feat(tvix/cli): Add derivation file dumping functionality r/8482

Provides a derivation file dumping functionality for tvix-cli that can
be used when passing the --drv-dumpdir CLI arg to tvix-cli.

This will dump all the known derivation files into the specified
directory, making it easier to debug derivation divergences between Tvix
generated drvs and the drvs generated by Nix.

Supersedes: https://cl.tvl.fyi/c/depot/+/11265

Change-Id: I0e10b26eba22032b84ac543af0d4150ad87aed3e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12192
Autosubmit: Ilan Joselevich <personal@ilanjoselevich.com>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

1 files changed, 11 insertions, 0 deletions

diff --git a/tvix/cli/src/args.rs b/tvix/cli/src/args.rs
index 5b8f6b48b134..36f9a6a262dd 100644
--- a/tvix/cli/src/args.rs
+++ b/tvix/cli/src/args.rs
@@ -72,4 +72,15 @@ pub struct Args {
 
     #[arg(long, env, default_value = "dummy://")]
     pub build_service_addr: String,
+
+    /// An optional path in which Derivations encountered during evaluation
+    /// are dumped into, after evaluation. If it doesn't exist, the directory is created.
+    ///
+    /// Files dumped there are named like they would show up in `/nix/store`,
+    /// if produced by Nix. Existing files are not overwritten.
+    ///
+    /// This is only for debugging and diffing purposes for post-eval inspection;
+    /// Tvix does not read from these.
+    #[clap(long)]
+    pub drv_dumpdir: Option<PathBuf>,
 }