grep: short-circuit max-count zero

src/lib.rs

@@ -419,6 +419,12 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     };
 
     let matcher = Matcher::compile(&config)?;
+
+    // grep with -m 0 should not open any file
+    if config.max_count == Some(0) {
+        return Err(ExitCode::new(1));
+    }
+
     // An empty pattern matches every line; with `-v`, GNU grep selects no lines
     // and exits as "no match" without reading any input files.
     if invert_match && patterns.iter().any(|pattern| pattern.is_empty()) {

tests/test_grep.rs

@@ -432,6 +432,21 @@ fn max_count() {
         .fails_with_code(1)
         .no_stdout();
 
+    let (_s, mut c) = ucmd();
+    c.args(&["-m", "0", "a", "missing"])
+        .fails_with_code(1)
+        .no_output();
+
+    let (_s, mut c) = ucmd();
+    c.args(&["-m", "1", "a", "missing"])
+        .fails_with_code(2)
+        .stderr_contains("grep: missing:");
+
+    let (_s, mut c) = ucmd();
+    c.args(&["-m", "0", "-e", "["])
+        .fails_with_code(2)
+        .stderr_contains("invalid pattern");
+
     // -A trailing context still printed after the cutoff.
     let (_s, mut c) = ucmd();
     c.args(&["-m", "1", "-A", "2", "match"])