Cleanup example metadata parsing utilities(#20251) (#20252)

## Which issue does this PR close?

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- Closes #https://github.com/apache/datafusion/issues/20251.

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datafusion-examples/src/utils/csv_to_parquet.rs

@@ -18,9 +18,8 @@
 use std::path::{Path, PathBuf};
 
 use datafusion::dataframe::DataFrameWriteOptions;
-use datafusion::error::Result;
+use datafusion::error::{DataFusionError, Result};
 use datafusion::prelude::{CsvReadOptions, SessionContext};
-use datafusion_common::DataFusionError;
 use tempfile::TempDir;
 use tokio::fs::create_dir_all;
 

datafusion-examples/src/utils/datasets/mod.rs

@@ -18,8 +18,7 @@
 use std::path::PathBuf;
 
 use arrow_schema::SchemaRef;
-use datafusion::error::Result;
-use datafusion_common::DataFusionError;
+use datafusion::error::{DataFusionError, Result};
 
 pub mod cars;
 pub mod regex;
@@ -50,10 +49,11 @@ impl ExampleDataset {
     }
 
     pub fn path_str(&self) -> Result<String> {
-        self.path().to_str().map(String::from).ok_or_else(|| {
+        let path = self.path();
+        path.to_str().map(String::from).ok_or_else(|| {
             DataFusionError::Execution(format!(
                 "CSV directory path is not valid UTF-8: {}",
-                self.path().display()
+                path.display()
             ))
         })
     }

datafusion-examples/src/utils/example_metadata/discover.rs

@@ -20,10 +20,12 @@
 //! An example group is defined as a directory containing a `main.rs` file
 //! under the examples root. This module is intentionally filesystem-focused
 //! and does not perform any parsing or rendering.
+//! Discovery fails if no valid example groups are found.
 
 use std::fs;
 use std::path::{Path, PathBuf};
 
+use datafusion::common::exec_err;
 use datafusion::error::Result;
 
 /// Discovers all example group directories under the given root.
@@ -35,10 +37,15 @@ pub fn discover_example_groups(root: &Path) -> Result<Vec<PathBuf>> {
         let entry = entry?;
         let path = entry.path();
 
-        if path.is_dir() && path.join("main.rs").exists() {
+        if path.is_dir() && path.join("main.rs").is_file() {
             groups.push(path);
         }
     }
+
+    if groups.is_empty() {
+        return exec_err!("No example groups found under: {}", root.display());
+    }
+
     groups.sort();
     Ok(groups)
 }
@@ -47,6 +54,8 @@ pub fn discover_example_groups(root: &Path) -> Result<Vec<PathBuf>> {
 mod tests {
     use super::*;
 
+    use crate::utils::example_metadata::test_utils::assert_exec_err_contains;
+
     use std::fs::{self, File};
 
     use tempfile::TempDir;
@@ -66,10 +75,29 @@ mod tests {
         fs::create_dir(&group2)?;
 
         let groups = discover_example_groups(root)?;
-
         assert_eq!(groups.len(), 1);
         assert_eq!(groups[0], group1);
+        Ok(())
+    }
 
+    #[test]
+    fn discover_example_groups_errors_if_main_rs_is_a_directory() -> Result<()> {
+        let tmp = TempDir::new()?;
+        let root = tmp.path();
+        let group = root.join("group");
+        fs::create_dir(&group)?;
+        fs::create_dir(group.join("main.rs"))?;
+
+        let err = discover_example_groups(root).unwrap_err();
+        assert_exec_err_contains(err, "No example groups found");
+        Ok(())
+    }
+
+    #[test]
+    fn discover_example_groups_errors_if_none_found() -> Result<()> {
+        let tmp = TempDir::new()?;
+        let err = discover_example_groups(tmp.path()).unwrap_err();
+        assert_exec_err_contains(err, "No example groups found");
         Ok(())
     }
 }

datafusion-examples/src/utils/example_metadata/model.rs

@@ -25,7 +25,16 @@ use std::path::Path;
 
 use datafusion::error::{DataFusionError, Result};
 
