pythongh-75229: limit ensurepip prefix overrides to explicit requests

Keep root-only installs using the interpreter's default prefix while
preserving the cross-build fix for Makefile-driven installs that pass an
explicit prefix.

Author: mcepl

Lib/ensurepip/__init__.py

@@ -165,32 +165,20 @@ def _bootstrap(*, root=None, upgrade=False, user=False,
         if verbosity:
             args += ["-" + "v" * verbosity]
 
+        if root:
+            args += ["--root", root]
+
         if user:
             # --user is mutually exclusive with --root/--prefix,
             # pip will enforce this.
             args += ["--user"]
-        else:
-            # Handle installation paths.
-            # If --root is given but not --prefix, we default to a prefix of "/"
-            # so that the install happens at the root of the --root directory.
-            # Otherwise, pip would use the configured sys.prefix, e.g.
-            # /usr/local, and install into ${root}/usr/local/.
-            effective_prefix = prefix
-            if root and not prefix:
-                effective_prefix = "/"
-
-            if root:
-                args += ["--root", root]
-
-            if effective_prefix:
-                args += ["--prefix", effective_prefix]
-
-                # Force the script shebang to point to the correct, final
-                # executable path. This is necessary when --root is used.
-                executable_path = (
-                    Path(effective_prefix) / "bin" / Path(sys.executable).name
-                )
-                args += ["--executable", os.fsdecode(executable_path)]
+        elif prefix:
+            args += ["--prefix", prefix]
+
+            # Force the script shebang to point to the correct, final
+            # executable path. This is necessary when --root is used.
+            executable_path = Path(prefix) / "bin" / Path(sys.executable).name
+            args += ["--executable", os.fsdecode(executable_path)]
 
         return _run_pip([*args, "pip"], [os.fsdecode(tmp_wheel_path)])
 

Lib/test/test_ensurepip.py

@@ -94,9 +94,7 @@ def test_bootstrapping_with_root(self):
         self.run_pip.assert_called_once_with(
             [
                 "install", "--no-cache-dir", "--no-index", "--find-links",
-                unittest.mock.ANY, "--root", "/foo/bar/", "--prefix", "/",
-                "--executable", unittest.mock.ANY,
-                "pip",
+                unittest.mock.ANY, "--root", "/foo/bar/", "pip",
             ],
             unittest.mock.ANY,
         )
@@ -112,6 +110,18 @@ def test_bootstrapping_with_prefix(self):
             unittest.mock.ANY,
         )
 
+    def test_bootstrapping_with_root_and_prefix(self):
+        ensurepip.bootstrap(root="/foo/root/", prefix="/foo/prefix/")
+        self.run_pip.assert_called_once_with(
+            [
+                "install", "--no-cache-dir", "--no-index", "--find-links",
+                unittest.mock.ANY, "--root", "/foo/root/",
+                "--prefix", "/foo/prefix/", "--executable",
+                unittest.mock.ANY, "pip",
+            ],
+            unittest.mock.ANY,
+        )
+
     def test_bootstrapping_with_user(self):
         ensurepip.bootstrap(user=True)