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.

crates/bundles/build.rs

@@ -0,0 +1,35 @@
+// Copyright 2016-2021 the Tectonic Project
+// Licensed under the MIT License.
+
+use std::env;
+
+/// The current hardcoded default prefix for tectonic's web bundle.
+const TECTONIC_WEB_BUNDLE_PREFIX_DEFAULT: &str = "https://relay.fullyjustified.net";
+
+/// Environment variable names to look for the bundle URLs.
+const LOCKED_VAR: &str = "TECTONIC_WEB_BUNDLE_LOCKED";
+const PREFIX_VAR: &str = "TECTONIC_WEB_BUNDLE_PREFIX";
+
+/// Sets the environment variables for the default web bundle.
+///
+/// `${TECTONIC_WEB_BUNDLE_PREFIX}` would lead to a url in the form of
+/// `${TECTONIC_WEB_BUNDLE_PREFIX}/default_bundle.tar`, while the optional
+/// "locked" url, `${TECTONIC_WEB_BUNDLE_LOCKED}`, can be used to pin the
+/// default bundle to a specific version if specified. This would be useful
+/// for reproducible builds.
+fn web_bundle_presets() {
+    // load from env
+    let web_bundle_locked = env::var(LOCKED_VAR).unwrap_or("".into());
+    let web_bundle_prefix = match env::var(PREFIX_VAR) {
+        Ok(x) if !x.is_empty() => x,
+        _ => TECTONIC_WEB_BUNDLE_PREFIX_DEFAULT.into(),
+    };
+
+    // export to rustc
+    println!("cargo:rustc-env={LOCKED_VAR}={web_bundle_locked}");
+    println!("cargo:rustc-env={PREFIX_VAR}={web_bundle_prefix}");
+}
+
+fn main() {
+    web_bundle_presets();
+}

crates/bundles/src/lib.rs

@@ -267,12 +267,23 @@ pub fn detect_bundle(
 /// 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 `bundles/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())
+        .expect("TECTONIC_WEB_BUNDLE_PREFIX must be defined at compile time");
+
+    // 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")
     }
 }