subprocess, lua: export whether the process was killed by us

We want to distinguish actual errors, and just aborting the program
intentionally.

Also be a bit more careful with handling the wait() exit status: do not
called WEXITSTATUS() without checking WIFEXITED() first.

DOCS/man/lua.rst

@@ -630,6 +630,10 @@ strictly part of the guaranteed API.
             On Windows, ``killed`` is only returned when the process has been
             killed by mpv as a result of ``cancellable`` being set to ``true``.
 
+        ``killed_by_us``
+            Set to ``true`` if the process has been killed by mpv as a result
+            of ``cancellable`` being set to ``true``.
+
     In all cases, ``mp.resume_all()`` is implicitly called.
 
 ``utils.parse_json(str [, trail])``

osdep/subprocess-posix.c

@@ -64,6 +64,8 @@ int mp_subprocess(char **args, struct mp_cancel *cancel, void *ctx,
     int p_stderr[2] = {-1, -1};
     int devnull = -1;
     pid_t pid = -1;
+    bool spawned = false;
+    bool killed_by_us = false;
 
     if (on_stdout && mp_make_cloexec_pipe(p_stdout) < 0)
         goto done;
@@ -89,6 +91,7 @@ int mp_subprocess(char **args, struct mp_cancel *cancel, void *ctx,
         pid = -1;
         goto done;
     }
+    spawned = true;
 
     close(p_stdout[1]);
     p_stdout[1] = -1;
@@ -124,6 +127,7 @@ int mp_subprocess(char **args, struct mp_cancel *cancel, void *ctx,
         }
         if (fds[2].revents) {
             kill(pid, SIGKILL);
+            killed_by_us = true;
             break;
         }
     }
@@ -143,12 +147,15 @@ done:
     close(p_stderr[1]);
     close(devnull);
 
-    if (WIFEXITED(status) && WEXITSTATUS(status) != 127) {
+    if (!spawned || (WIFEXITED(status) && WEXITSTATUS(status) == 127)) {
+        *error = "init";
+        status = -1;
+    } else if (WIFEXITED(status)) {
         *error = NULL;
         status = WEXITSTATUS(status);
     } else {
-        *error = WEXITSTATUS(status) == 127 ? "init" : "killed";
-        status = -1;
+        *error = "killed";
+        status = killed_by_us ? MP_SUBPROCESS_EKILLED_BY_US : -1;
     }
 
     return status;

osdep/subprocess-win.c

@@ -357,6 +357,7 @@ int mp_subprocess(char **args, struct mp_cancel *cancel, void *ctx,
             if (pi.hProcess) {
                 TerminateProcess(pi.hProcess, 1);
                 *error = "killed";
+                status = MP_SUBPROCESS_EKILLED_BY_US;
                 goto done;
             }
             break;

osdep/subprocess.h

@@ -28,6 +28,8 @@ typedef void (*subprocess_read_cb)(void *ctx, char *data, size_t size);
 int mp_subprocess(char **args, struct mp_cancel *cancel, void *ctx,
                   subprocess_read_cb on_stdout, subprocess_read_cb on_stderr,
                   char **error);
+// mp_subprocess return values. -1 is a generic error code.
+#define MP_SUBPROCESS_EKILLED_BY_US -2
 
 struct mp_log;
 void mp_subprocess_detached(struct mp_log *log, char **args);

player/lua.c

@@ -1243,6 +1243,8 @@ static int script_subprocess(lua_State *L)
     lua_setfield(L, -2, "status"); // res
     lua_pushlstring(L, cb_ctx.output.start, cb_ctx.output.len); // res d
     lua_setfield(L, -2, "stdout"); // res
+    lua_pushboolean(L, status == MP_SUBPROCESS_EKILLED_BY_US); // res b
+    lua_setfield(L, -2, "killed_by_us"); // res
     return 1;
 }