subscriber: add Targets filter, a lighter-weight EnvFilter (#1550)

This branch adds a new `Targets` filter to `tracing_subscriber`. The
`Targets` filter is very similar to `EnvFilter`, but it _only_ consists
of filtering directives consisting of a target and level. Because it
doesn't support filtering on field names, span contexts, or field
values, the implementation is *much* simpler, and it doesn't require the
`env_filter` feature flag. Also, `Targets` can easily implement the
`Filter` trait for per-layer filtering, while adding a `Filter`
implementation for `EnvFilter` will require additional effort.

Because the `Targets` filter doesn't allow specifiyng span or
field-value filters, the syntax for parsing one from a string is
significantly simpler than `EnvFilter`'s. Therefore, it can have a very
simple handwritten parser implementation that doesn't require the
`regex` crate. This should be useful for users who are concerned about
the number of dependencies required by `EnvFilter`.

The new implementation is quite small, as it mostly uses the same code
as the static filter subset of `EnvFilter`. This code was factored out
into a shared module for use in both `EnvFilter` and `Targets`. The code
required for _dynamic_ filtering with `EnvFilter` (i.e. on fields and
spans) is still in the `filter::env` module and is only enabled by the
`env-filter` feature flag.

I'm open to renaming the new type; I thought `filter::Targets` seemed
good, but would also be willing to go with `TargetFilter` or something.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/filter/directive.rs

