--- a/rust/hg-core/src/revlog/index.rs	Tue Dec 07 18:12:13 2021 +0000
+++ b/rust/hg-core/src/revlog/index.rs	Tue Dec 07 18:57:43 2021 +0000
@@ -84,6 +84,7 @@
     /// Offsets of starts of index blocks.
     /// Only needed when the index is interleaved with data.
     offsets: Option<Vec<usize>>,
+    uses_generaldelta: bool,
 }
 
 impl Index {
@@ -100,6 +101,11 @@
             return Err(HgError::corrupted("unsupported revlog version"));
         }
 
+        // This is only correct because we know version is REVLOGV1.
+        // In v2 we always use generaldelta, while in v0 we never use
+        // generaldelta. Similar for [is_inline] (it's only used in v1).
+        let uses_generaldelta = header.format_flags().uses_generaldelta();
+
         if header.format_flags().is_inline() {
             let mut offset: usize = 0;
             let mut offsets = Vec::new();
@@ -119,6 +125,7 @@
                 Ok(Self {
                     bytes,
                     offsets: Some(offsets),
+                    uses_generaldelta,
                 })
             } else {
                 Err(HgError::corrupted("unexpected inline revlog length")
@@ -128,10 +135,15 @@
             Ok(Self {
                 bytes,
                 offsets: None,
+                uses_generaldelta,
             })
         }
     }
 
+    pub fn uses_generaldelta(&self) -> bool {
+        self.uses_generaldelta
+    }
+
     /// Value of the inline flag.
     pub fn is_inline(&self) -> bool {
         self.offsets.is_some()
@@ -259,7 +271,7 @@
     }
 
     /// Return the revision upon which the data has been derived.
-    pub fn base_revision(&self) -> Revision {
+    pub fn base_revision_or_base_of_delta_chain(&self) -> Revision {
         // TODO Maybe return an Option when base_revision == rev?
         //      Requires to add rev to IndexEntry
 
@@ -297,7 +309,7 @@
         offset: usize,
         compressed_len: usize,
         uncompressed_len: usize,
-        base_revision: Revision,
+        base_revision_or_base_of_delta_chain: Revision,
     }
 
     #[cfg(test)]
@@ -311,7 +323,7 @@
                 offset: 0,
                 compressed_len: 0,
                 uncompressed_len: 0,
-                base_revision: 0,
+                base_revision_or_base_of_delta_chain: 0,
             }
         }
 
@@ -350,8 +362,11 @@
             self
         }
 
-        pub fn with_base_revision(&mut self, value: Revision) -> &mut Self {
-            self.base_revision = value;
+        pub fn with_base_revision_or_base_of_delta_chain(
+            &mut self,
+            value: Revision,
+        ) -> &mut Self {
+            self.base_revision_or_base_of_delta_chain = value;
             self
         }
 
@@ -374,7 +389,9 @@
             bytes.extend(&[0u8; 2]); // Revision flags.
             bytes.extend(&(self.compressed_len as u32).to_be_bytes());
             bytes.extend(&(self.uncompressed_len as u32).to_be_bytes());
-            bytes.extend(&self.base_revision.to_be_bytes());
+            bytes.extend(
+                &self.base_revision_or_base_of_delta_chain.to_be_bytes(),
+            );
             bytes
         }
     }
@@ -480,14 +497,16 @@
     }
 
     #[test]
-    fn test_base_revision() {
-        let bytes = IndexEntryBuilder::new().with_base_revision(1).build();
+    fn test_base_revision_or_base_of_delta_chain() {
+        let bytes = IndexEntryBuilder::new()
+            .with_base_revision_or_base_of_delta_chain(1)
+            .build();
         let entry = IndexEntry {
             bytes: &bytes,
             offset_override: None,
         };
 
-        assert_eq!(entry.base_revision(), 1)
+        assert_eq!(entry.base_revision_or_base_of_delta_chain(), 1)
     }
 
     #[test]

