Greatly simplify parsing using a specialized serde deserializer

..for the "service" field

src/main.rs

@@ -4,7 +4,7 @@ use std::io::BufReader;
 use std::collections::{HashMap, HashSet};
 use std::path::{Path, PathBuf};
 use std::process::ExitCode;
-use anyhow::{anyhow, Result};
+use anyhow::Result;
 
 mod parsing;
 use parsing::*;
@@ -73,8 +73,8 @@ impl Ctx {
     pub fn add_file(&mut self, file: &Path) -> anyhow::Result<()> {
         let cfg: Config = {
             let file = File::open(file)?;
-            let body: hcl::Body = hcl::from_reader(BufReader::new(file))?;
-            Config::from_body(&body).map_err(|s| anyhow!("{}", s))?
+            let config: Config = hcl::from_reader(BufReader::new(file))?;
+            config
         };
 
         for (jobname, job) in cfg.jobs {

src/parsing.rs

@@ -1,5 +1,10 @@
-use serde::Deserialize;
-use hcl::{Structure, Body, Map, Expression};
+use serde::de::{
+    value::{MapAccessDeserializer, SeqAccessDeserializer},
+    MapAccess, SeqAccess, Visitor,
+};
+use serde::{Deserialize, Deserializer};
+use std::fmt;
+use std::marker::PhantomData;
 
 #[derive(Deserialize, Debug)]
 pub struct Config {
@@ -10,142 +15,68 @@ pub struct Config {
 #[derive(Deserialize, Debug)]
 pub struct Job {
     pub datacenters: Vec<String>,
-    #[serde(rename = "group")]
+    #[serde(rename = "group", default = "hcl::Map::new")]
     pub groups: hcl::Map<String, Group>,
 }
 
 #[derive(Deserialize, Debug)]
 pub struct Group {
-    #[serde(rename = "task")]
+    #[serde(rename = "task", default = "hcl::Map::new")]
     pub tasks: hcl::Map<String, Task>,
 }
 
 #[derive(Deserialize, Debug)]
 pub struct Task {
-    #[serde(rename = "service")]
+    #[serde(rename = "service", deserialize_with = "vec_or_map", default = "Vec::new")]
     pub services: Vec<Service>,
 }
 
 #[derive(Deserialize, Debug)]
 pub struct Service {
     pub name: Option<String>,
+    #[serde(default = "Vec::new")]
     pub tags: Vec<String>,
 }
 
-// lots of manual boilerplate because I can't figure out how to use the
-// automated deserialization for blocks without labels (e.g. "service") that may
-// appear one or several times:
-// https://github.com/martinohmann/hcl-rs/issues/380
-
-impl Config {
-    pub fn from_body(b: &Body) -> Result<Self, String> {
-        Ok(Config { jobs: blocks_from_body(b, "job", Job::from_body)? })
-    }
-}
-
-impl Job {
-    fn from_body(b: &Body) -> Result<Self, String> {
-        let datacenters =
-            strings_from_expr(&attribute_from_body(b, "datacenters")?)?;
-        let groups = blocks_from_body(b, "group", Group::from_body)?;
-        Ok(Job { datacenters, groups })
-    }
-}
-
-impl Group {
-    fn from_body(b: &Body) -> Result<Self, String> {
-        Ok(Group { tasks: blocks_from_body(b, "task", Task::from_body)? })
-    }
-}
-
-impl Task {
-    fn from_body(b: &Body) -> Result<Self, String> {
-        Ok(Task { services: blocks_nolabel_from_body(b, "service", Service::from_body)? })
-    }
-}
-
-impl Service {
-    fn from_body(b: &Body) -> Result<Self, String> {
-        let name =
-            attribute_from_body(b, "name").ok()
-            .map(|e| string_from_expr(&e)).transpose()?;
-        let tags = match attribute_from_body(b, "tags") {
-            Err(_) => Vec::new(),
-            Ok(tags) => tags_from_expr(&tags)?,
-        };
-        Ok(Service { name, tags })
-    }
-}
-
-fn blocks_from_body<T, F>(b: &Body, id: &str, f: F) -> Result<Map<String, T>, String>
-    where F: Fn(&Body) -> Result<T, String>
+// In .hcl files, a field can sometimes container either a record (for a single
+// item), or a sequence of records (for several items). This is not something
+// that the built-in hcl deserializer handles. Instead, the following
+// deserializer can be attached to fields that follow this schema with the
+// "deserialize_with" serde attribute.
+//
+// Thanks to the hcl-rs maintener for the code snippet
+// (https://github.com/martinohmann/hcl-rs/issues/380#issuecomment-2456546232)
+fn vec_or_map<'de, T, D>(deserializer: D) -> Result<Vec<T>, D::Error>
+where
+    T: Deserialize<'de>,
+    D: Deserializer<'de>,
 {
-    let mut blocks = Map::new();
-    for s in &b.0 {
-        if let Structure::Block(block) = s {
-            if block.identifier.as_str() == id {
-                if block.labels.len() != 1 {
-                    return Err(format!("{} block: unexpected number of labels", id))
-                }
-                blocks.insert(block.labels[0].as_str().to_string(), f(&block.body)?);
-            }
-        }
-    }
-    Ok(blocks)
-}
+    struct VecOrMap<T>(PhantomData<fn() -> T>);
 
