delta-gitu 0.1.2

//! The git command whose output gitu is paging.
//!
//! Set as git's pager, gitu is handed a patch and told nothing about the
//! question it answers — not the subcommand, not the revs, not the options. The
//! command is in the process tree, though: git spawns its pager as a child, and
//! a shell pipeline puts the two side by side. Finding it there is what lets
//! gitu ask git the same question again — for more of the file around each
//! change — and know what the patch is a diff *of*, which is what decides
//! whether staging part of it means anything.

use crate::Res;
use crate::error::Error;
use crate::git::diff::DiffType;
use std::iter;
use std::path::Path;
use std::process::Command;
use sysinfo::{Pid, Process, ProcessRefreshKind, ProcessesToUpdate, System, UpdateKind};

/// The subcommands whose output gitu structures.
const SUBCOMMANDS: [&str; 3] = ["diff", "show", "log"];

/// The subcommands an edited command may ask for. gitu re-runs the command it
/// holds on every rebuild — staging a hunk re-asks it — so a command that
/// changed anything would run repeatedly and unbidden.
const READ_ONLY: [&str; 10] = [
    "diff", "show", "log", "blame", "grep", "shortlog", "reflog", "ls-files", "describe", "status",
];

/// How far above gitu to look for the git process: git may spawn its pager
/// through a shell, and that shell through another.
const ANCESTORS: usize = 3;

/// A git invocation gitu can put again.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) struct GitCommand {
    argv: Vec<String>,
    /// Where the subcommand sits in `argv`. What precedes it is git's own
    /// (`-c k=v`, `--git-dir=…`) and has to stay in front of it.
    subcommand: usize,
}

impl GitCommand {
    /// The invocation `argv` is, if it is one whose output gitu structures.
    pub(crate) fn of(argv: Vec<String>) -> Option<Self> {
        let program = Path::new(argv.first()?).file_stem()?.to_str()?;
        if !program.eq_ignore_ascii_case("git") {
            return None;
        }

        let subcommand = argv
            .iter()
            .position(|arg| SUBCOMMANDS.contains(&arg.as_str()))?;
        Some(Self { argv, subcommand })
    }

    /// What the patch is a diff of, which is what decides whether staging or
    /// unstaging part of it means anything (see [`DiffType`]). Only a `git diff`
    /// of the working tree or of the index is either of those; naming a rev
    /// makes it a comparison of two trees, as `git show` and `git log -p`
    /// always are.
    pub(crate) fn diff_type(&self) -> DiffType {
        if self.subcommand() != "diff" {
            return DiffType::TreeToTree;
        }

        let args: Vec<&str> = self.arguments().take_while(|arg| *arg != "--").collect();
        if args.contains(&"--cached") || args.contains(&"--staged") {
            DiffType::IndexToTree
        } else if args.iter().any(|arg| !arg.starts_with('-')) {
            // The value of a separated option (`-U 8`) reads as a rev here, so
            // such a diff is taken for a comparison of two trees. Nothing reads
            // the other way round, so the mistake only ever narrows the ops.
            DiffType::TreeToTree
        } else {
            DiffType::WorkdirToIndex
        }
    }

    /// The command again, asking for `context` around each change (`-U8`,
    /// `-W`) in place of whatever it originally asked for. `None` puts the
    /// question exactly as git put it.
    pub(crate) fn command(&self, context: Option<&str>) -> Command {
        let words = self.words(context);
        let mut command = Command::new(words[0]);
        command.args(&words[1..=self.subcommand]);
        // gitu parses what comes back, so an external differ must not stand in
        // for the patch, and git's colours have to be off however the user has
        // configured them.
        command.args(["--no-ext-diff", "--no-color"]);
        command.args(&words[self.subcommand + 1..]);
        command
    }

    /// The command as gitu is putting it, for the user to edit. The flags gitu
    /// forces are left out: they are not the question, and not the user's to
    /// change.
    pub(crate) fn line(&self, context: Option<&str>) -> String {
        let words = self.words(context);
        iter::once("git")
            .chain(words[1..].iter().copied())
            .map(quoted)
            .collect::<Vec<_>>()
            .join(" ")
    }

    /// The command an edit of [`Self::line`] asks for. git's program and its own
    /// options are fixed — gitu's ops act on the repository gitu opened, and an
    /// edit pointing git elsewhere would make the view and the ops disagree —
    /// and the subcommand must be one that only reports.
    pub(crate) fn edited(&self, line: &str) -> Res<Self> {
        let words = shell_words::split(line).map_err(|_| Error::EditedCommandQuotes)?;
        let own = &self.argv[1..self.subcommand];

        let [program, rest @ ..] = words.as_slice() else {
            return Err(Error::EditedCommandFixed);
        };
        if program != "git" || !rest.starts_with(own) {
            return Err(Error::EditedCommandFixed);
        }

        match rest.get(own.len()) {
            None => return Err(Error::EditedCommandFixed),
            Some(subcommand) if !READ_ONLY.contains(&subcommand.as_str()) => {
                return Err(Error::EditedCommandWrites(subcommand.clone()));
            }
            Some(_) => (),
        }

