Fix isolated environment scripts path on Debian

The scripts path was looked up passing explicitly the scheme to be
used using "nt" on Windows and "posix_prefix" everywhere else.
However, when the isolated build environment is created, packages are
installed using the default scheme for the platform. On most platforms
this works because normally "nt" and "posix_prefix" are the default
schemes.

However, Debian customizes sysconfig to use a "posix_local" scheme by
default and under this scheme the scripts path does not match the one
of the "posix_prefix" scheme. This results in scripts installed as
part of the build dependencies not to be found during the build, as
reported here mesonbuild/meson-python#109
and here https://bugs.debian.org/1019293.

The problem can be solved omitting to specify a scheme when looking up
the scripts path. To future proof the path lookup, use the "venv"
scheme if available as done in pypa#11598. For uniformity use similar
functions as used to lookup the library paths.

news/11623.bugfix.rst

@@ -0,0 +1 @@
+Fix scripts path in isolated build environment on Debian.

src/pip/_internal/build_env.py

@@ -19,6 +19,7 @@
 from pip import __file__ as pip_location
 from pip._internal.cli.spinners import open_spinner
 from pip._internal.locations import (
+    get_isolated_environment_bin_path,
     get_isolated_environment_lib_paths,
     get_platlib,
     get_purelib,
@@ -37,10 +38,7 @@ class _Prefix:
     def __init__(self, path: str) -> None:
         self.path = path
         self.setup = False
-        self.bin_dir = get_paths(
-            "nt" if os.name == "nt" else "posix_prefix",
-            vars={"base": path, "platbase": path},
-        )["scripts"]
+        self.bin_dir = get_isolated_environment_bin_path(path)
         self.lib_dirs = get_isolated_environment_lib_paths(path)
 
 

src/pip/_internal/locations/__init__.py

@@ -28,6 +28,7 @@
     "get_major_minor_version",
     "get_platlib",
     "get_isolated_environment_lib_paths",
+    "get_isolated_environment_bin_path",
     "get_purelib",
     "get_scheme",
     "get_src_prefix",
@@ -526,3 +527,7 @@ def get_isolated_environment_lib_paths(prefix: str) -> List[str]:
         _log_context(prefix=prefix)
 
     return old_lib_paths
+
+
+def get_isolated_environment_bin_path(prefix: str) -> str:
+    return _sysconfig.get_isolated_environment_bin_path(prefix)

src/pip/_internal/locations/_sysconfig.py

@@ -220,3 +220,12 @@ def get_isolated_environment_lib_paths(prefix: str) -> typing.Tuple[str, str]:
     else:
         paths = sysconfig.get_paths(vars=vars)
     return (paths["purelib"], paths["platlib"])
+
+
+def get_isolated_environment_bin_path(prefix: str) -> str:
+    vars = {"base": prefix, "platbase": prefix}
+    if "venv" in sysconfig.get_scheme_names():
+        paths = sysconfig.get_paths(vars=vars, scheme="venv")
+    else:
+        paths = sysconfig.get_paths(vars=vars)
+    return paths["scripts"]