subscriber: add EnvFilter::builder, option to disable regex (#2035)

## Motivation

Currently, `EnvFilter` will always interpret all field value filter
directives that are not numeric,or boolean literals as regular
expressions that are matched against a field's `fmt::Debug` output. In
many cases, it may not be desirable to use regular expressions in this
case, so users may prefer to perform literal matching of `fmt::Debug`
output against a string value instead. Currently, this is not possible.

## Solution

This branch introduces the ability to control whether an `EnvFilter`
interprets field value `fmt::Debug` match filters as regular expressions
or as literal strings. When matching literal `fmt::Debug` patterns, the
string is matched without requiring a temporary allocation for the
formatted representation by implementing `fmt::Write` for a matcher type
and "writing" the field's `Debug` output to it. This is similar to the
technique already used for matching regular expression patterns.

Since there is not currently a nice way to specify configurations prior
to parsing an `EnvFilter` from a string or environment variable, I've
also added a builder API. This allows setting things like whether field
value filters should use strict matching or regular expressions.

## Notes

Ideally, I think we would want the filter language to allow specifying
whether a field value filter should be interpreted as a regular
expression or as a literal string match. Instead of having a global
toggle between regular expressions and literal strings, we would
introduce new syntax for indicating that a value match pattern is a
regular expression. This way, a single filter can have both regular
expression and literal string value matchers. The `with_regex(false)`
configuration would just return an error any time the regex syntax was
used when parsing the filter string.

However, this would be a breaking change in behavior. Currently, field
value filters are interpreted as regex by default, so changing the
parser to only interpret a value filter as a regex if there's additional
syntax indicating it's a regex would break existing filter
configurations that rely on regex matching.

In `tracing-subscriber` 0.4, we should definitely consider introducing
new syntax to indicate a match pattern is a regex, and change the
`with_regex` method's behavior to disallow the use of that syntax. For
now, however, making it a global toggle at least allows users to control
whether or not we use regex matching, so this is a significant
improvement for v0.3.x.

