fix: avoid validating forest car twice in AnyCar::new

src/db/car/any.rs

@@ -34,8 +34,10 @@ impl<ReaderT: RandomAccessFileReader> AnyCar<ReaderT> {
     /// `.forest.car.zst`. This call may block for an indeterminate amount of
     /// time while data is decoded and indexed.
     pub fn new(reader: ReaderT) -> Result<Self> {
-        if super::ForestCar::is_valid(&reader) {
-            return Ok(AnyCar::Forest(super::ForestCar::new(reader)?));
+        if let Ok(validation_result) = super::ForestCar::validate_car(&reader) {
+            return Ok(
+                super::ForestCar::new_from_validation_result(reader, validation_result)?.into(),
+            );
         }
 
         // Maybe use a tempfile for this in the future.
@@ -46,7 +48,7 @@ impl<ReaderT: RandomAccessFileReader> AnyCar<ReaderT> {
         }
 
         if let Ok(plain_car) = super::PlainCar::new(reader) {
-            return Ok(AnyCar::Plain(plain_car));
+            return Ok(plain_car.into());
         }
         Err(Error::new(
             ErrorKind::InvalidData,

src/db/car/forest.rs

@@ -104,7 +104,14 @@ pub struct ForestCar<ReaderT> {
 
 impl<ReaderT: super::RandomAccessFileReader> ForestCar<ReaderT> {
     pub fn new(reader: ReaderT) -> io::Result<ForestCar<ReaderT>> {
-        let (header, index_start_pos, index_size_bytes) = Self::validate_car(&reader)?;
+        let validation_result = Self::validate_car(&reader)?;
+        Self::new_from_validation_result(reader, validation_result)
+    }
+
+    pub(super) fn new_from_validation_result(
+        reader: ReaderT,
+        (header, index_start_pos, index_size_bytes): (CarV1Header, u64, u64),
+    ) -> io::Result<ForestCar<ReaderT>> {
         let indexed = index::Reader::new(index::ZstdSkipFramesEncodedDataReader::new(
             positioned_io::Slice::new(reader, index_start_pos, Some(index_size_bytes)),
         )?)?;
@@ -138,7 +145,7 @@ impl<ReaderT: super::RandomAccessFileReader> ForestCar<ReaderT> {
         Self::validate_car(reader).is_ok()
     }
 
-    fn validate_car(reader: &ReaderT) -> io::Result<(CarV1Header, u64, u64)> {
+    pub(super) fn validate_car(reader: &ReaderT) -> io::Result<(CarV1Header, u64, u64)> {
         let mut cursor = SizeCursor::new(&reader);
         cursor.seek(SeekFrom::End(-(ForestCarFooter::SIZE as i64)))?;
         let index_end_pos = cursor.position();

src/db/car/many.rs

@@ -26,6 +26,7 @@ use fvm_ipld_blockstore::Blockstore;
 use parking_lot::RwLock;
 use std::cmp::Ord;
 use std::collections::BinaryHeap;
+use std::path::Path;
 use std::{path::PathBuf, sync::Arc};
 
 struct WithHeaviestEpoch {
@@ -124,12 +125,20 @@ impl<WriterT> ManyCar<WriterT> {
 
     pub fn read_only_files(&self, files: impl Iterator<Item = PathBuf>) -> anyhow::Result<()> {
         for file in files {
-            self.read_only(AnyCar::new(EitherMmapOrRandomAccessFile::open(file)?)?)?;
+            self.read_only_file(file)?;
         }
-
         Ok(())
     }
 
+    pub fn read_only_file(&self, file: impl AsRef<Path>) -> anyhow::Result<()> {
+        (|| {
+            self.read_only(AnyCar::new(EitherMmapOrRandomAccessFile::open(
+                file.as_ref(),
+            )?)?)
+        })()
+        .map_err(|e| anyhow::anyhow!("failed to load CAR at {}: {e}", file.as_ref().display()))
+    }
+
     pub fn heaviest_tipset_key(&self) -> anyhow::Result<Option<TipsetKey>> {
         Ok(self
             .read_only