skip stdenv rebuilds

Before attempting builds check if the stdenv for this branch is
available in the cache.  If this is not the case then either the merge
request is a mass rebuild or it's targeting a branch like staging.

Author: LnL7

ofborg/src/nix.rs

@@ -4,12 +4,15 @@ use crate::ofborg::partition_result;
 
 use std::collections::HashMap;
 use std::env;
+use std::error::Error;
 use std::ffi::OsStr;
 use std::fmt;
 use std::fs;
+use std::io;
 use std::io::{BufRead, BufReader, Seek, SeekFrom};
 use std::path::Path;
 use std::process::{Command, Stdio};
+use tracing::{debug, info};
 
 use tempfile::tempfile;
 
@@ -137,6 +140,47 @@ impl Nix {
         n
     }
 
+    pub fn safely_query_cache_for_attr(
+        &self,
+        nixpkgs: &Path,
+        file: File,
+        attr: String,
+    ) -> Result<bool, Box<dyn Error>> {
+        let mut command = self.safe_command::<&OsStr>(&Operation::Instantiate, nixpkgs, &[], &[]);
+        self.set_attrs_command(&mut command, file, vec![attr]);
+        let output = command
+            .stderr(Stdio::piped())
+            .stdout(Stdio::piped())
+            .output()?;
+        debug!("{}", String::from_utf8(output.stderr)?.trim());
+
+        let drv = String::from_utf8(output.stdout)?;
+        let output = Command::new("nix-store")
+            .args(&["-q", "--binding", "out"])
+            .arg(drv.trim())
+            .stderr(Stdio::piped())
+            .stdout(Stdio::piped())
+            .output()?;
+        debug!("{}", String::from_utf8(output.stderr)?.trim());
+        if !output.status.success() {
+            let err = io::Error::new(io::ErrorKind::Other, "Could not evaluate stdenv");
+            return Err(Box::new(err));
+        }
+
+        let out = String::from_utf8(output.stdout)?;
+        info!("stdenv {}", out);
+        let output = Command::new("nix-store")
+            .args(&["--option", "store", "https://cache.nixos.org"])
+            .args(&["-q", "--size"])
+            .arg(out.trim())
+            .stderr(Stdio::piped())
+            .stdout(Stdio::null())
+            .output()?;
+        debug!("{}", String::from_utf8(output.stderr)?.trim());
+
+        Ok(output.status.success())
+    }
+
     pub fn safely_partition_instantiable_attrs(
         &self,
         nixpkgs: &Path,

ofborg/src/tasks/build.rs

@@ -325,10 +325,7 @@ impl notifyworker::SimpleNotifyWorker for BuildWorker {
             return;
         }
 
-        info!(
-            "Got path: {:?}, determining which ones we can build ",
-            refpath
-        );
+        info!("Determining which attributes we can build");
         let (can_build, cannot_build) = self.nix.safely_partition_instantiable_attrs(
             refpath.as_ref(),
             buildfile,
@@ -341,6 +338,25 @@ impl notifyworker::SimpleNotifyWorker for BuildWorker {
             .map(|(attr, _)| attr)
             .collect();
 
+        info!("Checking for stdenv rebuild");
+        match self.nix.safely_query_cache_for_attr(
+            refpath.as_ref(),
+            buildfile,
+            String::from("stdenv"),
+        ) {
+            Ok(false) => {
+                info!(
+                    "Skip build: '{}', Cannot build: '{}'",
+                    can_build.join(", "),
+                    cannot_build_attrs.join(", ")
+                );
+                actions.build_not_attempted(cannot_build_attrs);
+                return;
+            }
+            Ok(true) => (),
+            Err(err) => error!("Failed to detect stdenv rebuild: {:?}", err),
+        }
+
         info!(
             "Can build: '{}', Cannot build: '{}'",
             can_build.join(", "),