Ensure the release binary doesn't block when the primary is unavailable

If the primary is in read-only mode then the leader database in
`DATABASE_URL` may be unavailable. To avoid blocking the release phase,
the command exits with an error message. The error will prevent deploys
and rollbacks (no change there), but Heroku will allow changes to config
vars even if the release fails.

Author: jtgeibel

src/admin/migrate.rs

@@ -1,4 +1,4 @@
-use anyhow::Error;
+use anyhow::{anyhow, Error};
 
 static CATEGORIES_TOML: &'static str = include_str!("../boot/categories.toml");
 diesel_migrations::embed_migrations!("./migrations");
@@ -8,7 +8,22 @@ diesel_migrations::embed_migrations!("./migrations");
 pub struct Opts;
 
 pub fn run(_opts: Opts) -> Result<(), Error> {
+    let config = crate::Config::default();
+
+    if config.db_primary_config.read_only_mode {
+        // The service is undergoing maintenance or mitigating an outage.
+        // Exit early to ensure we don't accidentally block the release phase.
+        //
+        // Heroku will update configuration variables even if the release command fails.
+        // This will prevent deploys while in this mode, even if there are no new migrations.
+        // https://devcenter.heroku.com/articles/release-phase#release-command-failure
+        return Err(anyhow!(
+            "Cannot run migrations or sync categories in read-only mode"
+        ));
+    }
+
     println!("==> migrating the database");
+    // The primary is online, access directly via `DATABASE_URL`.
     let conn = crate::db::connect_now()?;
     embedded_migrations::run_with_output(&conn, &mut std::io::stdout())?;
 

src/bin/crates-admin.rs

@@ -26,17 +26,17 @@ enum SubCommand {
     Migrate(migrate::Opts),
 }
 
-fn main() {
+fn main() -> anyhow::Result<()> {
     let opts: Opts = Opts::parse();
 
-    match opts.command {
+    Ok(match opts.command {
         SubCommand::DeleteCrate(opts) => delete_crate::run(opts),
         SubCommand::DeleteVersion(opts) => delete_version::run(opts),
         SubCommand::Populate(opts) => populate::run(opts),
         SubCommand::RenderReadmes(opts) => render_readmes::run(opts),
-        SubCommand::TestPagerduty(opts) => test_pagerduty::run(opts).unwrap(),
+        SubCommand::TestPagerduty(opts) => test_pagerduty::run(opts)?,
         SubCommand::TransferCrates(opts) => transfer_crates::run(opts),
         SubCommand::VerifyToken(opts) => verify_token::run(opts).unwrap(),
-        SubCommand::Migrate(opts) => migrate::run(opts).unwrap(),
-    }
+        SubCommand::Migrate(opts) => migrate::run(opts)?,
+    })
 }