From 3774a9d8d487384b1bd75d75da738ff2d3d04bfe Mon Sep 17 00:00:00 2001
From: Evan Rittenhouse <erittenhouse@cloudflare.com>
Date: Tue, 6 Feb 2024 08:07:56 -0600
Subject: [PATCH] telemetry: add capability to forward `log` facade logs to
 Foundations

---
 Cargo.toml                           |  3 ++
 foundations/Cargo.toml               |  6 +++
 foundations/src/telemetry/log/mod.rs | 57 ++++++++++++++++++++++++++++
 3 files changed, 66 insertions(+)

diff --git a/Cargo.toml b/Cargo.toml
index f4ab584..d2d2f27 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -38,6 +38,7 @@ governor = "0.6"
 hyper = { version = "0.14", default-features = false }
 indexmap = "2.0.0"
 ipnetwork = "0.20"
+log = "0.4.20"
 once_cell = "1.5"
 parking_lot = "0.12"
 proc-macro2 = { version = "1", default-features = false }
@@ -61,6 +62,8 @@ slab = "0.4.9"
 slog = "2.4"
 slog-async = "2.3"
 slog-json = "2.3"
+slog-scope = "4.4.0"
+slog-stdlog = "4.1.1"
 slog-term = "2.4"
 tempfile = "3.7"
 tokio = "1.31.0"
diff --git a/foundations/Cargo.toml b/foundations/Cargo.toml
index cdb743a..f11d775 100644
--- a/foundations/Cargo.toml
+++ b/foundations/Cargo.toml
@@ -98,10 +98,13 @@ telemetry-server = [
 # Enables logging functionality.
 logging = [
     "dep:governor",
+    "dep:log",
     "dep:once_cell",
     "dep:parking_lot",
     "dep:slog-async",
     "dep:slog-json",
+    "dep:slog-scope",
+    "dep:slog-stdlog",
     "dep:slog-term",
     "dep:slog",
     "dep:thread_local",
@@ -162,6 +165,7 @@ hyper = { workspace = true, optional = true, features = [
     "server",
 ] }
 indexmap = { workspace = true, optional = true, features = ["serde"] }
+log = { workspace = true, optional = true }
 once_cell = { workspace = true, optional = true }
 parking_lot = { workspace = true, optional = true }
 prometheus = { workspace = true, optional = true, features = ["process"] }
@@ -179,6 +183,8 @@ slab = { workspace = true, optional = true }
 slog = { workspace = true, optional = true, features = ["max_level_trace"] }
 slog-async = { workspace = true, optional = true }
 slog-json = { workspace = true, optional = true }
+slog-scope = { workspace = true, optional = true }
+slog-stdlog = { workspace = true, optional = true }
 slog-term = { workspace = true, optional = true }
 socket2 = { workspace = true, optional = true }
 thread_local = { workspace = true, optional = true }
diff --git a/foundations/src/telemetry/log/mod.rs b/foundations/src/telemetry/log/mod.rs
index 731fbbd..76b6904 100644
--- a/foundations/src/telemetry/log/mod.rs
+++ b/foundations/src/telemetry/log/mod.rs
@@ -21,9 +21,14 @@ use self::internal::current_log;
 use crate::telemetry::log::init::build_log_with_drain;
 use crate::telemetry::settings::LogVerbosity;
 use crate::Result;
+use log::Level as std_level;
 use slog::{Level, Logger, OwnedKV};
+use slog_scope::{set_global_logger, GlobalLoggerGuard};
 use std::sync::Arc;
 
+static mut GLOBAL_LOGGER_GUARD: Option<GlobalLoggerGuard> = None;
+static GLOBAL_LOGGER_CAPTURE: parking_lot::Once = parking_lot::Once::new();
+
 #[cfg(any(test, feature = "testing"))]
 pub use self::testing::TestLogRecord;
 
@@ -59,6 +64,58 @@ pub fn slog_logger() -> Arc<parking_lot::RwLock<Logger>> {
     current_log()
 }
 
+/// Capture logs from the [`log`](https://docs.rs/log/latest/log/) crate by forwarding them into
+/// the current [`slog::Drain`]. Due to the intricacies and subtleties of this method, **you should
+/// be very careful when using it**. Note also that this method can only be called once.
+///
+/// # Note
+///
+/// After calling this method, all `log` logs will be forwarded to the [`slog::Drain`] in use for
+/// the rest of the program's lifetime.
+///
+/// # Examples
+/// ```
+/// use foundations::telemetry::TelemetryContext;
+/// use foundations::telemetry::log::capture_global_log_logs;
+/// use log::warn as log_warn;
+///
+/// let cx = TelemetryContext::test();
+/// capture_global_log_logs();
+/// for i in 0..16 {
+///     log_warn!("{}", i);
+/// }
+///
+/// assert_eq!(cx.log_records().len(), 16);
+/// ```
+pub fn capture_global_log_logs() {
+    unsafe {
+        // SAFETY: mutating a `static mut` is generally unsafe, but since we're guarding it behind
+        // a call_once, we should be fine.
+        GLOBAL_LOGGER_CAPTURE.call_once(|| {
+            let curr_logger = Arc::clone(&slog_logger()).read().clone();
+            let scope_guard = set_global_logger(curr_logger);
+
+            // Convert slog::Level from Foundations settings to log::Level
+            let normalized_level = match verbosity().0 {
+                Level::Critical | Level::Error => std_level::Error,
+                Level::Warning => std_level::Warn,
+                Level::Info => std_level::Info,
+                Level::Debug => std_level::Debug,
+                Level::Trace => std_level::Trace,
+            };
+
+            slog_stdlog::init_with_level(normalized_level).unwrap();
+
+            // Storing the scope guard in a static global guard means that logs will be forwarded
+            // to the slog::Drain for the entirety of the program's lifetime. This prevents users
+            // from accidentally calling the method twice and triggering log::SetLoggerErrors, or
+            // attempting to log messages after dropping the guard and triggering
+            // slog_scope::NoLoggerSet.
+            GLOBAL_LOGGER_GUARD = Some(scope_guard);
+        });
+    }
+}
+
 // NOTE: `#[doc(hidden)]` + `#[doc(inline)]` for `pub use` trick is used to prevent these macros
 // to show up in the crate's top level docs.