feat: introduce codex_execpolicy crate for defining "safe" commands (#634)

As described in detail in `codex-rs/execpolicy/README.md` introduced in
this PR, `execpolicy` is a tool that lets you define a set of _patterns_
used to match [`execv(3)`](https://linux.die.net/man/3/execv)
invocations. When a pattern is matched, `execpolicy` returns the parsed
version in a structured form that is amenable to static analysis.

The primary use case is to define patterns match commands that should be
auto-approved by a tool such as Codex. This supports a richer pattern
matching mechanism that the sort of prefix-matching we have done to
date, e.g.:


5e40d9d221/codex-cli/src/approvals.ts (L333-L354)

Note we are still playing with the API and the `system_path` option in
particular still needs some work.

codex-rs/execpolicy/src/policy.rs

@@ -0,0 +1,103 @@
+use multimap::MultiMap;
+use regex::Error as RegexError;
+use regex::Regex;
+
+use crate::error::Error;
+use crate::error::Result;
+use crate::policy_parser::ForbiddenProgramRegex;
+use crate::program::PositiveExampleFailedCheck;
+use crate::ExecCall;
+use crate::Forbidden;
+use crate::MatchedExec;
+use crate::NegativeExamplePassedCheck;
+use crate::ProgramSpec;
+
+pub struct Policy {
+    programs: MultiMap<String, ProgramSpec>,
+    forbidden_program_regexes: Vec<ForbiddenProgramRegex>,
+    forbidden_substrings_pattern: Option<Regex>,
+}
+
+impl Policy {
+    pub fn new(
+        programs: MultiMap<String, ProgramSpec>,
+        forbidden_program_regexes: Vec<ForbiddenProgramRegex>,
+        forbidden_substrings: Vec<String>,
+    ) -> std::result::Result<Self, RegexError> {
+        let forbidden_substrings_pattern = if forbidden_substrings.is_empty() {
+            None
+        } else {
+            let escaped_substrings = forbidden_substrings
+                .iter()
+                .map(|s| regex::escape(s))
+                .collect::<Vec<_>>()
+                .join("|");
+            Some(Regex::new(&format!("({escaped_substrings})"))?)
+        };
+        Ok(Self {
+            programs,
+            forbidden_program_regexes,
+            forbidden_substrings_pattern,
+        })
+    }
+
+    pub fn check(&self, exec_call: &ExecCall) -> Result<MatchedExec> {
+        let ExecCall { program, args } = &exec_call;
+        for ForbiddenProgramRegex { regex, reason } in &self.forbidden_program_regexes {
+            if regex.is_match(program) {
+                return Ok(MatchedExec::Forbidden {
+                    cause: Forbidden::Program {
+                        program: program.clone(),
+                        exec_call: exec_call.clone(),
+                    },
+                    reason: reason.clone(),
+                });
+            }
+        }
+
+        for arg in args {
+            if let Some(regex) = &self.forbidden_substrings_pattern {
+                if regex.is_match(arg) {
+                    return Ok(MatchedExec::Forbidden {
+                        cause: Forbidden::Arg {
+                            arg: arg.clone(),
+                            exec_call: exec_call.clone(),
+                        },
+                        reason: format!("arg `{}` contains forbidden substring", arg),
+                    });
+                }
+            }
+        }
+
+        let mut last_err = Err(Error::NoSpecForProgram {
+            program: program.clone(),
+        });
+        if let Some(spec_list) = self.programs.get_vec(program) {
+            for spec in spec_list {
+                match spec.check(exec_call) {
+                    Ok(matched_exec) => return Ok(matched_exec),
+                    Err(err) => {
+                        last_err = Err(err);
+                    }
+                }
+            }
+        }
+        last_err
+    }
+
+    pub fn check_each_good_list_individually(&self) -> Vec<PositiveExampleFailedCheck> {
+        let mut violations = Vec::new();
+        for (_program, spec) in self.programs.flat_iter() {
+            violations.extend(spec.verify_should_match_list());
+        }
+        violations
+    }
+
+    pub fn check_each_bad_list_individually(&self) -> Vec<NegativeExamplePassedCheck> {
+        let mut violations = Vec::new();
+        for (_program, spec) in self.programs.flat_iter() {
+            violations.extend(spec.verify_should_not_match_list());
+        }
+        violations
+    }
+}