--- a/rust/hg-core/src/revlog/revlog.rs	Tue Dec 07 18:12:13 2021 +0000
+++ b/rust/hg-core/src/revlog/revlog.rs	Tue Dec 07 18:57:43 2021 +0000
@@ -191,11 +191,20 @@
         // Todo return -> Cow
         let mut entry = self.get_entry(rev)?;
         let mut delta_chain = vec![];
-        while let Some(base_rev) = entry.base_rev {
+
+        // The meaning of `base_rev_or_base_of_delta_chain` depends on
+        // generaldelta. See the doc on `ENTRY_DELTA_BASE` in
+        // `mercurial/revlogutils/constants.py` and the code in
+        // [_chaininfo] and in [index_deltachain].
+        let uses_generaldelta = self.index.uses_generaldelta();
+        while let Some(base_rev) = entry.base_rev_or_base_of_delta_chain {
+            let base_rev = if uses_generaldelta {
+                base_rev
+            } else {
+                entry.rev - 1
+            };
             delta_chain.push(entry);
-            entry = self
-                .get_entry(base_rev)
-                .map_err(|_| RevlogError::corrupted())?;
+            entry = self.get_entry_internal(base_rev)?;
         }
 
         // TODO do not look twice in the index
@@ -291,14 +300,26 @@
             bytes: data,
             compressed_len: index_entry.compressed_len(),
             uncompressed_len: index_entry.uncompressed_len(),
-            base_rev: if index_entry.base_revision() == rev {
+            base_rev_or_base_of_delta_chain: if index_entry
+                .base_revision_or_base_of_delta_chain()
+                == rev
+            {
                 None
             } else {
-                Some(index_entry.base_revision())
+                Some(index_entry.base_revision_or_base_of_delta_chain())
             },
         };
         Ok(entry)
     }
+
+    /// when resolving internal references within revlog, any errors
+    /// should be reported as corruption, instead of e.g. "invalid revision"
+    fn get_entry_internal(
+        &self,
+        rev: Revision,
+    ) -> Result<RevlogEntry, RevlogError> {
+        return self.get_entry(rev).map_err(|_| RevlogError::corrupted());
+    }
 }
 
 /// The revlog entry's bytes and the necessary informations to extract
@@ -309,7 +330,7 @@
     bytes: &'a [u8],
     compressed_len: usize,
     uncompressed_len: usize,
-    base_rev: Option<Revision>,
+    base_rev_or_base_of_delta_chain: Option<Revision>,
 }
 
 impl<'a> RevlogEntry<'a> {
@@ -375,7 +396,7 @@
     /// Tell if the entry is a snapshot or a delta
     /// (influences on decompression).
     fn is_delta(&self) -> bool {
-        self.base_rev.is_some()
+        self.base_rev_or_base_of_delta_chain.is_some()
     }
 }
 

--- a/tests/test-rhg-no-generaldelta.t	Tue Dec 07 18:12:13 2021 +0000
+++ b/tests/test-rhg-no-generaldelta.t	Tue Dec 07 18:57:43 2021 +0000
@@ -1,4 +1,3 @@
-
   $ NO_FALLBACK="env RHG_ON_UNSUPPORTED=abort"
 
   $ cat << EOF >> $HGRCPATH
@@ -21,9 +20,27 @@
   1 0 prev
   2 1 prev
 
-rhg breaks on non-generaldelta revlogs:
+rhg works on non-generaldelta revlogs:
 
-  $ $NO_FALLBACK hg cat f -r . | f --sha256 --size
-  abort: corrupted revlog (rhg !)
-  size=0, sha256=e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (rhg !)
-  size=58, sha256=0cf0386dd4813cc3b957ea790146627dfc0ec42ad3fcf47221b9842e4d5764c1 (no-rhg !)
+  $ $NO_FALLBACK hg cat f -r .
+  1
+  2
+  3
+  4
+  5
+  6
+  7
+  8
+  9
+  10
+  11
+  12
+  13
+  14
+  15
+  16
+  17
+  18
+  19
+  20
+  footer