Fixes rust-lang#55775 -- fixed regression in Command::exec's handling of PATH.

This restores the previous behavior where if env_clear() or env_remove("PATH") was used we fall back to a default PATH of "/bin:/usr/bin"

Author: Alex Gaynor

src/libstd/sys/unix/process/process_unix.rs

@@ -135,14 +135,15 @@ impl Command {
             Some(envp) => {
                 match envp.get_items().iter().find(|var| var.as_bytes().starts_with(b"PATH=")) {
                     Some(p) => &p.as_bytes()[5..],
-                    None => return None,
+                    // Chosen to match what glibc does if there's no PATH variable
+                    None => b"/bin:/usr/bin",
                 }
             },
             // maybe_envp is None if the process isn't changing the parent's env at all.
             None => {
                 match parent_path.as_ref() {
                     Some(p) => p.as_bytes(),
-                    None => return None,
+                    None => b"/bin:/usr/bin",
                 }
             },
         };

src/test/run-pass/command-exec.rs

@@ -55,6 +55,16 @@ fn main() {
                 println!("passed");
             }
 
+            "exec-test6" => {
+                let err = Command::new("echo").arg("passed").env_clear().exec();
+                panic!("failed to spawn: {}", err);
+            }
+
+            "exec-test7" => {
+                let err = Command::new("echo").arg("passed").env_remove("PATH").exec();
+                panic!("failed to spawn: {}", err);
+            }
+
             _ => panic!("unknown argument: {}", arg),
         }
         return
@@ -84,4 +94,14 @@ fn main() {
     assert!(output.status.success());
     assert!(output.stderr.is_empty());
     assert_eq!(output.stdout, b"passed\n");
+
+    let output = Command::new(&me).arg("exec-test6").output().unwrap();
+    assert!(output.status.success());
+    assert!(output.stderr.is_empty());
+    assert_eq!(output.stdout, b"passed\n");
+
+    let output = Command::new(&me).arg("exec-test7").output().unwrap();
+    assert!(output.status.success());
+    assert!(output.stderr.is_empty());
+    assert_eq!(output.stdout, b"passed\n");
 }