sdk: Fix reading strings

Fatshark has a few weird string fields, where they provide a length
field, but then sometimes write a shorter, NUL-terminated string into
that same field and adding padding up to the "advertised" length.
To properly read those strings, we can't rely on just the length field
anymore, but need to check for a NUL, too.

lib/sdk/src/binary.rs

@@ -43,6 +43,7 @@ impl<T: FromBinary> FromBinary for Vec<T> {
 }
 
 pub mod sync {
+    use std::ffi::CStr;
     use std::io::{self, Read, Seek, SeekFrom};
 
     use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
@@ -165,25 +166,13 @@ pub mod sync {
         }
 
         fn read_string_len(&mut self, len: usize) -> Result<String> {
-            let mut buf = vec![0; len];
+            let pos = self.stream_position();
-            let res = self
-                .read_exact(&mut buf)
-                .map_err(Report::new)
-                .and_then(|_| {
-                    String::from_utf8(buf).map_err(|err| {
-                        let ascii = String::from_utf8_lossy(err.as_bytes()).to_string();
-                        let bytes = format!("{:?}", err.as_bytes());
-                        Report::new(err)
-                            .with_section(move || bytes.header("Bytes:"))
-                            .with_section(move || ascii.header("ASCII:"))
-                    })
-                });
 
+            let res = read_string_len(self, len);
             if res.is_ok() {
                 return res;
             }
 
-            let pos = self.stream_position();
             if pos.is_ok() {
                 res.with_section(|| {
                     format!("{pos:#X} ({pos})", pos = pos.unwrap()).header("Position: ")
@@ -243,4 +232,22 @@ pub mod sync {
 
         Err(err).with_section(|| format!("{pos:#X} ({pos})").header("Position: "))
     }
+
+    fn read_string_len(mut r: impl Read, len: usize) -> Result<String> {
+        let mut buf = vec![0; len];
+        r.read_exact(&mut buf)
+            .wrap_err_with(|| format!("Failed to read {} bytes", len))?;
+
+        let res = match CStr::from_bytes_until_nul(&buf) {
+            Ok(s) => {
+                let s = s.to_str()?;
+                Ok(s.to_string())
+            }
+            Err(_) => String::from_utf8(buf.clone()).map_err(Report::new),
+        };
+
+        res.wrap_err("Invalid binary for UTF8 string")
+            .with_section(|| format!("{}", String::from_utf8_lossy(&buf)).header("ASCI:"))
+            .with_section(|| format!("{:x?}", buf).header("Bytes:"))
+    }
 }