feat: exec parser for desktop entries

Feature-rich Exec= parser implementation for .desktop files
* backslash stripping (e.g '\ ' -> ' ')
* percentage stripping (e.g '%u' -> '', '%%' -> '%')
* quote string handling (e.g 'arg1 "arg 2"' -> "arg1", "arg 2")
The current implementation strips all "percentage codes", instead
of handling them.
Argument count is limited at 100.

src/auth.c

@@ -15,6 +15,185 @@
 #include "unistd.h"
 #include "util.h"
 
+// constants for exec string parsing
+// ARG_LENGTH is the initial length of a parsed argument
+#define MAX_ARGS 100
+#define ARG_LENGTH 64
+
+// parse Exec=/bin/prog arg1 arg2\ with\ spaces
+void free_parsed_args(char** args, int arg_count) {
+  if (!args) return;
+  for (int i = 0; i < arg_count; i++) {
+    free(args[i]);
+  }
+  free((void*)args);
+}
+
+/* small closure-like struct to pass state to helper functions */
+struct ctx {
+  char** pcur;
+  size_t* pcur_len;
+  size_t* pcur_cap;
+  char*** pargv;
+  int* pargc;
+};
+/* append_char(state, ch) -> 0 on error, 1 on success */
+int append_char(struct ctx* st, char ch) {
+  char** pcur = st->pcur;
+  size_t* plen = st->pcur_len;
+  size_t* pcap = st->pcur_cap;
+  if (*plen + 1 >= *pcap) {
+    size_t newcap = *pcap ? (*pcap) * 2 : ARG_LENGTH;
+    char* cur = (char*)realloc(*pcur, newcap);
+    if (!cur) return 0;
+    *pcur = cur;
+    *pcap = newcap;
+  }
+  (*pcur)[(*plen)++] = ch;
+  return 1;
+}
+
+/* push_arg(state) -> 0 on error, 1 on success */
+int push_arg(struct ctx* st) {
+  char** pcur = st->pcur;
+  size_t* plen = st->pcur_len;
+  size_t* pcap = st->pcur_cap;
+  char*** pargv = st->pargv;
+  int* pargc = st->pargc;
+
+  if (*pargc > MAX_ARGS) {
+    return 1;
+  }
+  if (!*pcur) {
+    char* empty = strdup("");
+    if (!empty) return 0;
+    char** na = (char**)realloc((void*)*pargv, sizeof(char*) * ((*pargc) + 1));
+    if (!na) {
+      free(empty);
+      return 0;
+    }
+    *pargv = na;
+    (*pargv)[(*pargc)++] = empty;
+    return 1;
+  }
+  if (!append_char(st, '\0')) return 0;
+  char* final = (char*)realloc(*pcur, *plen);
+  if (!final) final = *pcur;
+  *pcur = NULL;
+  *plen = 0;
+  *pcap = 0;
+  char** na = (char**)realloc((void*)*pargv, sizeof(char*) * ((*pargc) + 1));
+  if (!na) {
+    free(final);
+    return 0;
+  }
+  *pargv = na;
+  (*pargv)[(*pargc)++] = final;
+  return 1;
+}
+
+/* Return codes:
+   0 = success
+   1 = bad args
+   2 = memory
+   3 = syntax
+
+  Important: call free_parsed_args afterwards to free the passed ***args
+*/
+// NOLINTBEGIN(readability-function-cognitive-complexity)
+int parse_exec_string(const char* exec_s, char*** args, int* arg_count) {
+  if (!exec_s || !args || !arg_count) return 1;
+  *args = NULL;
+  *arg_count = 0;
+
+  size_t len = strlen(exec_s);
+  size_t idx = 0;
+  char* cur = NULL;
+  size_t cur_len = 0;
+  size_t cur_cap = 0;
+  char** argv = NULL;
+  int argc = 0;
+  int in_quote = 0;
+
+  struct ctx ctx;
+  ctx.pcur = &cur;
+  ctx.pcur_len = &cur_len;
+  ctx.pcur_cap = &cur_cap;
+  ctx.pargv = &argv;
+  ctx.pargc = &argc;
+
+  while (idx < len) {
+    char cur_c = exec_s[idx];
+    if (!in_quote && (cur_c == ' ' || cur_c == '\t' || cur_c == '\n')) {
+      if (cur_cap) {
+        if (!push_arg(&ctx)) goto nomem;
+      }
+      idx++;
+      continue;
+    }
+    if (!in_quote && cur_c == '"') {
+      in_quote = 1;
+      idx++;
+      continue;
+    }
+    if (in_quote && cur_c == '"') {
+      in_quote = 0;
+      idx++;
+      continue;
+    }
+
+    if (cur_c == '\\') {
+      if (idx + 1 >= len) goto syntax_err;
+      if (!append_char(&ctx, exec_s[idx + 1])) goto nomem;
+      idx += 2;
+      continue;
+    }
+
+    if (cur_c == '%') {
+      if (idx + 1 >= len) goto syntax_err;
+      if (exec_s[idx + 1] == '%') {
+        if (!append_char(&ctx, '%')) goto nomem;
+        idx += 2;
+        continue;
+      }
+      /* drop any %X */
+      idx += 2;
+      continue;
+    }
+
+    if (!append_char(&ctx, cur_c)) goto nomem;
+    idx++;
+  }
+
+  if (in_quote) goto syntax_err;
+  if (cur_cap) {
+    if (!push_arg(&ctx)) goto nomem;
+  }
+  char** na = (char**)realloc((void*)argv, sizeof(char*) * (argc + 1));
+  if (!na) goto nomem;
+  argv = na;
+  argv[argc] = NULL;
+
+  *args = argv;
+  *arg_count = argc;
+  return 0;
+
+nomem:
+  if (cur) free(cur);
+  free_parsed_args(argv, argc);
+  *args = NULL;
+  *arg_count = 0;
+  return 2;
+
+syntax_err:
+  if (cur) free(cur);
+  free_parsed_args(argv, argc);
+  *args = NULL;
+  *arg_count = 0;
+  return 3;
+}
+// NOLINTEND(readability-function-cognitive-complexity)
+
 int pam_conversation(int num_msg, const struct pam_message** msg,
                      struct pam_response** resp, void* appdata_ptr) {
   struct pam_response* reply =
@@ -237,19 +416,29 @@ bool launch(char* user, char* passwd, struct session session, void (*cb)(void),
 
     *reach_session = true;
 
-    // TODO: these will be different due to TryExec
-    // and, Exec/TryExec might contain spaces as args
+    // TODO: test existence of executable with TryExec
+    char** args = NULL;
+    int arg_count = 0;
+    int parse_status = parse_exec_string(session.exec, &args, &arg_count);
+    if (parse_status != 0 || arg_count == 0 || !args[0]) {
+      (void)fprintf(stderr, "failure parsing exec string '%s': %d\n",
+                    session.exec ? session.exec : "NULL", parse_status);
+      free_parsed_args(args, arg_count);
+      return false;
+    }
+
     printf("\x1b[0m");
     // NOLINTNEXTLINE(bugprone-branch-clone)
     if (session.type == SHELL) {
       clear_screen();
       (void)fflush(stdout);
-      execlp(session.exec, session.exec, NULL);
+      execvp(args[0], args);
     } else if (session.type == XORG || session.type == WAYLAND) {
       clear_screen();
       (void)fflush(stdout);
-      execlp(session.exec, session.exec, NULL);
+      execvp(args[0], args);
     }
+    free_parsed_args(args, arg_count);
     perror("execl error");
     (void)fputs("failure calling session\n", stderr);
   } else {