tracing: don't emit log events if a subscriber has been set

tracing/Cargo.toml

@@ -28,7 +28,7 @@ keywords = ["logging", "tracing", "metrics", "async"]
 edition = "2018"
 
 [dependencies]
-tracing-core = { version = "0.1.5", path = "../tracing-core", default-features = false }
+tracing-core = { version = "0.1.6", default-features = false }
 log = { version = "0.4", optional = true }
 tracing-attributes = "0.1.2"
 cfg-if = "0.1.9"

tracing/src/lib.rs

@@ -577,10 +577,16 @@
 //!
 //! * A set of features controlling the [static verbosity level].
 //! * `log`: causes trace instrumentation points to emit [`log`] records as well
-//!   as trace events. This is intended for use in libraries whose users may be
-//!   using either `tracing` or `log`.
+//!   as trace events, if a default `tracing` subscriber has not been set. This
+//!   is intended for use in libraries whose users may be using either `tracing`
+//!   or `log`.
 //!   **Note:** `log` support will not work when `tracing` is renamed in `Cargo.toml`,
 //!   due to oddities in macro expansion.
+//! * `log-always`: Emit `log` records from all `tracing` spans and events, even
+//!   a `tracing` subscriber has been set. This should be set only by
+//!   applications which intend to collect traces and logs separately; if an
+//!   adapter is used to convert `log` records into `tracing` events, this will
+//!   cause duplicate events to occur.
 //! * `std`: Depend on the Rust standard library (enabled by default).
 //!
 //!   `no_std` users may disable this feature with `default-features = false`:
@@ -669,8 +675,6 @@ pub mod subscriber;
 pub mod __macro_support {
     pub use crate::stdlib::sync::atomic::{AtomicUsize, Ordering};
 
-    pub const LOG_ENABLED: bool = cfg!(feature = "log");
-
     #[cfg(feature = "std")]
     pub use crate::stdlib::sync::Once;
 

tracing/src/macros.rs

@@ -2291,23 +2291,43 @@ macro_rules! __mk_format_args {
 #[macro_export]
 macro_rules! __tracing_log {
     (target: $target:expr, $level:expr, $($field:tt)+ ) => {
-        use $crate::log;
-        let level = $crate::level_to_log!(&$level);
-        if level <= log::STATIC_MAX_LEVEL {
-            let log_meta = log::Metadata::builder()
-                .level(level)
-                .target($target)
-                .build();
-            let logger = log::logger();
-            if logger.enabled(&log_meta) {
-                logger.log(&log::Record::builder()
-                    .file(Some(file!()))
-                    .module_path(Some(module_path!()))
-                    .line(Some(line!()))
-                    .metadata(log_meta)
-                    .args($crate::__mk_format_args!($($field)+))
-                    .build());
+        if $crate::__tracing_should_log!() {
+            use $crate::log;
+            let level = $crate::level_to_log!(&$level);
+            if level <= log::STATIC_MAX_LEVEL {
+                let log_meta = log::Metadata::builder()
+                    .level(level)
+                    .target($target)
+                    .build();
+                let logger = log::logger();
+                if logger.enabled(&log_meta) {
+                    logger.log(&log::Record::builder()
+                        .file(Some(file!()))
+                        .module_path(Some(module_path!()))
+                        .line(Some(line!()))
+                        .metadata(log_meta)
+                        .args($crate::__mk_format_args!($($field)+))
+                        .build());
+                }
             }
         }
     };
 }
+
+#[cfg(all(feature = "log", not(feature = "log-always")))]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __tracing_should_log {
+    () => {
+        !$crate::dispatcher::has_been_set()
+    };
+}
+
+#[cfg(all(feature = "log", feature = "log-always"))]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __tracing_should_log {
+    () => {
+        true
+    };
+}

tracing/src/span.rs

@@ -386,6 +386,20 @@ pub struct Entered<'a> {
 
 // ===== impl Span =====
 
+macro_rules! if_log {
+    ($e:expr;) => {
+        if_log! { $e }
+    };
+    ($if_log:expr) => {
+        #[cfg(feature = "log")]
+        {
+            if __tracing_should_log!() {
+                $if_log
+            }
+        }
+    };
+}
+
 impl Span {
     /// Constructs a new `Span` with the given [metadata] and set of
     /// [field values].
@@ -505,8 +519,9 @@ impl Span {
             meta: Some(meta),
         };
 
-        #[cfg(feature = "log")]
-        span.log(format_args!("++ {}; {}", meta.name(), FmtAttrs(attrs)));
+        if_log! {
+            span.log(format_args!("++ {}; {}", meta.name(), FmtAttrs(attrs)));
+        }
 
         span
     }
@@ -588,8 +603,7 @@ impl Span {
             inner.subscriber.enter(&inner.id);
         }
 
-        #[cfg(feature = "log")]
-        {
+        if_log! {
             if let Some(ref meta) = self.meta {
                 self.log(format_args!("-> {}", meta.name()));
             }
@@ -690,8 +704,7 @@ impl Span {
             inner.record(&record);
         }
 
-        #[cfg(feature = "log")]
-        {
+        if_log! {
             if let Some(ref meta) = self.meta {
                 self.log(format_args!("{}; {}", meta.name(), FmtValues(&record)));
             }
@@ -839,13 +852,12 @@ impl Drop for Span {
             ref subscriber,
         }) = self.inner
         {
-            if subscriber.try_close(id.clone()) {
-                #[cfg(feature = "log")]
-                {
-                    if let Some(ref meta) = self.meta {
-                        self.log(format_args!("-- {}", meta.name()));
-                    }
-                }
+            subscriber.try_close(id.clone());
+        }
+
+        if_log! {
+            if let Some(ref meta) = self.meta {
+                self.log(format_args!("-- {}", meta.name()));
             }
         }
     }
@@ -924,8 +936,7 @@ impl<'a> Drop for Entered<'a> {
             inner.subscriber.exit(&inner.id);
         }
 
-        #[cfg(feature = "log")]
-        {
+        if_log! {
             if let Some(ref meta) = self.span.meta {
                 self.span.log(format_args!("<- {}", meta.name()));
             }