feat(users/Profpatsch): add die_* helpers for semantic exit errors r/2176

There is this semantic exit code schema championed by execline and
skaware tooling, and we refined and documented it a bit in lorri
https://github.com/nix-community/lorri/blob/d1d673d42090f0cfe8ab9b92b465315a9e7d30a3/src/ops/mod.rs#L24-L35
in the past.

This just transcribes the error messages into simple helper functions.

Applies the functions to the places where we would panic or die
`sys::exit()` instead.

Change-Id: I15ca05cd6f99a25a3378518be94110eab416354e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2475
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

5 files changed, 80 insertions, 28 deletions

diff --git a/users/Profpatsch/execline/exec_helpers.rs b/users/Profpatsch/execline/exec_helpers.rs
index 4e4149882b40..3e74ffc72210 100644
--- a/users/Profpatsch/execline/exec_helpers.rs
+++ b/users/Profpatsch/execline/exec_helpers.rs
@@ -36,6 +36,59 @@ pub fn exec_into_args<'a, 'b, Args, Arg, Env, Key, Val>(current_prog_name: &str,
     let env = env_additions.into_iter().collect::<Vec<(Key, Val)>>();
     let env = env.iter().map(|(k,v)| (OsStr::from_bytes(k.as_ref()), OsStr::from_bytes(v.as_ref())));
     let err = std::process::Command::new(prog).args(args).envs(env).exec();
-    panic!("{}: exec failed: {:?}", current_prog_name, err);
+    die_missing_executable(current_prog_name, format!("exec failed: {:?}", err));
 }
 
+/// Exit 1 to signify a generic expected error
+/// (e.g. something that sometimes just goes wrong, like a nix build).
+pub fn die_expected_error<S>(current_prog_name: &str, msg: S) -> !
+where S: AsRef<str>
+{
+  die_with(1, current_prog_name, msg)
+}
+
+/// Exit 100 to signify a user error (“the user is holding it wrong”).
+/// This is a permanent error, if the program is executed the same way
+/// it should crash with 100 again.
+pub fn die_user_error<S>(current_prog_name: &str, msg: S) -> !
+where S: AsRef<str>
+{
+    die_with(100, current_prog_name, msg)
+}
+
+/// Exit 101 to signify an unexpected crash (failing assertion or panic).
+/// This is the same exit code that `panic!()` emits.
+pub fn die_panic<S>(current_prog_name: &str, msg: S) -> !
+where S: AsRef<str>
+{
+    die_with(101, current_prog_name, msg)
+}
+
+/// Exit 111 to signify a temporary error (such as resource exhaustion)
+pub fn die_temporary<S>(current_prog_name: &str, msg: S) -> !
+where S: AsRef<str>
+{
+    die_with(111, current_prog_name, msg)
+}
+
+/// Exit 126 to signify an environment problem
+/// (the user has set up stuff incorrectly so the program cannot work)
+pub fn die_environment_problem<S>(current_prog_name: &str, msg: S) -> !
+where S: AsRef<str>
+{
+    die_with(126, current_prog_name, msg)
+}
+
+/// Exit 127 to signify a missing executable.
+pub fn die_missing_executable<S>(current_prog_name: &str, msg: S) -> !
+where S: AsRef<str>
+{
+    die_with(127, current_prog_name, msg)
+}
+
+fn die_with<S>(status: i32, current_prog_name: &str, msg: S) -> !
+    where S: AsRef<str>
+{
+    eprintln!("{}: {}", current_prog_name, msg.as_ref());
+    std::process::exit(status)
+}
diff --git a/users/Profpatsch/netencode/default.nix b/users/Profpatsch/netencode/default.nix
index fb1d2c2ef831..b9822d48225a 100644
--- a/users/Profpatsch/netencode/default.nix
+++ b/users/Profpatsch/netencode/default.nix
@@ -30,7 +30,10 @@ let
 
