Reject unreopenable source before checked save stages files

crates/oa-core/src/persistence.rs

@@ -3,6 +3,9 @@ use serde::{Deserialize, Serialize};
 use std::{io::Write, path::Path};
 use thiserror::Error;
 mod json_codec;
+#[cfg(test)]
+#[path = "persistence/source_save_tests.rs"]
+mod source_save_tests;
 #[path = "migration_v2/mod.rs"]
 pub(crate) mod v2;
 pub(crate) use v2::adapter::migrate as migrate_v2_limited;
@@ -948,19 +951,54 @@ fn save_checked_profile(
     expected: Option<&[u8]>,
     profile: json_codec::Profile,
     depth_128: bool,
+) -> Result<Vec<u8>, FileError> {
+    save_checked_profile_with_source(path, p, expected, profile, depth_128, |project| {
+        crate::language::format(project).into_bytes()
+    })
+}
+// Keep the source writer replaceable only at this private seam so regressions
+// can prove that malformed or lossy output never reaches filesystem staging.
+fn save_checked_profile_with_source(
+    path: &Path,
+    p: &Project,
+    expected: Option<&[u8]>,
+    profile: json_codec::Profile,
+    depth_128: bool,
+    source: impl FnOnce(&Project) -> Vec<u8>,
 ) -> Result<Vec<u8>, FileError> {
     validate_project(p)?;
-    let mut bytes = if path
+    let is_source = path
         .extension()
-        .is_some_and(|ext| ext.eq_ignore_ascii_case("arcadia"))
-    {
-        crate::language::format(p).into_bytes()
+        .is_some_and(|ext| ext.eq_ignore_ascii_case("arcadia"));
+    let mut bytes = if is_source {
+        source(p)
     } else {
         json_codec::encode(p, profile, depth_128)?
     };
     if !bytes.ends_with(b"\n") {
         bytes.push(b'\n');
     }
+    if is_source {
+        // Check exactly the final bytes, including their newline, through the
+        // public reader before creating a temp file or replacing a destination.
+        let decoded = decode_with_report_limited(
+            &bytes,
+            crate::language::ParseLimits {
+                max_source_bytes: profile.bytes,
+                ..crate::language::ParseLimits::UNLIMITED
+            },
+        )?;
+        if decoded.project != *p {
+            return Err(FileError::Invalid(
+                "Generated source does not reproduce the complete requested project.".into(),
+            ));
+        }
+        if decoded.migration.is_some() {
+            return Err(FileError::Invalid(
+                "Generated source unexpectedly requires migration.".into(),
+            ));
+        }
+    }
     let parent = path
         .parent()
         .filter(|p| !p.as_os_str().is_empty())