Add function replace_prefix_nullterm for use on mach-o rpaths. (#15347)

This recovers the old behavior of replace_prefix_bin that was
modified to work with elf binaries by prefixing os.sep to new prefix
until length is the same as old prefix.

lib/spack/spack/relocate.py

@@ -400,8 +400,8 @@ def replace_prefix_text(path_name, old_dir, new_dir):
 def replace_prefix_bin(path_name, old_dir, new_dir):
     """
     Attempt to replace old install prefix with new install prefix
-    in binary files by replacing with null terminated string
-    that is the same length unless the old path is shorter
+    in binary files by prefixing new install prefix with os.sep
+    until the lengths of the prefixes are the same.
     """
 
     def replace(match):
@@ -429,6 +429,38 @@ def replace(match):
         f.truncate()
 
 
+def replace_prefix_nullterm(path_name, old_dir, new_dir):
+    """
+    Attempt to replace old install prefix with new install prefix
+    in binary files by replacing with null terminated string
+    that is the same length unless the old path is shorter
+    Used on linux to replace mach-o rpaths
+    """
+
+    def replace(match):
+        occurances = match.group().count(old_dir.encode('utf-8'))
+        olen = len(old_dir.encode('utf-8'))
+        nlen = len(new_dir.encode('utf-8'))
+        padding = (olen - nlen) * occurances
+        if padding < 0:
+            return data
+        return match.group().replace(old_dir.encode('utf-8'),
+                                     new_dir.encode('utf-8')) + b'\0' * padding
+    with open(path_name, 'rb+') as f:
+        data = f.read()
+        f.seek(0)
+        original_data_len = len(data)
+        pat = re.compile(old_dir.encode('utf-8') + b'([^\0]*?)\0')
+        if not pat.search(data):
+            return
+        ndata = pat.sub(replace, data)
+        if not len(ndata) == original_data_len:
+            raise BinaryStringReplacementException(
+                path_name, original_data_len, len(ndata))
+        f.write(ndata)
+        f.truncate()
+
+
 def relocate_macho_binaries(path_names, old_dir, new_dir, allow_root):
     """
     Change old_dir to new_dir in LC_RPATH of mach-o files (on macOS)
@@ -466,8 +498,7 @@ def relocate_macho_binaries(path_names, old_dir, new_dir, allow_root):
             modify_object_macholib(path_name, placeholder, new_dir)
             modify_object_macholib(path_name, old_dir, new_dir)
         if len(new_dir) <= len(old_dir):
-            replace_prefix_bin(path_name, old_dir,
-                               new_dir)
+            replace_prefix_nullterm(path_name, old_dir, new_dir)
         else:
             tty.warn('Cannot do a binary string replacement'
                      ' with padding for %s'