-use crate::utils::example_metadata::{parse_main_rs_docs, render::ABBREVIATIONS};
+use crate::utils::example_metadata::parse_main_rs_docs;
+
+/// Well-known abbreviations used to preserve correct capitalization
+/// when generating human-readable documentation titles.
+const ABBREVIATIONS: &[(&str, &str)] = &[
+    ("dataframe", "DataFrame"),
+    ("io", "IO"),
+    ("sql", "SQL"),
+    ("udf", "UDF"),
+];
 
 /// A group of related examples (e.g. `builtin_functions`, `udf`).
 ///

datafusion-examples/src/utils/example_metadata/parser.rs

@@ -21,15 +21,16 @@
 //! and their associated metadata (file name and description), enforcing
 //! a strict ordering and structure to avoid ambiguous documentation.
 
-use std::path::Path;
-use std::{collections::HashSet, fs};
+use std::{collections::HashSet, fs, path::Path};
 
-use datafusion_common::{DataFusionError, Result};
+use datafusion::common::exec_err;
+use datafusion::error::Result;
 use nom::{
-    IResult, Parser,
+    Err, IResult, Parser,
     bytes::complete::{tag, take_until, take_while},
     character::complete::multispace0,
     combinator::all_consuming,
+    error::{Error, ErrorKind},
     sequence::{delimited, preceded},
 };
 
@@ -77,19 +78,13 @@ fn parse_metadata_line(input: &str) -> IResult<&str, (&str, &str)> {
     let content = payload
         .strip_prefix("(")
         .and_then(|s| s.strip_suffix(")"))
-        .ok_or_else(|| {
-            nom::Err::Error(nom::error::Error::new(payload, nom::error::ErrorKind::Tag))
-        })?;
+        .ok_or_else(|| Err::Error(Error::new(payload, ErrorKind::Tag)))?;
 
     let (file, desc) = content
         .strip_prefix("file:")
-        .ok_or_else(|| {
-            nom::Err::Error(nom::error::Error::new(payload, nom::error::ErrorKind::Tag))
-        })?
+        .ok_or_else(|| Err::Error(Error::new(payload, ErrorKind::Tag)))?
         .split_once(", desc:")
-        .ok_or_else(|| {
-            nom::Err::Error(nom::error::Error::new(payload, nom::error::ErrorKind::Tag))
-        })?;
+        .ok_or_else(|| Err::Error(Error::new(payload, ErrorKind::Tag)))?;
 
     Ok((rest, (file.trim(), desc.trim())))
 }
@@ -119,18 +114,16 @@ pub fn parse_main_rs_docs(path: &Path) -> Result<Vec<ExampleEntry>> {
             let subcommand = match state {
                 ParserState::SeenSubcommand(s) => s,
                 ParserState::Idle => {
-                    return Err(DataFusionError::Execution(format!(
+                    return exec_err!(
                         "Metadata without preceding subcommand at {}:{}",
                         path.display(),
                         line_no + 1
-                    )));
+                    );
                 }
             };
 
             if !seen_subcommands.insert(subcommand) {
-                return Err(DataFusionError::Execution(format!(
-                    "Duplicate metadata for subcommand `{subcommand}`"
-                )));
+                return exec_err!("Duplicate metadata for subcommand `{subcommand}`");
             }
 
             entries.push(ExampleEntry {

datafusion-examples/src/utils/example_metadata/render.rs

@@ -85,15 +85,6 @@ cargo run --example dataframe -- dataframe
 ```
 "#;
 
-/// Well-known abbreviations used to preserve correct capitalization
-/// when generating human-readable documentation titles.
-pub const ABBREVIATIONS: &[(&str, &str)] = &[
-    ("dataframe", "DataFrame"),
-    ("io", "IO"),
-    ("sql", "SQL"),
-    ("udf", "UDF"),
-];
-
 /// Generates Markdown documentation for DataFusion examples.
 ///
 /// If `group` is `None`, documentation is generated for all example groups.