Force NUM_JOBS to fixed value if SOURCE_DATE_EPOCH is defined

Fixes #80

src/environment.rs

@@ -1,6 +1,6 @@
 use crate::util::ArrayDisplay;
 use crate::{fmt_option_str, write_str_variable, write_variable};
-use std::{collections, env, ffi, fmt, fs, io, process};
+use std::{borrow, collections, env, ffi, fmt, fs, io, process};
 
 pub struct EnvironmentMap(collections::HashMap<String, String>);
 
@@ -115,7 +115,11 @@ impl EnvironmentMap {
             w,
             "NUM_JOBS",
             "u32",
-            env::var("NUM_JOBS").unwrap(),
+            if env::var(crate::SOURCE_DATE_EPOCH).is_ok() {
+                borrow::Cow::Borrowed("1")
+            } else {
+                borrow::Cow::Owned(env::var("NUM_JOBS").unwrap())
+            },
             "The parallelism that was specified during compilation."
         );
         write_variable!(

src/krono.rs

@@ -24,7 +24,7 @@ pub fn strptime(s: &str) -> chrono::DateTime<chrono::offset::Utc> {
 }
 
 fn get_source_date_epoch_from_env() -> Option<chrono::DateTime<chrono::offset::Utc>> {
-    match std::env::var("SOURCE_DATE_EPOCH") {
+    match std::env::var(crate::SOURCE_DATE_EPOCH) {
         Ok(val) => {
             let ts = match val.parse::<i64>() {
                 Ok(ts) => ts,

src/lib.rs

@@ -319,6 +319,13 @@ pub use environment::CIPlatform;
 #[allow(dead_code)]
 type _READMETEST = ();
 
+/// If `SOURCE_DATE_EPOCH` is defined, it's value is used instead of
+/// `chrono::..::now()` as `BUILT_TIME_UTC`.
+/// The presence of `SOURCE_DATE_EPOCH` also soft-indicates that a
+/// reproducible build is desired, which we may or may not be able
+/// to honor.
+const SOURCE_DATE_EPOCH: &str = "SOURCE_DATE_EPOCH";
+
 macro_rules! write_variable {
     ($writer:expr, $name:expr, $datatype:expr, $value:expr, $doc:expr) => {
         writeln!(

tests/testbox_tests.rs

@@ -517,6 +517,7 @@ mod built_info {
 fn main() {
     assert_eq!(built::util::strptime(built_info::BUILT_TIME_UTC).to_rfc2822(),
               "Sat, 25 May 2024 12:15:59 +0000");
+    assert_eq!(built_info::NUM_JOBS, 1);
     println!("builttestsuccess");
 }"#,
     );