        Ok(Self {
            argv: iter::once(self.argv[0].clone())
                .chain(rest.iter().cloned())
                .collect(),
            subcommand: self.subcommand,
        })
    }

    /// The paths the command limits itself to, as the user writes them (see
    /// [`pattern_of`]). What follows `--` is pathspecs; a command that names
    /// none limits nothing.
    pub(crate) fn file_patterns(&self) -> Vec<String> {
        self.pathspecs().map(pattern_of).collect()
    }

    /// The same command asking for these paths in place of whatever it asked
    /// for before.
    pub(crate) fn asking_for(&self, patterns: &[String]) -> Self {
        let mut argv: Vec<String> = self.argv[..=self.subcommand].to_vec();
        argv.extend(
            self.arguments()
                .take_while(|arg| *arg != "--")
                .map(str::to_owned),
        );
        if !patterns.is_empty() {
            argv.push("--".to_owned());
            argv.extend(patterns.iter().map(|pattern| pathspec_of(pattern)));
        }

        Self {
            argv,
            subcommand: self.subcommand,
        }
    }

    fn pathspecs(&self) -> impl Iterator<Item = &str> {
        self.arguments().skip_while(|arg| *arg != "--").skip(1)
    }

    /// git's own options and the subcommand, then what is asked of it with
    /// `context` in place of whatever it said.
    fn words<'a>(&'a self, context: Option<&'a str>) -> Vec<&'a str> {
        let mut words: Vec<&str> = self.argv[..=self.subcommand]
            .iter()
            .map(String::as_str)
            .collect();
        words.extend(context);
        match context {
            Some(_) => words.extend(without_context(self.arguments())),
            None => words.extend(self.arguments()),
        }
        words
    }

    fn subcommand(&self) -> &str {
        &self.argv[self.subcommand]
    }

    /// What was asked of the subcommand.
    fn arguments(&self) -> impl Iterator<Item = &str> + Clone {
        self.argv[self.subcommand + 1..].iter().map(String::as_str)
    }
}

/// The pathspec a file pattern asks for. The language is git's own, less the
/// magic: a bare pattern limits the diff to what it matches, `!` excludes what
/// it matches, and a pattern that is already a pathspec is passed on as it is.
/// The wildcards are git's, so `*_test.go` matches at any depth and a directory
/// matches its subtree.
///
/// `:(top)` is not decoration: a pathspec is relative to git's working
/// directory and gitu asks from the root of the repository, where the paths a
/// diff names are also rooted.
fn pathspec_of(pattern: &str) -> String {
    match pattern {
        _ if pattern.starts_with(':') => pattern.to_owned(),
        _ if let Some(glob) = pattern.strip_prefix('!') => format!(":(top,exclude){glob}"),
        _ => format!(":(top){pattern}"),
    }
}

/// The pattern a pathspec came from, so that a command's paths can be offered
/// back for editing. A pathspec gitu did not write is shown as it is.
fn pattern_of(pathspec: &str) -> String {
    if let Some(glob) = pathspec.strip_prefix(":(top,exclude)") {
        return format!("!{glob}");
    }

    pathspec
        .strip_prefix(":(top)")
        .unwrap_or(pathspec)
        .to_owned()
}

/// The pattern that excludes one file and nothing else. A path holding
/// wildcards has to say it means them literally; anything else reads better,
/// and round-trips through [`pattern_of`], as a glob.
pub(crate) fn excluding(path: &str) -> String {
    if path.contains(['*', '?', '[', '\\']) {
        return format!(":(top,exclude,literal){path}");
    }

    format!("!{path}")
}

/// `word` as it has to be written to survive being read back, and to mean the
/// same thing if it is pasted into a shell. Revisions (`HEAD~2`, `x^!`) and
/// git's own `k=v` options are left bare; a pathspec's parentheses and
/// wildcards are not.
fn quoted(word: &str) -> String {
    const BARE: &str = "_@%+=:,./-~^";

    if !word.is_empty()
        && word
            .chars()
            .all(|c| c.is_alphanumeric() || BARE.contains(c))
    {
        return word.to_owned();
    }

    format!("'{}'", word.replace('\'', r"'\''"))
}

/// `args` with whatever they said about context dropped, so that what gitu asks
/// for is what git is asked for.
fn without_context<'a>(args: impl Iterator<Item = &'a str>) -> Vec<&'a str> {
    let mut kept = Vec::new();
    let mut value_follows = false;

