IT WORKS

src/args.rs

@@ -1,4 +1,7 @@
-use std::sync::Arc;
+use std::{
+    env,
+    sync::{Arc, LazyLock},
+};
 
 use clap::ColorChoice;
 
@@ -52,6 +55,7 @@ impl FromStr for NixOsOption {
 pub struct AppendCmd {
     #[arg(required = true)]
     pub name: Arc<str>,
+
     #[arg(required = true)]
     pub value: Arc<str>,
 }
@@ -67,6 +71,28 @@ pub enum Subcommand {
     Delta(DeltaCmd),
 }
 
+static DEFAULT_PATH: LazyLock<Box<OsStr>> = LazyLock::new(|| {
+    // This has to be in a let binding to keep the storage around.
+    let nixos_config = env::var_os("NIXOS_CONFIG");
+    let nixos_config = nixos_config
+        .as_deref()
+        .map(Path::new)
+        .unwrap_or(Path::new("/etc/nixos/configuration.nix"));
+
+    nixos_config
+        .parent()
+        .unwrap_or_else(|| {
+            error!(
+                "Your $NIXOS_CONFIG value doesn't make sense: {}. Ignoring.",
+                nixos_config.display(),
+            );
+            Path::new("/etc/nixos")
+        })
+        .join("dynamic.nix")
+        .into_os_string()
+        .into_boxed_os_str()
+});
+
 #[derive(Debug, Clone, PartialEq, clap::Parser)]
 #[command(version, about, author)]
 #[command(arg_required_else_help(true), args_override_self(true))]
@@ -75,8 +101,10 @@ pub struct Args {
     #[arg(long, global(true), default_value = "auto")]
     pub color: ColorChoice,
 
-    // FIXME: default to /etc/configuration.nix, or something?
-    #[arg(long, global(true), default_value = "./configuration.nix")]
+    /// The .nix file with dynamic overrides to modify.
+    /// [default: $(dirname ${NIXOS_CONFIG-/etc/nixos/configuration.nix})/dynamic.nix]
+    #[arg(long, global(true), default_value = &**DEFAULT_PATH)]
+    #[arg(hide_default_value(true))]
     pub file: Arc<OsStr>,
 
     #[command(subcommand)]

src/lib.rs

@@ -1,4 +1,7 @@
-use std::{iter, sync::Arc};
+use std::{
+    iter,
+    sync::{Arc, LazyLock},
+};
 
 pub(crate) mod prelude {
     #![allow(unused_imports)]
@@ -41,14 +44,38 @@ pub mod line;
 mod nixcmd;
 pub use line::Line;
 pub mod source;
-pub use source::SourceLine;
+pub use source::{SourceFile, SourceLine};
 
+#[cfg(all(not(feature = "regex-full"), not(feature = "regex-lite")))]
+compile_error!("At least one of features `regex-full` or `regex-lite` must be used");
+
+#[cfg(feature = "regex-full")]
+use regex as _regex;
+
+// Having both `regex-full` and `regex-lite` isn't an error; it's just wasteful.
+#[cfg(not(feature = "regex-full"))]
+use regex_lite as _regex;
+
+use _regex::Regex;
+
+use itertools::Itertools;
 use serde::{Deserialize, Serialize};
 
-use crate::source::SourceFile;
-
 pub const ASCII_WHITESPACE: &[char] = &['\t', '\n', '\x0C', '\r', ' '];
 
+/// Regex pattern to extract the priority in a `lib.mkOverride` call.
+static MK_OVERRIDE_RE: LazyLock<Regex> = LazyLock::new(|| {
+    // Named capture group: priority
+    // - Word boundary
+    // - Literal `mkOverride`
+    // - One or more whitespace characters
+    // - Literal open parenthesis
+    // - Named capture group "priority"
+    //   - One or more of: digit characters, or literal `-`
+    // - Literal close parenthesis
+    Regex::new(r"(?-u)\bmkOverride\s+\((?<priority>[\d-]+)\)").unwrap()
+});
+
 #[tracing::instrument(level = "debug")]
 pub fn do_delta(args: Arc<Args>, delta_args: DeltaCmd) -> Result<(), BoxDynError> {
     todo!();
@@ -65,17 +92,15 @@ pub fn do_append(args: Arc<Args>, append_args: AppendCmd) -> Result<(), BoxDynEr
         filepath.to_path_buf()
     };
 
-    // Get what file that thing is defined in.
-    let def_path = get_where(&append_args.name, &filepath)?;
-
     let mut opts = File::options();
     opts.read(true)
         .write(true)
         .create(false)
         .custom_flags(libc::O_CLOEXEC);
-    let source_file = SourceFile::open_from(Arc::from(def_path), opts)?;
 