Signed-off-by: Eliza Weisman <[email protected]>

tracing-subscriber/src/filter/env/builder.rs

@@ -0,0 +1,318 @@
+use super::{
+    directive::{self, Directive},
+    EnvFilter, FromEnvError,
+};
+use crate::sync::RwLock;
+use std::env;
+use thread_local::ThreadLocal;
+use tracing::level_filters::STATIC_MAX_LEVEL;
+
+/// A [builder] for constructing new [`EnvFilter`]s.
+///
+/// [builder]: https://rust-unofficial.github.io/patterns/patterns/creational/builder.html
+#[derive(Debug, Clone)]
+pub struct Builder {
+    regex: bool,
+    env: Option<String>,
+    default_directive: Option<Directive>,
+}
+
+impl Builder {
+    /// Sets whether span field values can be matched with regular expressions.
+    ///
+    /// If this is `true`, field filter directives will be interpreted as
+    /// regular expressions if they are not able to be interpreted as a `bool`,
+    /// `i64`, `u64`, or `f64` literal. If this is `false,` those field values
+    /// will be interpreted as literal [`std::fmt::Debug`] output instead.
+    ///
+    /// By default, regular expressions are enabled.
+    ///
+    /// **Note**: when [`EnvFilter`]s are constructed from untrusted inputs,
+    /// disabling regular expressions is strongly encouraged.
+    pub fn with_regex(self, regex: bool) -> Self {
+        Self { regex, ..self }
+    }
+
+    /// Sets a default [filtering directive] that will be added to the filter if
+    /// the parsed string or environment variable contains no filter directives.
+    ///
+    /// By default, there is no default directive.
+    ///
+    /// # Examples
+    ///
+    /// If [`parse`], [`parse_lossy`], [`from_env`], or [`from_env_lossy`] are
+    /// called with an empty string or environment variable, the default
+    /// directive is used instead:
+    ///
+    /// ```rust
+    /// # fn main() -> Result<(), Box<dyn std::error::Error>> {
+    /// use tracing_subscriber::filter::{EnvFilter, LevelFilter};
+    ///
+    /// let filter = EnvFilter::builder()
+    ///     .with_default_directive(LevelFilter::INFO.into())
+    ///     .parse("")?;
+    ///
+    /// assert_eq!(format!("{}", filter), "info");
+    /// # Ok(()) }
+    /// ```
+    ///
+    /// Note that the `lossy` variants ([`parse_lossy`] and [`from_env_lossy`])
+    /// will ignore any invalid directives. If all directives in a filter
+    /// string or environment variable are invalid, those methods will also use
+    /// the default directive:
+    ///
+    /// ```rust
+    /// use tracing_subscriber::filter::{EnvFilter, LevelFilter};
+    ///
+    /// let filter = EnvFilter::builder()
+    ///     .with_default_directive(LevelFilter::INFO.into())
+    ///     .parse_lossy("some_target=fake level,foo::bar=lolwut");
+    ///
+    /// assert_eq!(format!("{}", filter), "info");
+    /// ```
+    ///
+    ///
+    /// If the string or environment variable contains valid filtering
+    /// directives, the default directive is not used:
+    ///
+    /// ```rust
+    /// use tracing_subscriber::filter::{EnvFilter, LevelFilter};
+    ///
+    /// let filter = EnvFilter::builder()
+    ///     .with_default_directive(LevelFilter::INFO.into())
+    ///     .parse_lossy("foo=trace");
+    ///
+    /// // The default directive is *not* used:
+    /// assert_eq!(format!("{}", filter), "foo=trace");
+    /// ```
+    ///
+    /// Parsing a more complex default directive from a string:
+    ///
+    /// ```rust
+    /// # fn main() -> Result<(), Box<dyn std::error::Error>> {
+    /// use tracing_subscriber::filter::{EnvFilter, LevelFilter};
+    ///
+    /// let default = "myapp=debug".parse()
+    ///     .expect("hard-coded default directive should be valid");
+    ///
+    /// let filter = EnvFilter::builder()
+    ///     .with_default_directive(default)
+    ///     .parse("")?;
+    ///
+    /// assert_eq!(format!("{}", filter), "myapp=debug");
+    /// # Ok(()) }
+    /// ```
+    ///
+    /// [`parse_lossy`]: Self::parse_lossy
+    /// [`from_env_lossy`]: Self::from_env_lossy
+    /// [`parse`]: Self::parse
+    /// [`from_env`]: Self::from_env
+    pub fn with_default_directive(self, default_directive: Directive) -> Self {
+        Self {
+            default_directive: Some(default_directive),
+            ..self
+        }
+    }
+
+    /// Sets the name of the environment variable used by the [`from_env`],
+    /// [`from_env_lossy`], and [`try_from_env`] methods.
+    ///
+    /// By default, this is the value of [`EnvFilter::DEFAULT_ENV`]
+    /// (`RUST_LOG`).
+    ///
+    /// [`from_env`]: Self::from_env
+    /// [`from_env_lossy`]: Self::from_env_lossy
+    /// [`try_from_env`]: Self::try_from_env
+    pub fn with_env_var(self, var: impl ToString) -> Self {
+        Self {
+            env: Some(var.to_string()),
+            ..self
+        }
+    }
+
+    /// Returns a new [`EnvFilter`] from the directives in the given string,
+    /// *ignoring* any that are invalid.
+    pub fn parse_lossy<S: AsRef<str>>(&self, dirs: S) -> EnvFilter {
+        let directives =
+            dirs.as_ref()
+                .split(',')
+                .filter_map(|s| match Directive::parse(s, self.regex) {
+                    Ok(d) => Some(d),
+                    Err(err) => {
+                        eprintln!("ignoring `{}`: {}", s, err);
+                        None
+                    }
+                });
+        self.from_directives(directives)
+    }
+
+    /// Returns a new [`EnvFilter`] from the directives in the given string,
+    /// or an error if any are invalid.
+    pub fn parse<S: AsRef<str>>(&self, dirs: S) -> Result<EnvFilter, directive::ParseError> {
+        let dirs = dirs.as_ref();
+        if dirs.is_empty() {
+            return Ok(self.from_directives(std::iter::empty()));
+        }
+        let directives = dirs
+            .split(',')
+            .map(|s| Directive::parse(s, self.regex))
+            .collect::<Result<Vec<_>, _>>()?;
+        Ok(self.from_directives(directives))
+    }
+
+    /// Returns a new [`EnvFilter`] from the directives in the configured
+    /// environment variable, ignoring any directives that are invalid.
+    pub fn from_env_lossy(&self) -> EnvFilter {
