fix: read error while invoking formatter API from Python

The `pyo3-file` crate doesn't accept buffers that are shorter than 4 bytes (https://github.com/omerbenamram/pyo3-file/blob/415def8ebdc93899aa130cc23d75afa8baf0349b/src/lib.rs#L102-L106), this causes an issue when an empty vector is passed to the `read` function implemented by `pyo3-file`. This happens at https://github.com/VirusTotal/yara-x/blob/18e2707e2824440ba7878ee6e80308daf75c631c/fmt/src/lib.rs#L344.

The issue is fixed by forcing the vector to have some initial capacity larger than 4 bytes.

Closes #299

fmt/src/lib.rs

@@ -49,11 +49,11 @@ mod tests;
 #[allow(clippy::large_enum_variant)]
 pub enum Error {
     /// Error while reading from input.
-    #[error("read error")]
+    #[error("read error: {0}")]
     ReadError(io::Error),
 
     /// Error while writing to output.
-    #[error("write error")]
+    #[error("write error: {0}")]
     WriteError(io::Error),
 
     /// The input file contained invalid UTF-8.
@@ -339,7 +339,7 @@ impl Formatter {
         W: io::Write,
     {
         let mut invalid_utf8 = Option::None;
-        let mut in_buf = Vec::new();
+        let mut in_buf = Vec::with_capacity(256);
 
         input.read_to_end(&mut in_buf).map_err(Error::ReadError)?;
 

py/tests/test_api.py

@@ -275,12 +275,22 @@ def callback(msg):
   scanner.scan(b'')
   assert ok
 
+
 def test_format():
-    import io
-    expected_output = "rule test {\n  condition:\n    true\n}\n"
-    inp = io.StringIO("rule test {condition: true}")
-    output = io.StringIO()
-    fmt = yara_x.Formatter()
-    fmt.format(inp, output)
-    result = output.getvalue()
-    assert result == expected_output
+  import io
+  expected_output = (
+      '''rule test {
+  strings:
+    $a = "foo"
+
+  condition:
+    $a at 0
+}
+''')
+  inp = io.StringIO(
+      'rule test { strings: $a = "foo" condition: $a at 0 }')
+  output = io.StringIO()
+  fmt = yara_x.Formatter()
+  fmt.format(inp, output)
+  result = output.getvalue()
+  assert result == expected_output