-    let pri = get_highest_prio(&append_args.name, source_file.clone())?;
+    let source_file = SourceFile::open_from(Arc::from(filepath), opts)?;
+    let pri = get_where(source_file.clone())?;
+
     let new_pri = pri - 1;
 
     let new_pri_line = get_next_prio_line(
@@ -85,7 +110,7 @@ pub fn do_append(args: Arc<Args>, append_args: AppendCmd) -> Result<(), BoxDynEr
         append_args.value.into(),
     )?;
 
-    eprintln!("new_pri_line={new_pri_line}");
+    debug!("new_pri_line={new_pri_line}");
 
     write_next_prio(source_file, new_pri_line)?;
 
@@ -108,35 +133,30 @@ pub fn expr_for_configuration(source_file: &Path) -> OsString {
     .collect()
 }
 
-pub fn get_where(option_name: &str, configuration_nix: &Path) -> Result<Box<Path>, BoxDynError> {
-    let expr = expr_for_configuration(configuration_nix);
-    let attrpath = format!("options.{}.definitionsWithLocations", option_name);
-
-    let output = nixcmd::NixEvalExpr { expr, attrpath }
-        .into_command()
-        .output_checked_utf8()?;
-    let stdout = output.stdout();
-
-    let definitions: Box<[DefinitionWithLocation]> = serde_json::from_str(&stdout)?;
-    let last_location = definitions.into_iter().last().unwrap();
-
-    Ok(Box::from(last_location.file))
+fn maybe_extract_prio_from_line(line: &SourceLine) -> Option<i64> {
+    MK_OVERRIDE_RE
+        .captures(line.text_ref())
+        .map(|caps| caps.name("priority").unwrap().as_str())
+        .map(|prio_str| {
+            i64::from_str(prio_str).unwrap_or_else(|e| {
+                panic!(
+                    "lib.mkOverride called with non-integer {}: {}. Nix source code is wrong!\n{}",
+                    prio_str, e, line,
+                );
+            })
+        })
 }
 
-pub fn get_highest_prio(option_name: &str, source: SourceFile) -> Result<i64, BoxDynError> {
-    // Get the current highest priority.
+pub fn get_where(dynamic_nix: SourceFile) -> Result<i64, BoxDynError> {
+    let lines = dynamic_nix.lines()?;
+    let prio = lines
+        .into_iter()
+        .filter_map(maybe_extract_prio_from_line)
+        .sorted_unstable()
+        .next() // Priorities with lower integer values are "stronger" priorities.
+        .unwrap_or(0);
 
-    let expr = expr_for_configuration(&source.path());
-
-    // Get the highest priority, and the file its defined in.
-    let attrpath = format!("options.{}.highestPrio", option_name);
-    let output = nixcmd::NixEvalExpr { expr, attrpath }
-        .into_command()
-        .output_checked_utf8()?;
-    let stdout = output.stdout();
-    let highest_prio = i64::from_str(stdout.trim())?;
-
-    Ok(highest_prio)
+    Ok(prio)
 }
 
 pub fn get_next_prio_line(
@@ -146,15 +166,19 @@ pub fn get_next_prio_line(
     new_value: Arc<str>,
 ) -> Result<SourceLine, BoxDynError> {
     let source_lines = source.lines()?;
-    let last_line = source_lines.last();
-    assert_eq!(last_line.map(SourceLine::text).as_deref(), Some("]"));
-    let last_line = last_line.unwrap();
+    let penultimate = source_lines.get(source_lines.len() - 2);
+    // FIXME: don't rely on whitespace lol
+    debug_assert_eq!(penultimate.map(SourceLine::text).as_deref(), Some("  ];"));
+    let penultimate = penultimate.unwrap();
+
+    let new_generation = 0 - new_prio;
 
     let new_line = SourceLine {
-        line: last_line.line,
+        line: penultimate.line,
         path: source.path(),
         text: Arc::from(format!(
-            "    {option_name} = lib.mkOverride ({new_prio}) ({new_value});",
+            "      {} = lib.mkOverride ({}) ({}); # DYNIX GENERATION {}",
+            option_name, new_prio, new_value, new_generation,
         )),
     };
 
@@ -165,12 +189,12 @@ pub fn write_next_prio(mut source: SourceFile, new_line: SourceLine) -> Result<(
     let new_mod_start = SourceLine {
         line: new_line.line.prev(),
         path: source.path(),
-        text: Arc::from("  {"),
+        text: Arc::from("    {"),
     };
     let new_mod_end = SourceLine {
         line: new_line.line.next(),
         path: source.path(),
-        text: Arc::from("  }"),
+        text: Arc::from("    }"),
     };
 
     source.insert_lines(&[new_mod_start, new_line, new_mod_end])?;