refactor(tvix/nix-compat): -derivation::Hash, +NixHash r/6009

This stops using our own custom Hash structure, which was mostly only
used because we had to parse the JSON representation somehow.

Since cl/8217, there's a `NixHash` struct, which is better suited to
hold this data. Converting the format requires a bit of serde labor
though, but that only really matters when interacting with JSON
representations (which we mostly don't).

Change-Id: Idc5ee511e36e6726c71f66face8300a441b0bf4c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8304
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 111 insertions, 1 deletions

diff --git a/tvix/nix-compat/src/nixhash.rs b/tvix/nix-compat/src/nixhash.rs
index 4cb076ed16b0..0da90fe4ccf5 100644
--- a/tvix/nix-compat/src/nixhash.rs
+++ b/tvix/nix-compat/src/nixhash.rs
@@ -1,14 +1,124 @@
 use data_encoding::{BASE64, BASE64_NOPAD, HEXLOWER};
+use serde::ser::SerializeMap;
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use std::fmt::Display;
 use thiserror::Error;
 
 use crate::nixbase32;
 
+/// A Nix Hash can either be flat or recursive.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum NixHashWithMode {
+    Flat(NixHash),
+    Recursive(NixHash),
+}
+
+impl NixHashWithMode {
+    /// Formats a [NixHashWithMode] in the Nix default hash format,
+    /// which is the algo, followed by a colon, then the lower hex encoded digest.
+    /// In case the hash itself is recursive, a `r:` is added as prefix
+    pub fn to_nix_hash_string(&self) -> String {
+        match self {
+            NixHashWithMode::Flat(h) => h.to_nix_hash_string(),
+            NixHashWithMode::Recursive(h) => format!("r:{}", h.to_nix_hash_string()),
+        }
+    }
+}
+
+impl Serialize for NixHashWithMode {
+    /// map a NixHashWithMode into the serde data model.
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        let mut map = serializer.serialize_map(Some(2))?;
+        match self {
+            NixHashWithMode::Flat(h) => {
+                map.serialize_entry("hash", &nixbase32::encode(&h.digest))?;
+                map.serialize_entry("hashAlgo", &h.algo.to_string())?;
+            }
+            NixHashWithMode::Recursive(h) => {
+                map.serialize_entry("hash", &nixbase32::encode(&h.digest))?;
+                map.serialize_entry("hashAlgo", &format!("r:{}", &h.algo.to_string()))?;
+            }
+        };
+        map.end()
+    }
+}
+
+impl<'de> Deserialize<'de> for NixHashWithMode {
+    /// map the serde data model into a NixHashWithMode.
+    ///
+    /// The serde data model has a `hash` field (containing a digest in nixbase32),
+    /// and a `hashAlgo` field, containing the stringified hash algo.
+    /// In case the hash is recursive, hashAlgo also has a `r:` prefix.
+    ///
+    /// This is to match how `nix show-derivation` command shows them in JSON
+    /// representation.
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        // TODO: don't use serde_json here?
+        // TODO: serde seems to simply set `hash_with_mode` to None if hash
+        // and hashAlgo fail, but that should be a proper deserialization error
+        // that should be propagated to the user!
+
+        let json = serde_json::Value::deserialize(deserializer)?;
+        match json.as_object() {
+            None => Err(serde::de::Error::custom("couldn't parse as map"))?,
+            Some(map) => {
+                let digest: Vec<u8> = {
+                    if let Some(v) = map.get("hash") {
+                        if let Some(s) = v.as_str() {
+                            data_encoding::HEXLOWER
+                                .decode(s.as_bytes())
+                                .map_err(|e| serde::de::Error::custom(e.to_string()))?
+                        } else {
+                            return Err(serde::de::Error::custom(
+                                "couldn't parse 'hash' as string",
+                            ));
+                        }
+                    } else {
+                        return Err(serde::de::Error::custom("couldn't extract 'hash' key"));
+                    }
+                };
+
+                if let Some(v) = map.get("hashAlgo") {
+                    if let Some(s) = v.as_str() {
+                        match s.strip_prefix("r:") {
+                            Some(rest) => Ok(NixHashWithMode::Recursive(NixHash::new(
+                                HashAlgo::try_from(rest).map_err(|e| {
+                                    serde::de::Error::custom(format!("unable to parse algo: {}", e))
+                                })?,
+                                digest,
+                            ))),
+                            None => Ok(NixHashWithMode::Flat(NixHash::new(
+                                HashAlgo::try_from(s).map_err(|e| {
+                                    serde::de::Error::custom(format!("unable to parse algo: {}", e))
+                                })?,
+                                digest,
+                            ))),
+                        }
+                    } else {
+                        Err(serde::de::Error::custom(
+                            "couldn't parse 'hashAlgo' as string",
+                        ))
+                    }
+                } else {
+                    Err(serde::de::Error::custom("couldn't extract 'hashAlgo' key"))
+                }
+            }
+        }
+    }
+}
+
 /// Nix allows specifying hashes in various encodings, and magically just
 /// derives the encoding.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct NixHash {
     pub digest: Vec<u8>,
+
     pub algo: HashAlgo,
 }
 
@@ -26,7 +136,7 @@ impl NixHash {
 }
 
 /// This are the hash algorithms supported by cppnix.
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
 pub enum HashAlgo {
     Md5,
     Sha1,