[WIP] Undefined variable value yields '.' as node and may cause removing all files in tree.

CHANGES.txt

@@ -17,6 +17,10 @@ RELEASE  VERSION/DATE TO BE FILLED IN LATER
       "Multiple ways to build the same target were specified for:".  Now mingw will disable
       creating the symlinks (and adding version string to ) dlls.  It sets SHLIBNOVERSIONSYMLINKS,
       IMPLIBNOVERSIONSYMLINKS and LDMODULENOVERSIONSYMLINKS to True.
+    - Fix Issue #2801 - Handle calling Clean('$XYZ','$ABC') when either env['XYZ'] or env['ABC']
+      is undefined as that would yield either argument as '.'. This will lead to cleaning the wrong
+      (or all) files potentially when you request the wrong (or any target).
+
 
   From Daniel Moody:
     - Update CacheDir to use uuid for tmpfile uniqueness instead of pid.

SCons/Environment.py

@@ -32,17 +32,15 @@
 
 import copy
 import os
-import sys
 import re
 import shlex
+import sys
 from collections import UserDict
 
 import SCons.Action
 import SCons.Builder
 import SCons.Debug
-from SCons.Debug import logInstanceCreation
 import SCons.Defaults
-from SCons.Errors import UserError, BuildError
 import SCons.Memoize
 import SCons.Node
 import SCons.Node.Alias
@@ -54,6 +52,8 @@
 import SCons.Subst
 import SCons.Tool
 import SCons.Warnings
+from SCons.Debug import logInstanceCreation
+from SCons.Errors import UserError, BuildError
 from SCons.Util import (
     AppendPath,
     CLVar,
@@ -74,6 +74,7 @@
     uniquer_hashables,
 )
 
+
 class _Null:
     pass
 
@@ -2000,6 +2001,19 @@ def CacheDir(self, path):
 
     def Clean(self, targets, files):
         global CleanTargets
+
+        # Check for anything which evaluates to empty string, which would yield cleaning '.'
+        targets_strings = [(t, self.subst(t)) for t in flatten(targets) if is_String(t) and ('$' in t or t == '')]
+        files_strings = [(t, self.subst(t)) for t in flatten(files) if is_String(t) and ('$' in t or t == '')]
+        if any([s == '' for t, s in targets_strings]):
+            raise UserError(
+                "Targets specified to Clean() include one which evaluates to an empty string: [%s]" % ",".join(
+                    ["%s='%s'" % (str(t), s) for (t, s) in targets_strings]))
+        if any([s == '' for t, s in files_strings]):
+            raise UserError(
+                "Targets specified to Clean() include one which evaluates to an empty string: [%s]" % ",".join(
+                    ["%s='%s'" % (str(t), s) for (t, s) in files_strings]))
+
         tlist = self.arg2nodes(targets, self.fs.Entry)
         flist = self.arg2nodes(files, self.fs.Entry)
         for t in tlist: