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"

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

@@ -17,6 +17,16 @@ use ptr;
 use sys::cvt;
 use sys::process::process_common::*;
 
+#[cfg(all(not(target_os = "android"), not(target_env = "musl")))]
+const DEFAULT_PATH: &[u8] = b"/bin:/usr/bin";
+// Musl has a different default path.
+#[cfg(target_env = "musl")]
+const DEFAULT_PATH: &[u8] = b"/usr/local/bin:/bin:/usr/bin";
+// Android has a different default path.
+#[cfg(target_os = "android")]
+const DEFAULT_PATH: &[u8] = b"/sbin:/system/sbin:/system/bin:/system/xbin:\
+    /odm/bin:/vendor/bin:/vendor/xbin";
+
 ////////////////////////////////////////////////////////////////////////////////
 // Command
 ////////////////////////////////////////////////////////////////////////////////
@@ -135,14 +145,15 @@ 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,
+                    // If there's no PATH, fall back to the default (which varies by platform).
+                    None => DEFAULT_PATH,
                 }
             },
             // 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 => DEFAULT_PATH,
                 }
             },
         };

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");
 }