wc: fix counting files from pseudo-filesystem

.vscode/cspell.dictionaries/jargon.wordlist.txt

@@ -37,6 +37,8 @@ exponentiate
 eval
 falsey
 fileio
+filesystem
+filesystems
 flamegraph
 fullblock
 getfacl

src/uu/wc/src/count_fast.rs

@@ -80,7 +80,13 @@ pub(crate) fn count_bytes_fast<T: WordCountable>(handle: &mut T) -> (usize, Opti
         if let Ok(stat) = stat::fstat(fd) {
             // If the file is regular, then the `st_size` should hold
             // the file's size in bytes.
-            if (stat.st_mode & S_IFREG) != 0 {
+            // If stat.st_size = 0 then
+            //  - either the size is 0
+            //  - or the size is unknown.
+            // The second case happens for files in pseudo-filesystems. For
+            // example with /proc/version and /sys/kernel/profiling. So,
+            // if it is 0 we don't report that and instead do a full read.
+            if (stat.st_mode & S_IFREG) != 0 && stat.st_size > 0 {
                 return (stat.st_size as usize, None);
             }
             #[cfg(any(target_os = "linux", target_os = "android"))]

tests/by-util/test_wc.rs

@@ -1,3 +1,6 @@
+#[cfg(all(unix, not(target_os = "macos")))]
+use pretty_assertions::assert_ne;
+
 use crate::common::util::*;
 
 // spell-checker:ignore (flags) lwmcL clmwL ; (path) bogusfile emptyfile manyemptylines moby notrailingnewline onelongemptyline onelongword weirdchars
@@ -235,3 +238,10 @@ fn test_read_from_nonexistent_file() {
         .stderr_contains(MSG)
         .stdout_is("");
 }
+
+#[test]
+#[cfg(all(unix, not(target_os = "macos")))]
+fn test_files_from_pseudo_filesystem() {
+    let result = new_ucmd!().arg("-c").arg("/proc/version").succeeds();
+    assert_ne!(result.stdout_str(), "0 /proc/version\n");
+}