Speedup environment activation (#13557)

* Add a transaction around repeated calls to `spec.prefix` in the activation process
* cache the computation of home in the python package to speed up setting deps
* ensure that module-scope variables are only set *once* per module

lib/spack/spack/build_environment.py

@@ -422,6 +422,11 @@ def set_build_environment_variables(pkg, env, dirty):
 
 def _set_variables_for_single_module(pkg, module):
     """Helper function to set module variables for single module."""
+    # Put a marker on this module so that it won't execute the body of this
+    # function again, since it is not needed
+    marker = '_set_run_already_called'
+    if getattr(module, marker, False):
+        return
 
     jobs = spack.config.get('config:build_jobs') if pkg.parallel else 1
     jobs = min(jobs, multiprocessing.cpu_count())
@@ -489,6 +494,10 @@ def static_to_shared_library(static_lib, shared_lib=None, **kwargs):
 
     m.static_to_shared_library = static_to_shared_library
 
+    # Put a marker on this module so that it won't execute the body of this
+    # function again, since it is not needed
+    setattr(m, marker, True)
+
 
 def set_module_variables_for_package(pkg):
     """Populate the module scope of install() with some useful functions.

lib/spack/spack/environment.py

@@ -159,7 +159,8 @@ def activate(
             cmds += 'export PS1="%s ${PS1}";\n' % prompt
 
     if add_view and default_view_name in env.views:
-        cmds += env.add_default_view_to_shell(shell)
+        with spack.store.db.read_transaction():
+            cmds += env.add_default_view_to_shell(shell)
 
     return cmds
 
@@ -207,7 +208,8 @@ def deactivate(shell='sh'):
         cmds += 'fi;\n'
 
     if default_view_name in _active_environment.views:
-        cmds += _active_environment.rm_default_view_from_shell(shell)
+        with spack.store.db.read_transaction():
+            cmds += _active_environment.rm_default_view_from_shell(shell)
 
     tty.debug("Deactivated environmennt '%s'" % _active_environment.name)
     _active_environment = None

var/spack/repos/builtin/packages/python/package.py

@@ -163,6 +163,9 @@ class Python(AutotoolsPackage):
     _DISTUTIL_CACHE_FILENAME = 'sysconfig.json'
     _distutil_vars = None
 
+    # Used to cache home locations, since computing them might be expensive
+    _homes = {}
+
     # An in-source build with --enable-optimizations fails for python@3.X
     build_directory = 'spack-build'
 
@@ -622,8 +625,11 @@ def home(self):
         ``packages.yaml`` unknowingly. Query the python executable to
         determine exactly where it is installed."""
 
-        prefix = self.get_config_var('prefix')
-        return Prefix(prefix)
+        dag_hash = self.spec.dag_hash()
+        if dag_hash not in self._homes:
+            prefix = self.get_config_var('prefix')
+            self._homes[dag_hash] = Prefix(prefix)
+        return self._homes[dag_hash]
 
     @property
     def libs(self):