   netencode-rs-common = tests: imports.writers.rustSimpleLib {
     name = "netencode";
-    dependencies = [ nom ];
+    dependencies = [
+      nom
+      depot.users.Profpatsch.execline.exec-helpers
+    ];
     buildTests = tests;
     release = false;
     verbose = true;
@@ -101,13 +104,13 @@ let
     use netencode::dec::{Record, ScalarAsBytes, Decoder, DecodeError};
 
     fn main() {
-        let t = netencode::t_from_stdin_or_panic("record-splice-env");
+        let t = netencode::t_from_stdin_or_die_user_error("record-splice-env");
         let (_, prog) = exec_helpers::args_for_exec("record-splice-env", 0);
         match Record::<ScalarAsBytes>::dec(t) {
             Ok(map) => {
                 exec_helpers::exec_into_args("record-splice-env", prog, map);
             },
-            Err(DecodeError(err)) => panic!("{}", err),
+            Err(DecodeError(err)) => exec_helpers::die_user_error("record-splice-env", err),
         }
     }
   '';
diff --git a/users/Profpatsch/netencode/netencode.rs b/users/Profpatsch/netencode/netencode.rs
index 66f3245fcf99..f6158ae3e6a0 100644
--- a/users/Profpatsch/netencode/netencode.rs
+++ b/users/Profpatsch/netencode/netencode.rs
@@ -1,4 +1,5 @@
 extern crate nom;
+extern crate exec_helpers;
 
 use std::collections::HashMap;
 use std::io::{Write, Read};
@@ -116,15 +117,15 @@ pub fn text(s: String) -> T {
     T::Text(s)
 }
 
-pub fn t_from_stdin_or_panic(prog_name: &str) -> T {
+pub fn t_from_stdin_or_die_user_error(prog_name: &str) -> T {
     let mut buf = vec![];
     std::io::stdin().lock().read_to_end(&mut buf);
     match parse::t_t(&buf) {
         Ok((rest, t)) => match rest {
             b"" => t,
-            _ => panic!("{}: stdin contained some soup after netencode value: {:?}", prog_name, rest)
+            _ => exec_helpers::die_user_error(prog_name, format!("stdin contained some soup after netencode value: {:?}", rest))
         },
-        Err(err) => panic!("{}: unable to parse netencode from stdin: {:?}", prog_name, err)
+        Err(err) => exec_helpers::die_user_error(prog_name, format!("unable to parse netencode from stdin: {:?}", err))
     }
 }
 
diff --git a/users/Profpatsch/read-http/default.nix b/users/Profpatsch/read-http.nix
index 41fe1c7fedcc..aff1fa8662e1 100644
--- a/users/Profpatsch/read-http/default.nix
+++ b/users/Profpatsch/read-http.nix
@@ -10,11 +10,8 @@ let
       depot.users.Profpatsch.rust-crates.httparse
       depot.users.Profpatsch.netencode.netencode-rs
       depot.users.Profpatsch.arglib.netencode.rust
+      depot.users.Profpatsch.execline.exec-helpers
     ];
   } (builtins.readFile ./read-http.rs);
 
-in {
-  inherit
-    read-http
-    ;
-}
+in read-http
diff --git a/users/Profpatsch/read-http/read-http.rs b/users/Profpatsch/read-http.rs
index ab2c3887d7b5..de112f4c772d 100644
--- a/users/Profpatsch/read-http/read-http.rs
+++ b/users/Profpatsch/read-http.rs
@@ -2,10 +2,12 @@ extern crate httparse;
 extern crate netencode;
 extern crate arglib_netencode;
 extern crate ascii;
+extern crate exec_helpers;
 
 use std::os::unix::io::FromRawFd;
 use std::io::Read;
 use std::io::Write;
+use exec_helpers::{die_user_error, die_expected_error, die_temporary};
 
 use netencode::{U, T};
 