-fn blocks_nolabel_from_body<T, F>(b: &Body, id: &str, f: F) -> Result<Vec<T>, String>
-    where F: Fn(&Body) -> Result<T, String>
-{
-    let mut blocks = Vec::new();
-    for s in &b.0 {
-        if let Structure::Block(block) = s {
-            if block.identifier.as_str() == id {
-                blocks.push(f(&block.body)?)
-            }
-        }
-    }
-    Ok(blocks)
-}
+    impl<'de, T> Visitor<'de> for VecOrMap<T>
+    where
+        T: Deserialize<'de>,
+    {
+        type Value = Vec<T>;
 
-fn attribute_from_body(b: &Body, name: &str) -> Result<Expression, String>
-{
-    b.0.iter().find_map(|s| {
-        if let Structure::Attribute(attr) = s {
-            if attr.key.as_str() == name {
-                return Some(attr.expr.clone())
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+            formatter.write_str("sequence or map")
         }
-        }
-        return None
-    }).ok_or(format!("attribute {} not found", name))
-}
 
-fn string_from_expr(e: &Expression) -> Result<String, String>
-{
-    match e {
-        Expression::String(s) => Ok(s.to_string()),
-        _ => Err(format!("string expected, got {:?}", &e)),
+        fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
+        where
+            A: SeqAccess<'de>,
+        {
+            Deserialize::deserialize(SeqAccessDeserializer::new(seq))
         }
-}
 
-fn tags_from_expr(e: &Expression) -> Result<Vec<String>, String>
-{
-    match e {
-        Expression::Array(es) => {
-            Ok(es.into_iter().filter_map(|e| match string_from_expr(e) {
-                Ok(e) => Some(e),
-                Err(_) => { println!("note: ignoring tag {:?}", &e); None },
-            }).collect())
-        },
-        _ => Err(format!("array expected, got {:#?}", &e))
+        fn visit_map<M>(self, map: M) -> Result<Self::Value, M::Error>
+        where
+            M: MapAccess<'de>,
+        {
+            Deserialize::deserialize(MapAccessDeserializer::new(map)).map(|value| vec![value])
+        }
     }
-}
 
-fn strings_from_expr(e: &Expression) -> Result<Vec<String>, String>
-{
-    match e {
-        Expression::Array(es) =>
-            es.into_iter().map(string_from_expr).collect(),
-        _ => Err(format!("array expected, got {:#?}", &e))
-    }
+    deserializer.deserialize_any(VecOrMap(PhantomData))
 }