environment_modifications_for_specs: do not mutate spec.prefix (#41737)

Sometimes env variables computed in `setup_run_environment` depend on tests
w.r.t. files in `spec.prefix`, but Spack temporarily projects `spec.prefix` to
the view. 

This is problematic for two reasons:

1. Some packages iterate over `<prefix>/bin`: they expect only the current
   package's executables, but find all linked in the view, leading to false
   positives.
2. Some packages test for `os.path.islink(...)`, which is always true in a view

`gcc` is an example that does both.

This PR lets Spack compute the environment modifications using the original
prefix, and projects to the view afterwards

lib/spack/spack/user_environment.py

@@ -3,18 +3,14 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 import os
+import re
 import sys
-from contextlib import contextmanager
-from typing import Callable
-
-from llnl.util.lang import nullcontext
 
 import spack.build_environment
 import spack.config
 import spack.error
 import spack.spec
 import spack.util.environment as environment
-import spack.util.prefix as prefix
 from spack import traverse
 from spack.context import Context
 
@@ -70,22 +66,6 @@ def unconditional_environment_modifications(view):
     return env
 
 
-@contextmanager
-def projected_prefix(*specs: spack.spec.Spec, projection: Callable[[spack.spec.Spec], str]):
-    """Temporarily replace every Spec's prefix with projection(s)"""
-    prefixes = dict()
-    for s in traverse.traverse_nodes(specs, key=lambda s: s.dag_hash()):
-        if s.external:
-            continue
-        prefixes[s.dag_hash()] = s.prefix
-        s.prefix = prefix.Prefix(projection(s))
-
-    yield
-
-    for s in traverse.traverse_nodes(specs, key=lambda s: s.dag_hash()):
-        s.prefix = prefixes.get(s.dag_hash(), s.prefix)
-
-
 def environment_modifications_for_specs(
     *specs: spack.spec.Spec, view=None, set_package_py_globals: bool = True
 ):
@@ -102,16 +82,12 @@ def environment_modifications_for_specs(
             been built on a different but compatible OS)
     """
     env = environment.EnvironmentModifications()
-    topo_ordered = traverse.traverse_nodes(specs, root=True, deptype=("run", "link"), order="topo")
+    topo_ordered = list(
+        traverse.traverse_nodes(specs, root=True, deptype=("run", "link"), order="topo")
+    )
 
-    if view:
-        maybe_projected = projected_prefix(*specs, projection=view.get_projection_for_spec)
-    else:
-        maybe_projected = nullcontext()
-
-    with maybe_projected:
     # Static environment changes (prefix inspections)
-        for s in reversed(list(topo_ordered)):
+    for s in reversed(topo_ordered):
         static = environment.inspect_path(
             s.prefix, prefix_inspections(s.platform), exclude=environment.is_system_path
         )
@@ -121,7 +97,21 @@ def environment_modifications_for_specs(
     setup_context = spack.build_environment.SetupContext(*specs, context=Context.RUN)
     if set_package_py_globals:
         setup_context.set_all_package_py_globals()
-        dynamic = setup_context.get_env_modifications()
+    env.extend(setup_context.get_env_modifications())
-        env.extend(dynamic)
+
+    # Apply view projections if any.
+    if view:
+        prefix_to_prefix = {
+            s.prefix: view.get_projection_for_spec(s)
+            for s in reversed(topo_ordered)
+            if not s.external
+        }
+        # Avoid empty regex if all external
+        if not prefix_to_prefix:
+            return env
+        prefix_regex = re.compile("|".join(re.escape(p) for p in prefix_to_prefix.keys()))
+        for mod in env.env_modifications:
+            if isinstance(mod, environment.NameValueModifier):
+                mod.value = prefix_regex.sub(lambda m: prefix_to_prefix[m.group(0)], mod.value)
 
     return env