Read from env variables for the default web bundle

This commit makes it easy to override the default web bundle through the
environment variables:

- `TECTONIC_WEB_BUNDLE_PREFIX`
- `TECTONIC_WEB_BUNDLE_LOCKED`

The PREFIX variable makes it easy for people to track a mirrored bundle,
while the LOCKED variable ensures reproducible builds across all CLIs.

build.rs

@@ -7,7 +7,17 @@ fn main() {
     // Re-export $TARGET during the build so that our executable tests know
     // what environment variable CARGO_TARGET_@TARGET@_RUNNER to check when
     // they want to spawn off executables.
-
     let target = env::var("TARGET").unwrap();
     println!("cargo:rustc-env=TARGET={target}");
+
+    // Set the default web bundle for tectonic to use.
+    // The `${TECTONIC_WEB_BUNDLE_PREFIX}` would lead to a url in the form of
+    // `${TECTONIC_WEB_BUNDLE_PREFIX}/default_bundle.tar`, while the "locked"
+    // url, if specified, can be used to pin the default bundle to a specific
+    // version. This would be useful for reproducible builds. Empty variables
+    // lead to a hardcoded fallback, currently defined in `crates/bundles/src/lib.rs`.
+    let web_bundle_prefix = env::var("TECTONIC_WEB_BUNDLE_PREFIX").unwrap_or("".to_owned());
+    let web_bundle_locked = env::var("TECTONIC_WEB_BUNDLE_LOCKED").unwrap_or("".to_owned());
+    println!("cargo:rustc-env=TECTONIC_WEB_BUNDLE_PREFIX={web_bundle_prefix}");
+    println!("cargo:rustc-env=TECTONIC_WEB_BUNDLE_LOCKED={web_bundle_locked}");
 }

crates/bundles/src/lib.rs

@@ -112,12 +112,23 @@ impl<B: Bundle + ?Sized> Bundle for Box<B> {
 /// low-level reliability problems and was blocked in China. We now use a custom
 /// webservice.
 pub fn get_fallback_bundle_url(format_version: u32) -> String {
+    // Build time environment variables declared in `build.rs`:
+    let web_bundle_locked = option_env!("TECTONIC_WEB_BUNDLE_LOCKED").unwrap_or("");
+    let web_bundle_prefix = option_env!("TECTONIC_WEB_BUNDLE_PREFIX")
+        .filter(|x| !x.is_empty())
+        .unwrap_or("https://relay.fullyjustified.net");
+
+    // Simply return the locked url when it is specified:
+    if !web_bundle_locked.is_empty() {
+        return web_bundle_locked.to_owned();
+    }
+
     // Format version 32 (TeXLive 2021) was when we introduced versioning to the
     // URL.
     if format_version < 32 {
-        "https://relay.fullyjustified.net/default_bundle.tar".to_owned()
+        format!("{web_bundle_prefix}/default_bundle.tar")
     } else {
-        format!("https://relay.fullyjustified.net/default_bundle_v{format_version}.tar")
+        format!("{web_bundle_prefix}/default_bundle_v{format_version}.tar")
     }
 }