fixes: improve login environment handling (#114)

* chore(organization): allow nested header and c files

* feat: add shell login "middleware" & etc:
  * lidm now calls `bash` (or other shells, depends on PACKAGE cfg) in
  login mode as a session wrapper to source most env (can be disabled)
  * this fixes a lot of env problems with all `/etc/profile` and more

Extra:
* implemented a musl compatible version of `execvpe` and now lidm should
  search for PATH everywhere it needs to
* `search_path` now also checks if the found binary is properly
  executable
* lidm now uses `confstr` for a decent PATH default if none is found
* logs are unbuffered for cases where debug logs appear empty (exit
  without handlers moment)

* chore: one-time evaluate certain makefile vars

---------

Co-authored-by: grialion <48643945+grialion@users.noreply.github.com>

src/sessions.c

@@ -1,3 +1,4 @@
+#include <errno.h>
 #include <ftw.h>
 #include <limits.h>
 #include <stdbool.h>
@@ -6,13 +7,16 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <unistd.h>
 
+#include "config.h"
 #include "desktop.h"
 #include "desktop_exec.h"
 #include "log.h"
 #include "macros.h"
 #include "sessions.h"
-#include "util.h"
+#include "util/path.h"
+#include "util/vec.h"
 
 struct source_dir {
   enum SessionType type;
@@ -163,3 +167,54 @@ struct Vector get_avaliable_sessions() {
 
   return sessions;
 }
+
+int session_exec_exec(struct session_exec* NNULLABLE exec,
+                      char* NULLABLE* NNULLABLE envp) {
+  switch (exec->typ) {
+    case EXEC_SHELL: {
+      char* argv[] = {exec->shell, NULL};
+      return execvpe(exec->shell, argv, envp);
+    }
+    case EXEC_DESKTOP:
+      return execvpe(exec->desktop.args[0], exec->desktop.args, envp);
+    default:
+      __builtin_unreachable();
+  }
+}
+
+/// So the reason this doesn't use the user shell is because that can really be
+/// anything, also assuming it were fish for example, it can't execute posix
+/// shell files and that leaves it out of most `/etc/profile.d/` scripts.
+///
+/// I'll just default to bash for now as it's able to source almost everything
+/// and takes most flags. Maybe will try to make this more sophisticated in the
+/// future.
+///
+/// This respects errno. Even also works as any exec family function.
+#ifndef LOGIN_SHELL
+  #define LOGIN_SHELL "bash"
+#endif
+// This triggers login behavior
+#define LOGIN_SHELL_ARG0 "-" LOGIN_SHELL
+int session_exec_login_through_shell(struct session_exec* NNULLABLE exec,
+                                     char* NULLABLE* NNULLABLE envp) {
+  switch (exec->typ) {
+    case EXEC_SHELL: {
+      char* argv[] = {LOGIN_SHELL_ARG0, "-c", exec->shell, NULL};
+      return execvpe(LOGIN_SHELL, argv, envp);
+    }
+    case EXEC_DESKTOP: {
+      char* name = desktop_as_cmdline(exec->desktop.args);
+      if (!name) {
+        errno = EINVAL;
+        return -1;
+      }
+      char* argv[] = {LOGIN_SHELL_ARG0, "-c", name, NULL};
+      int ret = execvpe(LOGIN_SHELL, argv, envp);
+      free(name);
+      return ret;
+    }
+    default:
+      __builtin_unreachable();
+  }
+}