@@ -15,25 +17,21 @@ enum What {
 }
 
 fn main() -> std::io::Result<()> {
-    fn die<T: std::fmt::Display>(msg: T) -> ! {
-        eprintln!("{}", msg);
-        std::process::exit(1);
-    }
 
     let what : What = match arglib_netencode::arglib_netencode(None).unwrap() {
         T::Record(rec) => match rec.get("what") {
             Some(T::Text(t)) => match t.as_str() {
                 "request" => What::Request,
                 "response" => What::Response,
-                _ => die("read-http arglib: what should be either t:request or t:response"),
+                _ => die_user_error("read-http arglib", "`what` should be either t:request or t:response"),
             },
-            Some(o) => die(format!("read-http arglib: expected a record of text, got {:#?}", o)),
+            Some(o) => die_user_error("read-http arglib", format!("expected a record of text, got {:#?}", o)),
             None => {
                 eprintln!("read-http arglib: no `what` given, defaulting to Response");
                 What::Response
             }
         }
-        o => die(format!("read-http arglib: expected a record, got {:#?}", o))
+        o => die_user_error("read-http arglib", format!("expected a record, got {:#?}", o))
     };
 
     fn read_stdin_to_complete<F>(mut parse: F) -> ()
@@ -49,13 +47,13 @@ fn main() -> std::io::Result<()> {
                 Ok(size) => if size == 0 {
                     break;
                 },
-                Err(err) => panic!("could not read from stdin, {:?}", err)
+                Err(err) => die_temporary("read-http", format!("could not read from stdin, {:?}", err))
             }
             match parse(&buf) {
                 Ok(status) => {
                     res = status;
                 }
-                Err(err) => die(format!("httparse parsing failed: {:#?}", err))
+                Err(err) => die_temporary("read-http", format!("httparse parsing failed: {:#?}", err))
             }
         }
     }
@@ -84,7 +82,7 @@ fn main() -> std::io::Result<()> {
                         return Some(());
                     }
                 },
-                Some(Err(err)) => die(format!("error reading from stdin: {:?}", err)),
+                Some(Err(err)) => die_temporary("read-http", format!("error reading from stdin: {:?}", err)),
                 None => return None
             }
         }
@@ -101,10 +99,10 @@ fn main() -> std::io::Result<()> {
             match read_till_end_of_header(&mut buf, stdin.lock()) {
                 Some(()) => match req.parse(&buf) {
                     Ok(httparse::Status::Complete(_body_start)) => {},
-                    Ok(httparse::Status::Partial) => die("httparse should have gotten a full header"),
-                    Err(err) => die(format!("httparse response parsing failed: {:#?}", err))
+                    Ok(httparse::Status::Partial) => die_expected_error("read-http", "httparse should have gotten a full header"),
+                    Err(err) => die_expected_error("read-http", format!("httparse response parsing failed: {:#?}", err))
                 },
-                None => die(format!("httparse end of stdin reached before able to parse request headers"))
+                None => die_expected_error("read-http", format!("httparse end of stdin reached before able to parse request headers"))
             }
             let method = req.method.expect("method must be filled on complete parse");
             let path = req.path.expect("path must be filled on complete parse");
@@ -116,10 +114,10 @@ fn main() -> std::io::Result<()> {
             match read_till_end_of_header(&mut buf, stdin.lock()) {
                 Some(()) => match resp.parse(&buf) {
                     Ok(httparse::Status::Complete(_body_start)) => {},
-                    Ok(httparse::Status::Partial) => die("httparse should have gotten a full header"),
-                    Err(err) => die(format!("httparse response parsing failed: {:#?}", err))
+                    Ok(httparse::Status::Partial) => die_expected_error("read-http", "httparse should have gotten a full header"),
+                    Err(err) => die_expected_error("read-http", format!("httparse response parsing failed: {:#?}", err))
                 },
-                None => die(format!("httparse end of stdin reached before able to parse response headers"))
+                None => die_expected_error("read-http", format!("httparse end of stdin reached before able to parse response headers"))
             }
             let code = resp.code.expect("code must be filled on complete parse");
             let reason = resp.reason.expect("reason must be filled on complete parse");