perf(tsort): avoid redundant check on input

Author: anastygnome

src/uu/tsort/src/tsort.rs

@@ -10,12 +10,11 @@ use std::collections::{HashMap, VecDeque};
 use std::ffi::OsString;
 use std::fs::File;
 use std::io::{self, BufRead, BufReader};
-use std::path::Path;
 use string_interner::StringInterner;
 use string_interner::backend::StringBackend;
 use thiserror::Error;
 use uucore::display::Quotable;
-use uucore::error::{UError, UResult, USimpleError};
+use uucore::error::{UError, UIoError, UResult, USimpleError};
 use uucore::{format_usage, show, translate};
 
 // short types for switching interning behavior on the fly.
@@ -60,14 +59,33 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     if input == "-" {
         process_input(io::stdin().lock(), &mut g)?;
     } else {
+        let file = open_file(input)?;
+
+        let reader = BufReader::new(file);
+        process_input(reader, &mut g)?;
+    }
+
+    g.run_tsort();
+    Ok(())
+}
+
+fn open_file(input: &OsString) -> Result<File, UIoError> {
+    // Windows throws a permission denied error when trying to read a directory,
+    // so we need to check manually beforehand.
+    #[cfg(windows)]
+    {
+        use std::path::Path;
         let path = Path::new(&input);
         if path.is_dir() {
-            return Err(TsortError::IsDir(input.to_string_lossy().to_string()).into());
+            return Err(TsortError::IsDir(input.to_string_lossy().to_string()));
         }
 
-        let file = File::open(path)?;
-
-        // advise the OS we will access the data sequentially if available.
+        Ok(File::open(path)?)
+    }
+    // advise the OS that we are going to read the file sequentially, if available.
+    #[cfg(not(windows))]
+    {
+        let file = File::open(input)?;
         #[cfg(any(
             target_os = "linux",
             target_os = "android",
@@ -88,13 +106,8 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
             )
             .ok();
         }
-
-        let reader = BufReader::new(file);
-        process_input(reader, &mut g)?;
+        Ok(file)
     }
-
-    g.run_tsort();
-    Ok(())
 }
 
 pub fn uu_app() -> Command {
@@ -160,7 +173,13 @@ fn process_input<R: BufRead>(reader: R, graph: &mut Graph) -> Result<(), TsortEr
     // with tokens separated by whitespaces
 
     for line in reader.lines() {
-        let line = line?;
+        let line = line.map_err(|e| {
+            if e.kind() == io::ErrorKind::IsADirectory {
+                TsortError::IsDir(graph.name())
+            } else {
+                e.into()
+            }
+        })?;
         for token in line.split_whitespace() {
             // Intern the token and get a Sym
             let token_sym = graph.interner.get_or_intern(token);