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/tests/sed.rs

@@ -0,0 +1,83 @@
+extern crate codex_execpolicy;
+
+use codex_execpolicy::get_default_policy;
+use codex_execpolicy::ArgType;
+use codex_execpolicy::Error;
+use codex_execpolicy::ExecCall;
+use codex_execpolicy::MatchedArg;
+use codex_execpolicy::MatchedExec;
+use codex_execpolicy::MatchedFlag;
+use codex_execpolicy::MatchedOpt;
+use codex_execpolicy::Policy;
+use codex_execpolicy::Result;
+use codex_execpolicy::ValidExec;
+
+fn setup() -> Policy {
+    get_default_policy().expect("failed to load default policy")
+}
+
+#[test]
+fn test_sed_print_specific_lines() -> Result<()> {
+    let policy = setup();
+    let sed = ExecCall::new("sed", &["-n", "122,202p", "hello.txt"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec {
+                program: "sed".to_string(),
+                flags: vec![MatchedFlag::new("-n")],
+                args: vec![
+                    MatchedArg::new(1, ArgType::SedCommand, "122,202p")?,
+                    MatchedArg::new(2, ArgType::ReadableFile, "hello.txt")?,
+                ],
+                system_path: vec!["/usr/bin/sed".to_string()],
+                ..Default::default()
+            }
+        }),
+        policy.check(&sed)
+    );
+    Ok(())
+}
+
+#[test]
+fn test_sed_print_specific_lines_with_e_flag() -> Result<()> {
+    let policy = setup();
+    let sed = ExecCall::new("sed", &["-n", "-e", "122,202p", "hello.txt"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec {
+                program: "sed".to_string(),
+                flags: vec![MatchedFlag::new("-n")],
+                opts: vec![MatchedOpt::new("-e", "122,202p", ArgType::SedCommand).unwrap()],
+                args: vec![MatchedArg::new(3, ArgType::ReadableFile, "hello.txt")?],
+                system_path: vec!["/usr/bin/sed".to_string()],
+            }
+        }),
+        policy.check(&sed)
+    );
+    Ok(())
+}
+
+#[test]
+fn test_sed_reject_dangerous_command() {
+    let policy = setup();
+    let sed = ExecCall::new("sed", &["-e", "s/y/echo hi/e", "hello.txt"]);
+    assert_eq!(
+        Err(Error::SedCommandNotProvablySafe {
+            command: "s/y/echo hi/e".to_string(),
+        }),
+        policy.check(&sed)
+    );
+}
+
+#[test]
+fn test_sed_verify_e_or_pattern_is_required() {
+    let policy = setup();
+    let sed = ExecCall::new("sed", &["122,202p"]);
+    assert_eq!(
+        Err(Error::MissingRequiredOptions {
+            program: "sed".to_string(),
+            options: vec!["-e".to_string()],
+        }),
+        policy.check(&sed)
+    );
+}