@@ -0,0 +1,395 @@
+use crate::filter::level::{self, LevelFilter};
+use std::{cmp::Ordering, error::Error, fmt, iter::FromIterator, str::FromStr};
+use tracing_core::Metadata;
+/// Indicates that a string could not be parsed as a filtering directive.
+#[derive(Debug)]
+pub struct DirectiveParseError {
+    kind: ParseErrorKind,
+}
+
+/// A directive which will statically enable or disable a given callsite.
+///
+/// Unlike a dynamic directive, this can be cached by the callsite.
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub(crate) struct StaticDirective {
+    pub(in crate::filter) target: Option<String>,
+    pub(in crate::filter) field_names: FilterVec<String>,
+    pub(in crate::filter) level: LevelFilter,
+}
+
+#[cfg(feature = "smallvec")]
+pub(in crate::filter) type FilterVec<T> = smallvec::SmallVec<[T; 8]>;
+#[cfg(not(feature = "smallvec"))]
+pub(in crate::filter) type FilterVec<T> = Vec<T>;
+
+#[derive(Debug, PartialEq, Clone)]
+pub(in crate::filter) struct DirectiveSet<T> {
+    directives: FilterVec<T>,
+    pub(in crate::filter) max_level: LevelFilter,
+}
+
+pub(in crate::filter) trait Match {
+    fn cares_about(&self, meta: &Metadata<'_>) -> bool;
+    fn level(&self) -> &LevelFilter;
+}
+
+#[derive(Debug)]
+enum ParseErrorKind {
+    Field(Box<dyn Error + Send + Sync>),
+    Level(level::ParseError),
+    Other(Option<&'static str>),
+}
+
+// === impl DirectiveSet ===
+
+impl<T> DirectiveSet<T> {
+    pub(crate) fn is_empty(&self) -> bool {
+        self.directives.is_empty()
+    }
+
+    pub(crate) fn iter(&self) -> std::slice::Iter<'_, T> {
+        self.directives.iter()
+    }
+}
+
+impl<T: Ord> Default for DirectiveSet<T> {
+    fn default() -> Self {
+        Self {
+            directives: FilterVec::new(),
+            max_level: LevelFilter::OFF,
+        }
+    }
+}
+
+impl<T: Match + Ord> DirectiveSet<T> {
+    pub(crate) fn directives(&self) -> impl Iterator<Item = &T> {
+        self.directives.iter()
+    }
+
+    pub(crate) fn directives_for<'a>(
+        &'a self,
+        metadata: &'a Metadata<'a>,
+    ) -> impl Iterator<Item = &'a T> + 'a {
+        self.directives().filter(move |d| d.cares_about(metadata))
+    }
+
+    pub(crate) fn add(&mut self, directive: T) {
+        // does this directive enable a more verbose level than the current
+        // max? if so, update the max level.
+        let level = *directive.level();
+        if level > self.max_level {
+            self.max_level = level;
+        }
+        // insert the directive into the vec of directives, ordered by
+        // specificity (length of target + number of field filters). this
+        // ensures that, when finding a directive to match a span or event, we
+        // search the directive set in most specific first order.
+        match self.directives.binary_search(&directive) {
+            Ok(i) => self.directives[i] = directive,
+            Err(i) => self.directives.insert(i, directive),
+        }
+    }
+
+    #[cfg(test)]
+    pub(in crate::filter) fn into_vec(self) -> FilterVec<T> {
+        self.directives
+    }
+}
+
+impl<T: Match + Ord> FromIterator<T> for DirectiveSet<T> {
+    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
+        let mut this = Self::default();
+        this.extend(iter);
+        this
+    }
+}
+
+impl<T: Match + Ord> Extend<T> for DirectiveSet<T> {
+    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
+        for directive in iter.into_iter() {
+            self.add(directive);
+        }
+    }
+}
+
+// === impl Statics ===
+
+impl DirectiveSet<StaticDirective> {
+    pub(crate) fn enabled(&self, meta: &Metadata<'_>) -> bool {
+        let level = meta.level();
+        match self.directives_for(meta).next() {
+            Some(d) => d.level >= *level,
+            None => false,
+        }
+    }
+}
+
+// === impl StaticDirective ===
+
+impl StaticDirective {
+    pub(in crate::filter) fn new(
+        target: Option<String>,
+        field_names: FilterVec<String>,
+        level: LevelFilter,
+    ) -> Self {
+        Self {
+            target,
+            field_names,
+            level,
+        }
+    }
+}
+
+impl Ord for StaticDirective {
+    fn cmp(&self, other: &StaticDirective) -> Ordering {
+        // We attempt to order directives by how "specific" they are. This
+        // ensures that we try the most specific directives first when
+        // attempting to match a piece of metadata.
+
+        // First, we compare based on whether a target is specified, and the
+        // lengths of those targets if both have targets.
+        let ordering = self
+            .target
+            .as_ref()
+            .map(String::len)
+            .cmp(&other.target.as_ref().map(String::len))
+            // Then we compare how many field names are matched by each directive.
+            .then_with(|| self.field_names.len().cmp(&other.field_names.len()))
+            // Finally, we fall back to lexicographical ordering if the directives are
+            // equally specific. Although this is no longer semantically important,
+            // we need to define a total ordering to determine the directive's place
+            // in the BTreeMap.
+            .then_with(|| {
+                self.target
+                    .cmp(&other.target)
+                    .then_with(|| self.field_names[..].cmp(&other.field_names[..]))
+            })
+            .reverse();
+
+        #[cfg(debug_assertions)]
+        {
+            if ordering == Ordering::Equal {
+                debug_assert_eq!(
+                    self.target, other.target,
+                    "invariant violated: Ordering::Equal must imply a.target == b.target"
+                );
+                debug_assert_eq!(
+                    self.field_names, other.field_names,
+                    "invariant violated: Ordering::Equal must imply a.field_names == b.field_names"
+                );
+            }
+        }
+
+        ordering
+    }
+}
+
+impl PartialOrd for StaticDirective {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Match for StaticDirective {
