subscriber: unify span traversal (#1445)

Forward-port of #1431 and #1434

* subscriber: unify span traversal (#1431)

## Motivation

Fixes #1429 (forwardport from v0.1.x branch)

## Solution

Implemented as described in #1429, and migrated all internal uses of the
deprecated methods. All tests passed both before and after the migration
(9ec8130).

- Add a new method `SpanRef::scope(&self)` that returns a leaf-to-root
  `Iterator`, including the specified leaf
- Add a new method `Scope::from_root(self)` (where `Scope` is the type
  returned by `SpanRef::scope` defined earlier) that returns a
  root-to-leaf `Iterator` that ends at the current leaf (in other
  words: it's essentially the same as
  `Scope::collect::<Vec<_>>().into_iter().rev()`)
- Deprecate all existing iterators, since they can be replaced by the
  new unified mechanism:
  - `Span::parents` is equivalent to `Span::scope().skip(1)` (although
    the `skip` is typically a bug)
  - `Span::from_root` is equivalent to `Span::scope().skip(1).from_root()`
    (although the `skip` is typically a bug)
  - `Context::scope` is equivalent to
    `Context::lookup_current().scope().from_root()` (although the
    `lookup_current` is sometimes a bug, see also #1428)

* subscriber: add Context method for resolving an Event's SpanRef (#1434)

## Motivation

Fixes #1428 (forward-port from v0.1.x)

## Solution

Adds a new `Context::event_span` method as proposed in the issue.

No existing formatters were changed to use it yet, that seems like a
secondary issue (especially if they already work correctly).

tracing-error/src/subscriber.rs

@@ -98,8 +98,7 @@ where
         let span = collector
             .span(id)
             .expect("registry should have a span for the current ID");
-        let parents = span.parents();
-        for span in std::iter::once(span).chain(parents) {
+        for span in span.scope() {
             let cont = if let Some(fields) = span.extensions().get::<FormattedFields<F>>() {
                 f(span.metadata(), fields.fields.as_str())
             } else {

tracing-flame/src/lib.rs

@@ -394,12 +394,10 @@ where
 
         let first = ctx.span(id).expect("expected: span id exists in registry");
 
-        if !self.config.empty_samples && first.from_root().count() == 0 {
+        if !self.config.empty_samples && first.parent().is_none() {
             return;
         }
 
-        let parents = first.from_root();
-
         let mut stack = String::new();
 
         if !self.config.threads_collapsed {
@@ -408,9 +406,12 @@ where
             stack += "all-threads";
         }
 
-        for parent in parents {
-            stack += "; ";
-            write(&mut stack, parent, &self.config).expect("expected: write to String never fails");
+        if let Some(second) = first.parent() {
+            for parent in second.scope().from_root() {
+                stack += "; ";
+                write(&mut stack, parent, &self.config)
+                    .expect("expected: write to String never fails");
+            }
         }
 
         write!(&mut stack, " {}", samples.as_nanos())
@@ -440,28 +441,22 @@ where
 
         let samples = self.time_since_last_event();
         let first = expect!(ctx.span(&id), "expected: span id exists in registry");
-        let parents = first.from_root();
 
         let mut stack = String::new();
         if !self.config.threads_collapsed {
             THREAD_NAME.with(|name| stack += name.as_str());
         } else {
             stack += "all-threads";
         }
-        stack += "; ";
 
-        for parent in parents {
+        for parent in first.scope().from_root() {
+            stack += "; ";
             expect!(
                 write(&mut stack, parent, &self.config),
                 "expected: write to String never fails"
             );
-            stack += "; ";
         }
 
-        expect!(
-            write(&mut stack, first, &self.config),
-            "expected: write to String never fails"
-        );
         expect!(
             write!(&mut stack, " {}", samples.as_nanos()),
             "expected: write to String never fails"

tracing-journald/src/lib.rs

@@ -126,7 +126,7 @@ where
         let span = ctx.span(id).expect("unknown span");
         let mut buf = Vec::with_capacity(256);
 
-        let depth = span.parents().count();
+        let depth = span.scope().skip(1).count();
 
         writeln!(buf, "S{}_NAME", depth).unwrap();
         put_value(&mut buf, span.name().as_bytes());
@@ -143,7 +143,7 @@ where
 
     fn on_record(&self, id: &Id, values: &Record, ctx: Context<C>) {
         let span = ctx.span(id).expect("unknown span");
-        let depth = span.parents().count();
+        let depth = span.scope().skip(1).count();
         let mut exts = span.extensions_mut();
         let buf = &mut exts.get_mut::<SpanFields>().expect("missing fields").0;
         values.record(&mut SpanVisitor {
@@ -157,7 +157,11 @@ where
         let mut buf = Vec::with_capacity(256);
 
