Look up the top-level Kconfig file relative to $srctree

Due to an old design braino, the top-level Kconfig filename passed to
Kconfig.__init__() wasn't looked up relative to $srctree, breaking
out-of-tree usage for e.g. menuconfig. Fixing it required ugliness like

  srctree = os.environ.get("srctree", "")
  kconfiglib.Kconfig(os.path.join(srctree, "Kconfig"))

Change the behavior of Kconfig.__init__() to look up the top-level
Kconfig file relative to $srctree. This means that all Kconfig files
(both the top-level file and any source'd files) now use $srctree, which
makes the vast majority of scripts just work when running out-of-tree.

Also remove the note re. loading a subset of Kconfig files. Saying that
the top-level file and all source'd Kconfig files are looked up relative
to $srctree should make the behavior clear enough.

Print a note about the new behavior whenever the top-level Kconfig file
can't be opened, as this change could be breaking for some scripts.

This is a slight backwards-compatiblity break, so the major version will
be bumped.

kconfiglib.py

@@ -610,20 +610,14 @@ def __init__(self, filename="Kconfig", warn=True, warn_to_stderr=True,
           the right Kconfig is included from there (arch/$SRCARCH/Kconfig as of
           writing).
 
+          If $srctree is set, 'filename' will be looked up relative to it.
+          $srctree is also used to look up source'd files within Kconfig files.
+          See the class documentation.
+
           If you are using Kconfiglib via 'make scriptconfig', the filename of
           the base base Kconfig file will be in sys.argv[1]. It's currently
           always "Kconfig" in practice.
 
-          The $srctree environment variable is used to look up Kconfig files
-          referenced in Kconfig files if set. See the class documentation.
-
-          Note: '(o)source' statements in Kconfig files always work relative to
-          $srctree (or the current directory if $srctree is unset), even if
-          'filename' is a path with directories. This allows a subset of
-          Kconfig files to be loaded without breaking references to other
-          Kconfig files, e.g. by doing Kconfig("./sub/Kconfig"). sub/Kconfig
-          might expect to be sourced by ./Kconfig.
-
         warn (default: True):
           True if warnings related to this configuration should be generated.
           This can be changed later with Kconfig.enable/disable_warnings(). It
@@ -737,7 +731,7 @@ def __init__(self, filename="Kconfig", warn=True, warn_to_stderr=True,
         self.top_node.prompt = ("Main menu", self.y)
         self.top_node.parent = None
         self.top_node.dep = self.y
-        self.top_node.filename = os.path.relpath(filename, self.srctree)
+        self.top_node.filename = filename
         self.top_node.linenr = 1
 
         # Parse the Kconfig files
@@ -751,11 +745,17 @@ def __init__(self, filename="Kconfig", warn=True, warn_to_stderr=True,
         self._filestack = []
 
         # The current parsing location
-        self._filename = os.path.relpath(filename, self.srctree)
+        self._filename = filename
         self._linenr = 0
 
         # Open the top-level Kconfig file
-        self._file = self._open(filename, "r")
+        try:
+            self._file = self._open(os.path.join(self.srctree, filename), "r")
+        except IOError as e:
+            if self.srctree:
+                print(textwrap.fill(
+                    _INIT_SRCTREE_NOTE.format(self.srctree), 80))
+            raise
 
         try:
             # Parse everything
@@ -6014,3 +6014,12 @@ def _re_search(regex):
     GREATER:       ">",
     GREATER_EQUAL: ">=",
 }
+
+_INIT_SRCTREE_NOTE = """
+NOTE: Starting with Kconfiglib 10.0.0, the Kconfig filename passed to
+Kconfig.__init__() is looked up relative to the $srctree (which is set to '{}')
+instead of relative to the working directory. Previously, $srctree only applied
+to files being source'd within Kconfig files. This change makes running scripts
+out-of-tree work seamlessly, with no special coding required. Sorry for the
+backwards compatibility break!
+"""[1:]

testsuite.py

@@ -895,17 +895,17 @@ def verify_repr(item, s):
 <configuration with 14 symbols, main menu prompt "Main menu", srctree is current directory, config symbol prefix "CONFIG_", warnings disabled, printing of warnings to stderr enabled, undef. symbol assignment warnings disabled, redundant symbol assignment warnings enabled>
 """)
 
-    os.environ["srctree"] = "srctree value"
+    os.environ["srctree"] = "Kconfiglib"
     os.environ["CONFIG_"] = "CONFIG_ value"
 
-    c = Kconfig("Kconfiglib/tests/Krepr", warn=False)
+    c = Kconfig("tests/Krepr", warn=False)
     c.enable_warnings()
     c.disable_stderr_warnings()
     c.disable_redun_warnings()
     c.enable_undef_warnings()
 
     verify_repr(c, """
-<configuration with 14 symbols, main menu prompt "Main menu", srctree "srctree value", config symbol prefix "CONFIG_ value", warnings enabled, printing of warnings to stderr disabled, undef. symbol assignment warnings enabled, redundant symbol assignment warnings disabled>
+<configuration with 14 symbols, main menu prompt "Main menu", srctree "Kconfiglib", config symbol prefix "CONFIG_ value", warnings enabled, printing of warnings to stderr disabled, undef. symbol assignment warnings enabled, redundant symbol assignment warnings disabled>
 """)
 
     os.environ.pop("srctree", None)
@@ -987,7 +987,7 @@ def verify_locations(nodes, *expected_locs):
         os.environ["srctree"] = srctree
 
         # Has symbol with empty help text, so disable warnings
-        c = Kconfig("Kconfiglib/tests/Klocation", warn=False)
+        c = Kconfig("tests/Klocation", warn=False)
 
         verify_locations(c.syms["SINGLE_DEF"].nodes, "tests/Klocation:4")
 
@@ -1018,7 +1018,7 @@ def verify_locations(nodes, *expected_locs):
         # Test recursive 'source' detection
 
         try:
-            Kconfig("Kconfiglib/tests/Krecursive1")
+            Kconfig("tests/Krecursive1")
         except KconfigError:
             pass
         except:
@@ -1030,7 +1030,7 @@ def verify_locations(nodes, *expected_locs):
         # TODO: Make an exception test helper
 
         try:
-            Kconfig("Kconfiglib/tests/Kmissingsource")
+            Kconfig("tests/Kmissingsource")
         except KconfigError:
             pass
         except:
@@ -1039,7 +1039,7 @@ def verify_locations(nodes, *expected_locs):
             fail("'source' with missing file did not raise exception")
 
         try:
-            Kconfig("Kconfiglib/tests/Kmissingrsource")
+            Kconfig("tests/Kmissingrsource")
         except KconfigError:
             pass
         except:
@@ -1559,7 +1559,7 @@ def verify_range(sym_name, low, high, default):
            "defconfig_filename gave wrong file with $srctree unset")
 
     os.environ["srctree"] = "Kconfiglib/tests"
-    c = Kconfig("Kconfiglib/tests/Kdefconfig_srctree")
+    c = Kconfig("Kdefconfig_srctree")
     verify(c.defconfig_filename == "Kconfiglib/tests/sub/defconfig_in_sub",
            "defconfig_filename gave wrong file with $srctree set")