+    fn cares_about(&self, meta: &Metadata<'_>) -> bool {
+        // Does this directive have a target filter, and does it match the
+        // metadata's target?
+        if let Some(ref target) = self.target {
+            if !meta.target().starts_with(&target[..]) {
+                return false;
+            }
+        }
+
+        if meta.is_event() && !self.field_names.is_empty() {
+            let fields = meta.fields();
+            for name in &self.field_names {
+                if fields.field(name).is_none() {
+                    return false;
+                }
+            }
+        }
+
+        true
+    }
+
+    fn level(&self) -> &LevelFilter {
+        &self.level
+    }
+}
+
+impl Default for StaticDirective {
+    fn default() -> Self {
+        StaticDirective {
+            target: None,
+            field_names: FilterVec::new(),
+            level: LevelFilter::ERROR,
+        }
+    }
+}
+
+impl fmt::Display for StaticDirective {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut wrote_any = false;
+        if let Some(ref target) = self.target {
+            fmt::Display::fmt(target, f)?;
+            wrote_any = true;
+        }
+
+        if !self.field_names.is_empty() {
+            f.write_str("[")?;
+
+            let mut fields = self.field_names.iter();
+            if let Some(field) = fields.next() {
+                write!(f, "{{{}", field)?;
+                for field in fields {
+                    write!(f, ",{}", field)?;
+                }
+                f.write_str("}")?;
+            }
+
+            f.write_str("]")?;
+            wrote_any = true;
+        }
+
+        if wrote_any {
+            f.write_str("=")?;
+        }
+
+        fmt::Display::fmt(&self.level, f)
+    }
+}
+
+impl FromStr for StaticDirective {
+    type Err = DirectiveParseError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        // This method parses a filtering directive in one of the following
+        // forms:
+        //
+        // * `foo=trace` (TARGET=LEVEL)
+        // * `foo[{bar,baz}]=info` (TARGET[{FIELD,+}]=LEVEL)
+        // * `trace` (bare LEVEL)
+        // * `foo` (bare TARGET)
+        let mut split = s.split('=');
+        let part0 = split
+            .next()
+            .ok_or_else(|| DirectiveParseError::msg("string must not be empty"))?;
+
+        // Directive includes an `=`:
+        // * `foo=trace`
+        // * `foo[{bar}]=trace`
+        // * `foo[{bar,baz}]=trace`
+        if let Some(part1) = split.next() {
+            if split.next().is_some() {
+                return Err(DirectiveParseError::msg(
+                    "too many '=' in filter directive, expected 0 or 1",
+                ));
+            }
+
+            let mut split = part0.split("[{");
+            let target = split.next().map(String::from);
+            let mut field_names = FilterVec::new();
+            // Directive includes fields:
+            // * `foo[{bar}]=trace`
+            // * `foo[{bar,baz}]=trace`
+            if let Some(maybe_fields) = split.next() {
+                if split.next().is_some() {
+                    return Err(DirectiveParseError::msg(
+                        "too many '[{' in filter directive, expected 0 or 1",
+                    ));
+                }
+
+                let fields = maybe_fields.strip_suffix("}]").ok_or_else(|| {
+                    DirectiveParseError::msg("expected fields list to end with '}]'")
+                })?;
+                field_names.extend(fields.split(',').filter_map(|s| {
+                    if s.is_empty() {
+                        None
+                    } else {
+                        Some(String::from(s))
+                    }
+                }));
+            };
+            let level = part1.parse()?;
+            return Ok(Self {
+                level,
+                field_names,
+                target,
+            });
+        }
+
+        // Okay, the part after the `=` was empty, the directive is either a
+        // bare level or a bare target.
+        // * `foo`
+        // * `info`
+        Ok(match part0.parse::<LevelFilter>() {
+            Ok(level) => Self {
+                level,
+                target: None,
+                field_names: FilterVec::new(),
+            },
+            Err(_) => Self {
+                target: Some(String::from(part0)),
+                level: LevelFilter::TRACE,
+                field_names: FilterVec::new(),
+            },
+        })
+    }
+}
+
+// === impl ParseError ===
+
+impl DirectiveParseError {
+    pub(crate) fn new() -> Self {
+        DirectiveParseError {
+            kind: ParseErrorKind::Other(None),
+        }
+    }
+
+    pub(crate) fn msg(s: &'static str) -> Self {
+        DirectiveParseError {
+            kind: ParseErrorKind::Other(Some(s)),
+        }
+    }
+}
+
+impl fmt::Display for DirectiveParseError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.kind {
+            ParseErrorKind::Other(None) => f.pad("invalid filter directive"),
+            ParseErrorKind::Other(Some(msg)) => write!(f, "invalid filter directive: {}", msg),
+            ParseErrorKind::Level(ref l) => l.fmt(f),
+            ParseErrorKind::Field(ref e) => write!(f, "invalid field filter: {}", e),
+        }
+    }
+}
+
+impl Error for DirectiveParseError {
+    fn description(&self) -> &str {
+        "invalid filter directive"
+    }
+
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        match self.kind {
+            ParseErrorKind::Other(_) => None,
+            ParseErrorKind::Level(ref l) => Some(l),
+            ParseErrorKind::Field(ref n) => Some(n.as_ref()),
+        }
+    }
+}
+
+impl From<Box<dyn Error + Send + Sync>> for DirectiveParseError {
+    fn from(e: Box<dyn Error + Send + Sync>) -> Self {
+        Self {
+            kind: ParseErrorKind::Field(e),
+        }
+    }
+}
+
+impl From<level::ParseError> for DirectiveParseError {
+    fn from(l: level::ParseError) -> Self {
+        Self {
+            kind: ParseErrorKind::Level(l),
+        }
+    }
+}