fish-shell/crates/feature-flags/src/lib.rs

//! Flags to enable upcoming features

use fish_widestring::{L, WExt as _, wstr};
use std::{
    cell::RefCell,
    sync::atomic::{AtomicBool, Ordering},
};

/// The list of flags.
#[repr(u8)]
#[derive(Clone, Copy, PartialEq, Eq)]
pub enum FeatureFlag {
    /// Whether ^ is supported for stderr redirection.
    StderrNoCaret,

    /// Whether ? is supported as a glob.
    QuestionMarkNoGlob,

    /// Whether string replace -r double-unescapes the replacement.
    StringReplaceBackslash,

    /// Whether "&" is not-special if followed by a word character.
    AmpersandNoBgInToken,
    /// Whether "%self" is expanded to fish's pid
    RemovePercentSelf,

    /// Remove `test`'s one and zero arg mode (make `test -n` return false etc)
    TestRequireArg,

    /// Whether to write OSC 133 prompt markers
    MarkPrompt,

    /// Do not look up $TERM in terminfo database.
    IgnoreTerminfo,

    /// Whether we are allowed to query the TTY for extra information.
    QueryTerm,

    /// Do not try to work around incompatible terminal.
    OmitTermWorkarounds,
}

struct Features {
    // Values for the flags.
    // These are atomic to "fix" a race reported by tsan where tests of feature flags and other
    // tests which use them conceptually race.
    values: [AtomicBool; METADATA.len()],
}

/// Metadata about feature flags.
pub struct FeatureMetadata {
    /// The flag itself.
    pub flag: FeatureFlag,

    /// User-presentable short name of the feature flag.
    pub name: &'static wstr,

    /// Comma-separated list of feature groups.
    pub groups: &'static wstr,

    /// User-presentable description of the feature flag.
    pub description: &'static wstr,

    /// Default flag value.
    default_value: bool,

    /// Whether the value can still be changed or not.
    read_only: bool,
}

/// The metadata, indexed by flag.
pub const METADATA: &[FeatureMetadata] = &[
    FeatureMetadata {
        flag: FeatureFlag::StderrNoCaret,
        name: L!("stderr-nocaret"),
        groups: L!("3.0"),
        description: L!("^ no longer redirects stderr (historical, can no longer be changed)"),
        default_value: true,
        read_only: true,
    },
    FeatureMetadata {
        flag: FeatureFlag::QuestionMarkNoGlob,
        name: L!("qmark-noglob"),
        groups: L!("3.0"),
        description: L!("? no longer globs"),
        default_value: true,
        read_only: false,
    },
    FeatureMetadata {
        flag: FeatureFlag::StringReplaceBackslash,
        name: L!("regex-easyesc"),
        groups: L!("3.1"),
        description: L!("string replace -r needs fewer \\'s"),
        default_value: true,
        read_only: false,
    },
    FeatureMetadata {
        flag: FeatureFlag::AmpersandNoBgInToken,
        name: L!("ampersand-nobg-in-token"),
        groups: L!("3.4"),
        description: L!("& only backgrounds if followed by a separator"),
        default_value: true,
        read_only: false,
    },
    FeatureMetadata {
        flag: FeatureFlag::RemovePercentSelf,
        name: L!("remove-percent-self"),
        groups: L!("4.0"),
        description: L!("%self is no longer expanded (use $fish_pid)"),
        default_value: false,
        read_only: false,
    },
    FeatureMetadata {
        flag: FeatureFlag::TestRequireArg,
        name: L!("test-require-arg"),
        groups: L!("4.0"),
        description: L!("builtin test requires an argument"),
        default_value: false,
        read_only: false,
    },
    FeatureMetadata {
        flag: FeatureFlag::MarkPrompt,
        name: L!("mark-prompt"),
        groups: L!("4.0"),
        description: L!("write OSC 133 prompt markers to the terminal"),
        default_value: true,
        read_only: false,
    },
    FeatureMetadata {
        flag: FeatureFlag::IgnoreTerminfo,
        name: L!("ignore-terminfo"),
        groups: L!("4.1"),
        description: L!(
            "do not look up $TERM in terminfo database (historical, can no longer be changed)"
        ),
        default_value: true,
        read_only: true,
    },
    FeatureMetadata {
        flag: FeatureFlag::QueryTerm,
        name: L!("query-term"),
        groups: L!("4.1"),
        description: L!("query the TTY to enable extra functionality"),
        default_value: true,
        read_only: false,
    },
    FeatureMetadata {
        flag: FeatureFlag::OmitTermWorkarounds,
        name: L!("omit-term-workarounds"),
        groups: L!("4.3"),
        description: L!("skip workarounds for incompatible terminals"),
        default_value: false,
        read_only: false,
    },
];

thread_local!(
    static LOCAL_OVERRIDE_STACK: RefCell<Vec<(FeatureFlag, bool)>> =
        const { RefCell::new(Vec::new()) };
);

