Add cfg attribute for advanced seekable impls and make seek table cre…

…ation safe (#328)

* Fix missing cfg attribute on AdvancedSeekable impls

The missing cfg macros on the impls caused a compilation error when
compiling with feature `seekable` but without `std`

* Make seek table creation safe

The `SeekTable::try_from_seekable()` fn segfaulted when called with an
uninitialized seekable in zstd versions prior to v1.5.7. This is fixed
with v1.5.7, so the fn is safe to use now.

zstd-safe/src/seekable.rs

@@ -455,7 +455,9 @@ pub struct AdvancedSeekable<'a, F> {
     src: *mut F,
 }
 
+#[cfg(feature = "std")]
 unsafe impl<F> Send for AdvancedSeekable<'_, F> where F: Send {}
+#[cfg(feature = "std")]
 unsafe impl<F> Sync for AdvancedSeekable<'_, F> where F: Sync {}
 
 #[cfg(feature = "std")]
@@ -597,17 +599,11 @@ unsafe impl Send for SeekTable {}
 unsafe impl Sync for SeekTable {}
 
 impl SeekTable {
-    // `ZSTD_seekTable_create_fromSeekable` causes a segmentation fault when called with an
-    // uninitialized Seekable. This function is safe once the issue is resolved and released
-    // upstream.
-    // See https://github.com/facebook/zstd/issues/4200 and https://github.com/facebook/zstd/pull/4201
     /// Try to create a `SeekTable` from a `Seekable`.
     ///
     /// Memory constrained use cases that manage multiple archives benefit from retaining
     /// multiple archive seek tables without retaining a `Seekable` instance for each.
-    ///
-    /// May cause a segmentation fault when called wih an uninitialized `Seekable`.
-    pub unsafe fn try_from_seekable<'a>(
+    pub fn try_from_seekable<'a>(
         value: &Seekable<'a>,
     ) -> Result<Self, SeekTableCreateError> {
         // Safety: Just FFI

zstd-safe/src/tests.rs

@@ -209,14 +209,19 @@ fn test_seekable_seek_table() {
 
     let seekable_archive = new_seekable_archive(INPUT);
     let mut seekable = Seekable::create();
+
+    // Assert that creating a SeekTable from an uninitialized seekable errors.
+    // This led to segfaults with zstd versions prior v1.5.7
+    assert!(SeekTable::try_from_seekable(&seekable).is_err());
+
     seekable
         .init_buff(&seekable_archive)
         .map_err(zstd_safe::get_error_name)
         .unwrap();
 
     // Try to create a seek table from the seekable
     let seek_table =
-        unsafe { SeekTable::try_from_seekable(&seekable).unwrap() };
+        { SeekTable::try_from_seekable(&seekable).unwrap() };
 
     // Seekable and seek table should return the same results
     assert_eq!(seekable.num_frames(), seek_table.num_frames());