Set default log directives only when RUST_LOG is not set

Summary:
Normally, [log directives are set using the RUST_LOG environment variable](https://docs.rs/env_logger/latest/env_logger/#enabling-logging). We have some default log directives we want to set in supertd, and we currently set them unconditionally, which is surprising to someone expecting directives they assign to the RUST_LOG envvar to work! Instead, only set the default log directives when RUST_LOG is not set.

This diff also moves the place where we set the default log directives into init_tracing, so that we have uniform logging behavior across td binaries.

Reviewed By: Acesine

Differential Revision: D53673935

fbshipit-source-id: 5e5564a8c9c42488312f6220302c0310b631a560

btd/bin/main.rs

@@ -11,6 +11,6 @@ use td_util::cli::parse_args;
 use td_util::tracing::init_tracing;
 
 pub fn main() -> anyhow::Result<()> {
-    init_tracing(vec![]);
+    init_tracing();
     btd::main(parse_args()?)
 }

supertd/bin/main.rs

@@ -35,16 +35,7 @@ pub fn main(fb: FacebookInit) -> anyhow::Result<()> {
     // Avoid a warning in open source code
     let _unused_in_oss = fb;
 
-    // Enable info log by default, debug log for target determinator packages
-    init_tracing(vec![
-        "info".parse()?,
-        "btd=debug".parse()?,
-        "citadel=debug".parse()?,
-        "ranker=debug".parse()?,
-        "scheduler=debug".parse()?,
-        "targets=debug".parse()?,
-        "verse=debug".parse()?,
-    ]);
+    init_tracing();
 
     let mut command = Args::command();
     if std::env::var_os("SUPERTD_IGNORE_EXTRA_ARGUMENTS") == Some("1".into()) {

targets/bin/main.rs

@@ -11,6 +11,6 @@ use td_util::cli::parse_args;
 use td_util::tracing::init_tracing;
 
 pub fn main() -> anyhow::Result<()> {
-    init_tracing(vec![]);
+    init_tracing();
     targets::main(parse_args()?)
 }

td_util/src/tracing.rs

@@ -14,19 +14,30 @@ use std::io::stderr;
 use std::io::stdout;
 use std::io::IsTerminal;
 
-use tracing::metadata::LevelFilter;
-use tracing_subscriber::filter::Directive;
 use tracing_subscriber::filter::EnvFilter;
 use tracing_subscriber::layer::SubscriberExt;
 use tracing_subscriber::util::SubscriberInitExt;
 use tracing_subscriber::Layer;
 
 /// Set up tracing so it prints to stderr, and can be used for output.
 /// Most things should use `info` and `debug` level for showing messages.
-pub fn init_tracing(directives: Vec<Directive>) {
-    let mut env_filter = EnvFilter::from_default_env().add_directive(LevelFilter::DEBUG.into());
-    for directive in directives {
-        env_filter = env_filter.add_directive(directive);
+pub fn init_tracing() {
+    let mut env_filter = EnvFilter::from_default_env();
+    if std::env::var_os("RUST_LOG").is_none() {
+        // Enable info log by default, debug log for target determinator packages
+        let directives = vec![
+            "info",
+            "btd=debug",
+            "citadel=debug",
+            "ranker=debug",
+            "scheduler=debug",
+            "targets=debug",
+            "verse=debug",
+        ];
+        for directive in directives {
+            env_filter =
+                env_filter.add_directive(directive.parse().expect("bad hardcoded log directive"));
+        }
     }
 
     let layer = tracing_subscriber::fmt::layer()