_limit)
 
-    progress_finish_command = Command("progress_finish", [], progress_finish)
-    AlwaysBuild(progress_finish_command)
+    atexit.register(cache_finally)
 
 
 def dump(env):