fix(expandvars, LP: #2055348: replace bash shell expansion with custo…

…m routine (#222)

landscape/lib/format.py

@@ -1,5 +1,5 @@
 import inspect
-import subprocess
+import re
 
 
 def format_object(object):
@@ -35,19 +35,31 @@ def format_percent(percent):
 def expandvars(pattern: str, **kwargs) -> str:
     """Expand the pattern by replacing the params with values in `kwargs`.
 
-    This uses bash shell parameter expansion. Here are some examples of
-    possible patterns:
+    This implements a small subset of shell parameter expansion and the
+    patterns can only be in the following forms:
         - ${parameter}
         - ${parameter:offset} - start at `offset` to the end
         - ${parameter:offset:length} - start at `offset` to `offset + length`
+    For simplicity, `offset` and `length` MUST be positive values.
     """
-    values = {k: str(v) for k, v in kwargs.items()}
-    shell = subprocess.run(
-        f'echo -n "{pattern.lower()}"',
-        text=True,
-        capture_output=True,
-        shell=True,
-        executable="/usr/bin/bash",
-        env=values,
+    regex = re.compile(
+        r"\$\{([a-zA-Z][a-zA-Z0-9]*)(?::([0-9]+))?(?::([0-9]+))?\}",
+        re.MULTILINE,
     )
-    return shell.stdout
+    values = {k: str(v) for k, v in kwargs.items()}
+
+    def _replace(match):
+        param = match.group(1)
+        result = values[param.lower()]
+
+        offset, length = match.group(2), match.group(3)
+        if offset:
+            start = int(offset)
+            end = None
+            if length:
+                end = start + int(length)
+            return result[start:end]
+
+        return result
+
+    return re.sub(regex, _replace, pattern)

landscape/lib/tests/test_format.py

@@ -81,7 +81,7 @@ def test_expand_without_offset_and_length(self):
             "f315cab5:after",
         )
         self.assertEqual(
-            expandvars("be\\$fore:${serial}:after", serial="f315cab5"),
+            expandvars("be$fore:${serial}:after", serial="f315cab5"),
             "be$fore:f315cab5:after",
         )
 
@@ -100,7 +100,7 @@ def test_expand_with_offset(self):
         )
         self.assertEqual(
             expandvars(
-                "be\\$fore:${serial:7}:after",
+                "be$fore:${serial:7}:after",
                 serial="01234567890abcdefgh",
             ),
             "be$fore:7890abcdefgh:after",
@@ -121,7 +121,7 @@ def test_expand_with_offset_and_length(self):
         )
         self.assertEqual(
             expandvars(
-                "be\\$fore:${serial:7:2}:after",
+                "be$fore:${serial:7:2}:after",
                 serial="01234567890abcdefgh",
             ),
             "be$fore:78:after",