         // Record span fields
-        for span in ctx.scope() {
+        for span in ctx
+            .lookup_current()
+            .into_iter()
+            .flat_map(|span| span.scope().from_root())
+        {
             let exts = span.extensions();
             let fields = exts.get::<SpanFields>().expect("missing fields");
             buf.extend_from_slice(&fields.0);

tracing-subscriber/src/fmt/fmt_subscriber.rs

@@ -2,7 +2,7 @@ use crate::{
     field::RecordFields,
     fmt::{format, FormatEvent, FormatFields, MakeWriter, TestWriter},
     registry::{LookupSpan, SpanRef},
-    subscribe::{self, Context, Scope},
+    subscribe::{self, Context},
 };
 use format::{FmtSpan, TimingDisplay};
 use std::{
@@ -756,8 +756,10 @@ where
         F: FnMut(&SpanRef<'_, C>) -> Result<(), E>,
     {
         // visit all the current spans
-        for span in self.ctx.scope() {
-            f(&span)?;
+        if let Some(leaf) = self.ctx.lookup_current() {
+            for span in leaf.scope().from_root() {
+                f(&span)?;
+            }
         }
         Ok(())
     }
@@ -811,18 +813,6 @@ where
         self.ctx.lookup_current()
     }
 
-    /// Returns an iterator over the [stored data] for all the spans in the
-    /// current context, starting the root of the trace tree and ending with
-    /// the current span.
-    ///
-    /// [stored data]: SpanRef
-    pub fn scope(&self) -> Scope<'_, C>
-    where
-        C: for<'lookup> LookupSpan<'lookup>,
-    {
-        self.ctx.scope()
-    }
-
     /// Returns the current span for this formatter.
     pub fn current_span(&self) -> Current {
         self.ctx.current_span()

tracing-subscriber/src/fmt/format/json.rs

@@ -95,8 +95,10 @@ where
         use serde::ser::SerializeSeq;
         let mut serializer = serializer_o.serialize_seq(None)?;
 
-        for span in self.0.scope() {
-            serializer.serialize_element(&SerializableSpan(&span, self.1))?;
+        if let Some(leaf_span) = self.0.lookup_current() {
+            for span in leaf_span.scope().from_root() {
+                serializer.serialize_element(&SerializableSpan(&span, self.1))?;
+            }
         }
 
         serializer.end()

tracing-subscriber/src/fmt/format/mod.rs

@@ -9,7 +9,6 @@ use crate::{
 
 use std::{
     fmt::{self, Write},
-    iter,
     marker::PhantomData,
 };
 use tracing_core::{
@@ -650,10 +649,7 @@ where
             .and_then(|id| ctx.ctx.span(&id))
             .or_else(|| ctx.ctx.lookup_current());
 
-        let scope = span.into_iter().flat_map(|span| {
-            let parents = span.parents();
-            iter::once(span).chain(parents)
-        });
+        let scope = span.into_iter().flat_map(|span| span.scope());
         #[cfg(feature = "ansi")]
         let dimmed = if self.ansi {
             Style::new().dimmed()
@@ -876,9 +872,7 @@ where
             .and_then(|id| self.ctx.ctx.span(&id))
             .or_else(|| self.ctx.ctx.lookup_current());
 
-        let scope = span
-            .into_iter()
-            .flat_map(|span| span.from_root().chain(iter::once(span)));
+        let scope = span.into_iter().flat_map(|span| span.scope().from_root());
 
         for span in scope {
             write!(f, "{}", bold.paint(span.metadata().name()))?;

tracing-subscriber/src/fmt/format/pretty.rs

@@ -5,10 +5,7 @@ use crate::{
     registry::LookupSpan,
 };
 
-use std::{
-    fmt::{self, Write},
-    iter,
-};
+use std::fmt::{self, Write};
 use tracing_core::{
     field::{self, Field},
     Collect, Event, Level,
@@ -186,10 +183,7 @@ where
             .and_then(|id| ctx.span(&id))
             .or_else(|| ctx.lookup_current());
 
-        let scope = span.into_iter().flat_map(|span| {
-            let parents = span.parents();
-            iter::once(span).chain(parents)
-        });
+        let scope = span.into_iter().flat_map(|span| span.scope());
 
         for span in scope {
             let meta = span.metadata();