+        let var = env::var(self.env_var_name()).unwrap_or_default();
+        self.parse_lossy(var)
+    }
+
+    /// Returns a new [`EnvFilter`] from the directives in the in the configured
+    /// environment variable, or an error if the environment variable is not set
+    /// or contains invalid directives.
+    pub fn from_env(&self) -> Result<EnvFilter, FromEnvError> {
+        let var = env::var(self.env_var_name()).unwrap_or_default();
+        self.parse(var).map_err(Into::into)
+    }
+
+    /// Returns a new [`EnvFilter`] from the directives in the in the configured
+    /// environment variable, or an error if the environment variable is not set
+    /// or contains invalid directives.
+    pub fn try_from_env(&self) -> Result<EnvFilter, FromEnvError> {
+        let var = env::var(self.env_var_name())?;
+        self.parse(var).map_err(Into::into)
+    }
+
+    // TODO(eliza): consider making this a public API?
+    pub(super) fn from_directives(
+        &self,
+        directives: impl IntoIterator<Item = Directive>,
+    ) -> EnvFilter {
+        use tracing::Level;
+
+        let mut directives: Vec<_> = directives.into_iter().collect();
+        let mut disabled = Vec::new();
+        for directive in &mut directives {
+            if directive.level > STATIC_MAX_LEVEL {
+                disabled.push(directive.clone());
+            }
+            if !self.regex {
+                directive.deregexify();
+            }
+        }
+
+        if !disabled.is_empty() {
+            #[cfg(feature = "ansi_term")]
+            use ansi_term::{Color, Style};
+            // NOTE: We can't use a configured `MakeWriter` because the EnvFilter
+            // has no knowledge of any underlying subscriber or collector, which
+            // may or may not use a `MakeWriter`.
+            let warn = |msg: &str| {
+                #[cfg(not(feature = "ansi_term"))]
+                let msg = format!("warning: {}", msg);
+                #[cfg(feature = "ansi_term")]
+                let msg = {
+                    let bold = Style::new().bold();
+                    let mut warning = Color::Yellow.paint("warning");
+                    warning.style_ref_mut().is_bold = true;
+                    format!("{}{} {}", warning, bold.paint(":"), bold.paint(msg))
+                };
+                eprintln!("{}", msg);
+            };
+            let ctx_prefixed = |prefix: &str, msg: &str| {
+                #[cfg(not(feature = "ansi_term"))]
+                let msg = format!("{} {}", prefix, msg);
+                #[cfg(feature = "ansi_term")]
+                let msg = {
+                    let mut equal = Color::Fixed(21).paint("="); // dark blue
+                    equal.style_ref_mut().is_bold = true;
+                    format!(" {} {} {}", equal, Style::new().bold().paint(prefix), msg)
+                };
+                eprintln!("{}", msg);
+            };
+            let ctx_help = |msg| ctx_prefixed("help:", msg);
+            let ctx_note = |msg| ctx_prefixed("note:", msg);
+            let ctx = |msg: &str| {
+                #[cfg(not(feature = "ansi_term"))]
+                let msg = format!("note: {}", msg);
+                #[cfg(feature = "ansi_term")]
+                let msg = {
+                    let mut pipe = Color::Fixed(21).paint("|");
+                    pipe.style_ref_mut().is_bold = true;
+                    format!(" {} {}", pipe, msg)
+                };
+                eprintln!("{}", msg);
+            };
+            warn("some trace filter directives would enable traces that are disabled statically");
+            for directive in disabled {
+                let target = if let Some(target) = &directive.target {
+                    format!("the `{}` target", target)
+                } else {
+                    "all targets".into()
+                };
+                let level = directive
+                    .level
+                    .into_level()
+                    .expect("=off would not have enabled any filters");
+                ctx(&format!(
+                    "`{}` would enable the {} level for {}",
+                    directive, level, target
+                ));
+            }
+            ctx_note(&format!("the static max level is `{}`", STATIC_MAX_LEVEL));
+            let help_msg = || {
+                let (feature, filter) = match STATIC_MAX_LEVEL.into_level() {
+                    Some(Level::TRACE) => unreachable!(
+                        "if the max level is trace, no static filtering features are enabled"
+                    ),
+                    Some(Level::DEBUG) => ("max_level_debug", Level::TRACE),
+                    Some(Level::INFO) => ("max_level_info", Level::DEBUG),
+                    Some(Level::WARN) => ("max_level_warn", Level::INFO),
+                    Some(Level::ERROR) => ("max_level_error", Level::WARN),
+                    None => return ("max_level_off", String::new()),
+                };
+                (feature, format!("{} ", filter))
+            };
+            let (feature, earlier_level) = help_msg();
+            ctx_help(&format!(
+                "to enable {}logging, remove the `{}` feature",
+                earlier_level, feature
+            ));
+        }
+
+        let (dynamics, statics) = Directive::make_tables(directives);
+        let has_dynamics = !dynamics.is_empty();
+
+        let mut filter = EnvFilter {
+            statics,
+            dynamics,
+            has_dynamics,
+            by_id: RwLock::new(Default::default()),
+            by_cs: RwLock::new(Default::default()),
+            scope: ThreadLocal::new(),
+            regex: self.regex,
+        };
+
+        if !has_dynamics && filter.statics.is_empty() {
+            if let Some(ref default) = self.default_directive {
+                filter = filter.add_directive(default.clone());
+            }
+        }
+
+        filter
+    }
+
+    fn env_var_name(&self) -> &str {
+        self.env.as_deref().unwrap_or(EnvFilter::DEFAULT_ENV)
+    }
+}
+
+impl Default for Builder {
+    fn default() -> Self {
+        Self {
+            regex: true,
+            env: None,
+            default_directive: None,
+        }
+    }
+}