feat(tvix/derivation): add nix drv path generation to Derivation

This adds a function to generate the derivation path. The computation
is based on the Go implementation.

Change-Id: Iae89db4976f5fd9208f0453f73688689a245cd66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7729
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/derivation/src/derivation.rs

@@ -1,7 +1,10 @@
+use crate::nix_hash;
 use crate::output::Output;
 use crate::write;
 use serde::{Deserialize, Serialize};
-use std::{collections::BTreeMap, fmt, fmt::Write};
+use sha2::{Digest, Sha256};
+use std::{collections::BTreeMap, fmt, fmt::Write, iter::FromIterator};
+use tvix_store::nixbase32::NIXBASE32;
 
 #[derive(Serialize, Deserialize)]
 pub struct Derivation {
@@ -31,6 +34,72 @@ impl Derivation {
 
         Ok(())
     }
+
+    /// Returns the path of a Derivation struct.
+    ///
+    /// The path is calculated like this:
+    ///   - Write the fingerprint of the Derivation to the sha256 hash function.
+    ///     This is: `text:`,
+    ///     all d.InputDerivations and d.InputSources (sorted, separated by a `:`),
+    ///     a `:`,
+    ///     a `sha256:`, followed by the sha256 digest of the ATerm representation (hex-encoded)
+    ///     a `:`,
+    ///     the storeDir, followed by a `:`,
+    ///     the name of a derivation,
+    ///     a `.drv`.
+    ///   - Write the .drv A-Term contents to a hash function
+    ///   - Take the digest, run hash.CompressHash(digest, 20) on it.
+    ///   - Encode it with nixbase32
+    ///   - Construct the full path $storeDir/$nixbase32EncodedCompressedHash-$name.drv
+    pub fn calculate_derivation_path(&self, name: &str) -> String {
+        let mut hasher = Sha256::new();
+
+        // collect the list of paths from input_sources and input_derivations
+        // into a sorted list, and join them by :
+        hasher.update(write::TEXT_COLON);
+
+        let concat_inputs: Vec<String> = {
+            let mut inputs = self.input_sources.clone();
+            let input_derivation_keys: Vec<String> =
+                self.input_derivations.keys().cloned().collect();
+            inputs.extend(input_derivation_keys);
+            inputs.sort();
+            inputs
+        };
+
+        for input in concat_inputs {
+            hasher.update(input);
+            hasher.update(write::COLON);
+        }
+
+        // calculate the sha256 hash of the ATerm representation, and represent
+        // it as a hex-encoded string (prefixed with sha256:).
+        hasher.update(write::SHA256_COLON);
+
+        let digest = {
+            let mut derivation_hasher = Sha256::new();
+            derivation_hasher.update(self.to_string());
+            derivation_hasher.finalize()
+        };
+
+        hasher.update(format!("{:x}", digest));
+        hasher.update(write::COLON);
+        hasher.update(write::STORE_PATH_COLON);
+        hasher.update(name);
+        hasher.update(write::DOT_FILE_EXT);
+
+        let compressed = {
+            let aterm_digest = Vec::from_iter(hasher.finalize());
+            nix_hash::compress_hash(&aterm_digest, 20)
+        };
+
+        format!(
+            "{}-{}{}",
+            NIXBASE32.encode(&compressed),
+            name,
+            write::DOT_FILE_EXT
+        )
+    }
 }
 
 impl fmt::Display for Derivation {