setup.py: introduce TRITON_OFFLINE_BUILD

python/setup.py

@@ -113,6 +113,22 @@ def get_env_with_keys(key: list):
     return ""
 
 
+def is_offline_build() -> bool:
+    """
+    Downstream projects and distributions which bootstrap their own dependencies from scratch
+    and run builds in offline sandboxes
+    may set `TRITON_OFFLINE_BUILD` in the build environment to prevent any attempts at downloading
+    pinned dependencies from the internet or at using dependencies vendored in-tree.
+
+    Dependencies must be defined using respective search paths (cf. `syspath_var_name` in `Package`).
+    Missing dependencies lead to an early abortion.
+    Dependencies' compatibility is not verified.
+
+    Note that this flag isn't tested by the CI and does not provide any guarantees.
+    """
+    return os.environ.get("TRITON_OFFLINE_BUILD", "") != ""
+
+
 # --- third party packages -----
 
 
@@ -220,8 +236,14 @@ def get_thirdparty_packages(packages: list):
         if os.environ.get(p.syspath_var_name):
             package_dir = os.environ[p.syspath_var_name]
         version_file_path = os.path.join(package_dir, "version.txt")
-        if p.syspath_var_name not in os.environ and\
-           (not os.path.exists(version_file_path) or Path(version_file_path).read_text() != p.url):
+
+        input_defined = p.syspath_var_name not in os.environ
+        input_exists = input_defined and os.path.exists(version_file_path)
+        input_compatible = input_exists and Path(version_file_path).read_text() == p.url
+
+        if is_offline_build() and not input_defined:
+            raise RuntimeError(f"Requested an offline build but {p.syspath_var_name} is not set")
+        if not is_offline_build() and not input_compatible:
             with contextlib.suppress(Exception):
                 shutil.rmtree(package_root_dir)
             os.makedirs(package_root_dir, exist_ok=True)
@@ -245,6 +267,8 @@ def get_thirdparty_packages(packages: list):
 
 
 def download_and_copy(name, src_path, variable, version, url_func):
+    if is_offline_build():
+        return
     triton_cache_path = get_triton_cache_path()
     if variable in os.environ:
         return