    for arg in args {
        if std::mem::take(&mut value_follows) {
            continue;
        }
        match arg {
            "-U" | "--unified" => value_follows = true,
            "-W" | "--function-context" | "--no-function-context" => {}
            _ if arg.starts_with("-U") || arg.starts_with("--unified=") => {}
            _ => kept.push(arg),
        }
    }

    kept
}

/// The argv of the git command gitu is paging the output of, looked for in the
/// two places it can be: above gitu, which is where git puts its pager, and
/// beside it, which is where a shell pipeline puts it.
pub(crate) fn paging_for() -> Option<Vec<String>> {
    let mut system = System::new();
    system.refresh_processes_specifics(
        ProcessesToUpdate::All,
        true,
        ProcessRefreshKind::nothing().with_cmd(UpdateKind::Always),
    );

    let me = Pid::from_u32(std::process::id());

    let mut descendant = me;
    for _ in 0..ANCESTORS {
        let Some(ancestor) = system.process(descendant).and_then(Process::parent) else {
            break;
        };
        if let Some(argv) = git_argv(&system, ancestor) {
            return Some(argv);
        }
        descendant = ancestor;
    }

    // `git diff | gitu --pager`: the shell created both, so git is the sibling
    // of gitu rather than its ancestor. Two of them and there is no telling
    // which produced the input.
    let shell = system.process(me)?.parent()?;
    let mut siblings = system
        .processes()
        .iter()
        .filter(|(pid, process)| **pid != me && process.parent() == Some(shell))
        .filter_map(|(pid, _)| git_argv(&system, *pid));

    siblings.next().filter(|_| siblings.next().is_none())
}

/// The argv of `pid`, if it is a git command gitu can put again.
fn git_argv(system: &System, pid: Pid) -> Option<Vec<String>> {
    let argv: Vec<String> = system
        .process(pid)?
        .cmd()
        .iter()
        .map(|arg| arg.to_string_lossy().into_owned())
        .collect();

    GitCommand::of(argv).map(|git| git.argv)
}

#[cfg(test)]
mod tests {
    use super::*;

    fn git(args: &[&str]) -> GitCommand {
        GitCommand::of(args.iter().map(|arg| (*arg).to_owned()).collect()).unwrap()
    }

    fn rerun(args: &[&str], context: Option<&str>) -> Vec<String> {
        let command = git(args).command(context);
        std::iter::once(command.get_program())
            .chain(command.get_args())
            .map(|arg| arg.to_string_lossy().into_owned())
            .collect()
    }

    #[test]
    fn a_git_command_is_recognised_behind_its_own_options() {
        assert_eq!(
            git(&["/usr/bin/git", "-c", "color.diff=always", "diff"]).subcommand(),
            "diff"
        );
        assert_eq!(git(&["git.exe", "show", "HEAD"]).subcommand(), "show");
        assert_eq!(GitCommand::of(vec!["ls".to_owned()]), None);
        assert_eq!(GitCommand::of(vec!["git".to_owned()]), None);
        assert_eq!(
            GitCommand::of(["git", "status"].map(String::from).to_vec()),
            None
        );
    }

    /// Whether a patch is of the working tree, of the index, or of two trees is
    /// what the command says it is.
    #[test]
    fn the_command_says_what_the_patch_is_a_diff_of() {
        assert_eq!(git(&["git", "diff"]).diff_type(), DiffType::WorkdirToIndex);
        assert_eq!(
            git(&["git", "diff", "-U8", "--", "src/lib.rs"]).diff_type(),
            DiffType::WorkdirToIndex
        );
        assert_eq!(
            git(&["git", "diff", "--cached"]).diff_type(),
            DiffType::IndexToTree
        );
        assert_eq!(
            git(&["git", "diff", "main"]).diff_type(),
            DiffType::TreeToTree
        );
        assert_eq!(
            git(&["git", "show", "HEAD"]).diff_type(),
            DiffType::TreeToTree
        );
    }

    /// What the user is given to edit is the question as gitu is putting it,
    /// less the flags gitu forces on every ask.
    #[test]
    fn the_line_offered_for_editing_is_the_effective_command() {
        assert_eq!(git(&["git", "diff", "main"]).line(None), "git diff main");
        assert_eq!(
            git(&["/usr/bin/git", "-c", "color.ui=always", "diff"]).line(None),
            "git -c color.ui=always diff"
        );
        assert_eq!(
            git(&["git", "diff", "-U10", "main"]).line(Some("-U2")),
            "git diff -U2 main"
        );
        assert_eq!(
            git(&["git", "log", "-p", "--", "a file"]).line(None),
            "git log -p -- 'a file'"
        );
    }

