Catch runaway forked children from Lua scriptlets centrally

Commit 2d418ad added a safety catch
for runaway children from package scriptlets, but we have many other
ways to run Lua scriptlets too, and runaway children are equally harmful
in them all. Catch them all centrally by wrapping lua_pcall() instead.

Besides handling more cases, there are a couple of more subtle
improvements here: rpmlog() in a child process is undefined behavior
due to the locking it does, but also nonsensical - rpmlog() is more than
fprintf() and any warnings/errors it collects end up in the wrong process.
Check and log from the parent instead.

Finally, make it a warning instead of an error because we're not failing
the scriptlet because of this, and make the message more generic as
besides exec(), this can be avoided by waiting for the child in
the script too.

lib/rpmscript.c

@@ -160,17 +160,11 @@ static rpmRC runLuaScript(rpmPlugins plugins, ARGV_const_t prefixes,
     if (cwd != -1) {
 	mode_t oldmask = umask(0);
 	umask(oldmask);
-	pid_t pid = getpid();
 
 	if (chdir("/") == 0 &&
 		rpmluaRunScript(lua, script, sname, NULL, *argvp) == 0) {
 	    rc = RPMRC_OK;
 	}
-	if (pid != getpid()) {
-	    /* Terminate child process forked in lua scriptlet */
-	    rpmlog(RPMLOG_ERR, _("No exec() called after fork() in lua scriptlet\n"));
-	    _exit(EXIT_FAILURE);
-	}
 	/* This failing would be fatal, return something different for it... */
 	if (fchdir(cwd)) {
 	    rpmlog(RPMLOG_ERR, _("Unable to restore current directory: %m"));

rpmio/rpmlua.c

@@ -243,6 +243,25 @@ static int luaopt(int c, const char *oarg, int oint, void *data)
     return 0;
 }
 
+static int rpm_pcall(lua_State *L, int nargs, int nresults, int errfunc)
+{
+    pid_t pid = getpid();
+    int status;
+
+    int rc = lua_pcall(L, nargs, nresults, errfunc);
+
+    /* Terminate unhandled fork from Lua script */
+    if (pid != getpid())
+	_exit(1);
+
+    if (waitpid(0, &status, WNOHANG) == 0) {
+	rpmlog(RPMLOG_WARNING,
+		_("runaway fork() in Lua script\n"));
+    }
+
+    return rc;
+}
+
 int rpmluaRunScript(rpmlua lua, const char *script, const char *name,
 		    const char *opts, ARGV_t args)
 {
@@ -290,7 +309,7 @@ int rpmluaRunScript(rpmlua lua, const char *script, const char *name,
 	}
     }
 
-    if (lua_pcall(L, 2, LUA_MULTRET, 0) != 0) {
+    if (rpm_pcall(L, 2, LUA_MULTRET, 0) != 0) {
 	rpmlog(RPMLOG_ERR, _("lua script failed: %s\n"),
 		 lua_tostring(L, -1));
 	lua_pop(L, 1);
@@ -301,7 +320,7 @@ int rpmluaRunScript(rpmlua lua, const char *script, const char *name,
     if (nret > 0 && lua->printbuf) {
 	lua_getglobal(L, "print");
 	lua_insert(L, -(nret + 1));
-	if (lua_pcall(L, nret, 0, 0) != 0) {
+	if (rpm_pcall(L, nret, 0, 0) != 0) {
 	    rpmlog(RPMLOG_ERR, _("result print failed: %s\n"),
 		    lua_tostring(L, -1));
 	    lua_pop(L, 1);
@@ -326,7 +345,7 @@ int rpmluaRunScriptFile(rpmlua lua, const char *filename)
 		 lua_tostring(L, -1));
 	lua_pop(L, 1);
 	ret = -1;
-    } else if (lua_pcall(L, 0, 0, 0) != 0) {
+    } else if (rpm_pcall(L, 0, 0, 0) != 0) {
 	rpmlog(RPMLOG_ERR, _("lua script failed: %s\n"),
 		 lua_tostring(L, -1));
 	lua_pop(L, 1);
@@ -392,7 +411,7 @@ static void _rpmluaInteractive(lua_State *L, rpmluarl rlcb)
 	   break;
 	}
 	if (rc == 0)
-	    rc = lua_pcall(L, 0, 0, 0);
+	    rc = rpm_pcall(L, 0, 0, 0);
 	if (rc != 0) {
 	    fprintf(stderr, "%s\n", lua_tostring(L, -1));
 	    lua_pop(L, 1);
@@ -448,7 +467,7 @@ char *rpmluaCallStringFunction(rpmlua lua, const char *function, rpmhookArgs arg
     }
 
     /* run the call */
-    if (lua_pcall(L, args->argc, 1, 0) != 0) {
+    if (rpm_pcall(L, args->argc, 1, 0) != 0) {
 	rpmlog(RPMLOG_ERR, "%s: %s\n", function, lua_tostring(L, -1));
 	lua_pop(L, 1);
 	return NULL;
@@ -462,7 +481,7 @@ char *rpmluaCallStringFunction(rpmlua lua, const char *function, rpmhookArgs arg
     } else {
 	lua_getglobal(L, "tostring");
 	lua_insert(L, -2);
-	if (lua_pcall(L, 1, 1, 0) != 0) {
+	if (rpm_pcall(L, 1, 1, 0) != 0) {
 	    rpmlog(RPMLOG_ERR, "%s: %s\n", function, lua_tostring(L, -1));
 	    lua_pop(L, 1);
 	    return NULL;
@@ -664,7 +683,7 @@ static int rpmluaHookWrapper(rpmhookArgs args, void *data)
 	}
 	lua_rawseti(L, -2, i+1);
     }
-    if (lua_pcall(L, 1, 1, 0) != 0) {
+    if (rpm_pcall(L, 1, 1, 0) != 0) {
 	rpmlog(RPMLOG_ERR, _("lua hook failed: %s\n"),
 		 lua_tostring(L, -1));
 	lua_pop(L, 1);