1 files changed, 68 insertions, 0 deletions
diff --git a/config/rebuild_check.py b/config/rebuild_check.py
new file mode 100644
index 0000000000..a906fdc304
--- /dev/null
+++ b/config/rebuild_check.py
@@ -0,0 +1,68 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import errno
+import os
+
+
+def mtime(path):
+    try:
+        return os.stat(path).st_mtime
+    except OSError as e:
+        if e.errno == errno.ENOENT:
+            return -1
+        raise
+
+
+def rebuild_check(args):
+    target = args[0]
+    deps = args[1:]
+    t = mtime(target)
+    if t < 0:
+        print(target)
+        return
+
+    newer = []
+    removed = []
+    for dep in deps:
+        deptime = mtime(dep)
+        if deptime < 0:
+            removed.append(dep)
+        elif deptime > t:
+            newer.append(dep)
+
+    def format_filelist(filelist):
+        if not filelist:
+            return None
+
+        limit = 5
+        length = len(filelist)
+        if length < limit:
+            return ", ".join(filelist)
+
+        truncated = filelist[:limit]
+        remaining = length - limit
+
+        return "%s (and %d other files)" % (", ".join(truncated), remaining)
+
+    newer = format_filelist(newer)
+    removed = format_filelist(removed)
+
+    if newer and removed:
+        print(
+            "Rebuilding %s because %s changed and %s was removed"
+            % (target, newer, removed)
+        )
+    elif newer:
+        print("Rebuilding %s because %s changed" % (target, newer))
+    elif removed:
+        print("Rebuilding %s because %s was removed" % (target, removed))
+    else:
+        print("Rebuilding %s for an unknown reason" % target)
+
+
+if __name__ == "__main__":
+    import sys
+
+    rebuild_check(sys.argv[1:])