    #[test]
    fn an_edit_may_ask_git_anything_that_only_reports() {
        let edited = |line: &str| {
            git(&["/usr/bin/git", "diff", "main"])
                .edited(line)
                .map(|git| git.line(None))
        };

        assert_eq!(edited("git show HEAD~2").unwrap(), "git show HEAD~2");
        assert_eq!(edited("git log -p -- src").unwrap(), "git log -p -- src");
        assert_eq!(
            edited("git diff main -- ':(top,exclude)*_test.go'").unwrap(),
            "git diff main -- ':(top,exclude)*_test.go'"
        );
    }

    /// The program gitu found is the program gitu runs, whatever the edit says.
    #[test]
    fn an_edit_leaves_the_program_and_gits_own_options_alone() {
        let git = git(&["/usr/bin/git", "-c", "color.ui=always", "diff"]);
        let program = |line: &str| Ok::<_, Error>(git.edited(line)?.argv[0].clone());

        assert_eq!(
            program("git -c color.ui=always show").unwrap(),
            "/usr/bin/git"
        );
        assert!(matches!(
            program("/usr/bin/git -c color.ui=always show"),
            Err(Error::EditedCommandFixed)
        ));
        assert!(matches!(
            program("git show"),
            Err(Error::EditedCommandFixed)
        ));
        assert!(matches!(
            program("git -c color.ui=always --git-dir=/elsewhere show"),
            Err(Error::EditedCommandWrites(_))
        ));
    }

    /// The held command is re-run on every rebuild, so nothing that writes may
    /// be held.
    #[test]
    fn an_edit_that_would_change_the_repository_is_refused() {
        let git = git(&["git", "diff"]);

        assert!(matches!(
            git.edited("git commit --amend"),
            Err(Error::EditedCommandWrites(subcommand)) if subcommand == "commit"
        ));
        assert!(matches!(
            git.edited("git reset --hard"),
            Err(Error::EditedCommandWrites(_))
        ));
        assert!(matches!(git.edited("git"), Err(Error::EditedCommandFixed)));
        assert!(matches!(
            git.edited("git diff 'unbalanced"),
            Err(Error::EditedCommandQuotes)
        ));
    }

    /// The paths a command asks for are what the user wrote, so they can be
    /// offered back and asked again unchanged.
    #[test]
    fn the_paths_asked_for_round_trip_through_their_patterns() {
        let patterns = ["src", "!*_test.go", ":(top,exclude,literal)a[1].go"].map(String::from);
        let asked = git(&["git", "diff", "main"]).asking_for(&patterns);

        assert_eq!(
            asked.line(None),
            "git diff main -- ':(top)src' ':(top,exclude)*_test.go' ':(top,exclude,literal)a[1].go'"
        );
        assert_eq!(asked.file_patterns(), patterns);
        assert_eq!(asked.asking_for(&asked.file_patterns()), asked);
    }

    /// Asking for other paths replaces the ones asked for before, and asking
    /// for none leaves no `--` behind.
    #[test]
    fn the_paths_asked_for_are_replaced_wholesale() {
        let git = git(&["git", "diff", "-U8", "main", "--", "src"]);

        assert_eq!(
            git.asking_for(&["lib".to_owned()]).line(None),
            "git diff -U8 main -- ':(top)lib'"
        );
        assert_eq!(git.asking_for(&[]).line(None), "git diff -U8 main");
    }

    /// A file is hidden by name, and a name that holds wildcards has to say it
    /// means them literally.
    #[test]
    fn one_file_is_excluded_by_its_own_name() {
        assert_eq!(excluding("src/gen/a.pb.go"), "!src/gen/a.pb.go");
        assert_eq!(
            excluding("src/a[1].go"),
            ":(top,exclude,literal)src/a[1].go"
        );
    }

    #[test]
    fn asking_again_keeps_the_question_and_replaces_the_context() {
        assert_eq!(
            rerun(&["git", "-c", "color.ui=always", "diff", "main"], None),
            [
                "git",
                "-c",
                "color.ui=always",
                "diff",
                "--no-ext-diff",
                "--no-color",
                "main"
            ]
        );
        assert_eq!(
            rerun(&["git", "diff", "-U10", "main", "--", "src"], Some("-U2")),
            [
                "git",
                "diff",
                "--no-ext-diff",
                "--no-color",
                "-U2",
                "main",
                "--",
                "src"
            ]
        );
        assert_eq!(
            rerun(&["git", "diff", "--unified", "10"], Some("-W")),
            ["git", "diff", "--no-ext-diff", "--no-color", "-W"]
        );
        assert_eq!(
            rerun(&["git", "diff", "--function-context"], Some("-U3")),
            ["git", "diff", "--no-ext-diff", "--no-color", "-U3"]
        );
    }
}