/// The singleton shared feature set.
static FEATURES: Features = Features::new();

/// Perform a feature test on the global set of features.
pub fn feature_test(flag: FeatureFlag) -> bool {
    if let Some(value) = LOCAL_OVERRIDE_STACK.with(|stack| {
        for &(overridden_feature, value) in stack.borrow().iter().rev() {
            if flag == overridden_feature {
                return Some(value);
            }
        }
        None
    }) {
        return value;
    }
    FEATURES.test(flag)
}

/// Parses a comma-separated feature-flag string, updating ourselves with the values.
/// Feature names or group names may be prefixed with "no-" to disable them.
/// The special group name "all" may be used for those who like to live on the edge.
/// Unknown features are silently ignored.
pub fn set_from_string<'a>(str: impl Into<&'a wstr>) {
    FEATURES.set_from_string(str.into());
}

impl Features {
    const fn new() -> Self {
        // TODO: feature(const_array): use std::array::from_fn()
        use std::mem::{MaybeUninit, transmute};
        let values = {
            let mut data: [MaybeUninit<AtomicBool>; METADATA.len()] =
                [const { MaybeUninit::uninit() }; METADATA.len()];

            let mut i = 0;
            while i < METADATA.len() {
                data[i].write(AtomicBool::new(METADATA[i].default_value));
                i += 1;
            }

            // SAFETY: `data` is guaranteed initialized by the loop
            unsafe {
                transmute::<[MaybeUninit<AtomicBool>; METADATA.len()], [AtomicBool; METADATA.len()]>(
                    data,
                )
            }
        };
        Features { values }
    }

    fn test(&self, flag: FeatureFlag) -> bool {
        self.values[flag as usize].load(Ordering::SeqCst)
    }

    fn set(&self, flag: FeatureFlag, value: bool) {
        self.values[flag as usize].store(value, Ordering::SeqCst);
    }

    fn set_from_string(&self, str: &wstr) {
        for entry in str.split(',') {
            let entry = entry.trim();
            if entry.is_empty() {
                continue;
            }

            // A "no-" prefix inverts the sense.
            let (name, value) = match entry.strip_prefix("no-") {
                Some(suffix) => (suffix, false),
                None => (entry, true),
            };
            // Look for a feature with this name. If we don't find it, assume it's a group name and set
            // all features whose group contain it. Do nothing even if the string is unrecognized; this
            // is to allow uniform invocations of fish (e.g. disable a feature that is only present in
            // future versions).
            // The special name 'all' may be used for those who like to live on the edge.
            if let Some(md) = METADATA.iter().find(|md| md.name == name) {
                // Only change it if it's not read-only.
                // Don't complain if it is, this is typically set from a variable.
                if !md.read_only {
                    self.set(md.flag, value);
                }
            } else {
                for md in METADATA {
                    if (md.groups == name || name == L!("all")) && !md.read_only {
                        self.set(md.flag, value);
                    }
                }
            }
        }
    }
}

/// Run code with a feature overridden.
/// This should only be used in tests.
pub fn with_overridden_feature(flag: FeatureFlag, value: bool, test_fn: impl FnOnce()) {
    LOCAL_OVERRIDE_STACK.with(|stack| {
        stack.borrow_mut().push((flag, value));
        test_fn();
        stack.borrow_mut().pop();
    });
}

#[cfg(test)]
mod tests {
    use super::{FeatureFlag, Features, METADATA, feature_test, with_overridden_feature};
    use fish_widestring::L;

    #[test]
    fn test_feature_flags() {
        let f = Features::new();
        f.set_from_string(L!("stderr-nocaret,nonsense"));
        assert!(f.test(FeatureFlag::StderrNoCaret));
        f.set_from_string(L!("stderr-nocaret,no-stderr-nocaret,nonsense"));
        assert!(f.test(FeatureFlag::StderrNoCaret));

        // Ensure every metadata is represented once.
        let mut counts: [usize; METADATA.len()] = [0; METADATA.len()];
        for md in METADATA {
            counts[md.flag as usize] += 1;
        }
        for count in counts {
            assert_eq!(count, 1);
        }

        assert_eq!(
            METADATA[FeatureFlag::StderrNoCaret as usize].name,
            L!("stderr-nocaret")
        );
    }

    #[test]
    fn test_overridden_feature() {
        with_overridden_feature(FeatureFlag::QuestionMarkNoGlob, true, || {
            assert!(feature_test(FeatureFlag::QuestionMarkNoGlob));
        });

        with_overridden_feature(FeatureFlag::QuestionMarkNoGlob, false, || {
            assert!(!feature_test(FeatureFlag::QuestionMarkNoGlob));
        });

        with_overridden_feature(FeatureFlag::QuestionMarkNoGlob, false, || {
            with_overridden_feature(FeatureFlag::QuestionMarkNoGlob, true, || {
                assert!(feature_test(FeatureFlag::QuestionMarkNoGlob));
            });
        });
    }
}