Notify adb missing

There are many user who encounters missing adb.
To stop things happens again, we check it and show
sexy response to user.

Signed-off-by: yuchenlin <npes87184@gmail.com>

app/src/sys/unix/command.c

@@ -1,5 +1,7 @@
 #include "command.h"
 
+#include <errno.h>
+#include <fcntl.h>
 #include <signal.h>
 #include <stdlib.h>
 #include <sys/types.h>
@@ -7,18 +9,58 @@
 #include <unistd.h>
 #include "log.h"
 
-pid_t cmd_execute(const char *path, const char *const argv[]) {
-    pid_t pid = fork();
-    if (pid == -1) {
-        perror("fork");
+int cmd_execute(const char *path, const char *const argv[], pid_t *pid) {
+    int fd[2];
+    int ret = 0;
+
+    if (pipe(fd) == -1) {
+        perror("pipe");
         return -1;
     }
-    if (pid == 0) {
-        execvp(path, (char *const *)argv);
-        perror("exec");
+
+    *pid = fork();
+    if (*pid == -1) {
+        perror("fork");
+        ret = -1;
+        goto end;
+    }
+
+    if (*pid > 0) {
+        // parent close write side
+        close(fd[1]);
+        fd[1] = -1;
+        // wait for EOF or receive errno from child
+        if (read(fd[0], &ret, sizeof(int)) == -1) {
+            perror("read");
+            ret = -1;
+            goto end;
+        }
+    } else if (*pid == 0) {
+        // child close read side
+        close(fd[0]);
+        if (fcntl(fd[1], F_SETFD, FD_CLOEXEC) == 0) {
+            execvp(path, (char *const *)argv);
+        } else {
+            perror("fcntl");
+        }
+        // send errno to the parent
+        ret = errno;
+        if (write(fd[1], &ret, sizeof(int)) == -1) {
+            perror("write");
+        }
+        // close write side before exiting
+        close(fd[1]);
         _exit(1);
     }
-    return pid;
+
+end:
+    if (fd[0] != -1) {
+        close(fd[0]);
+    }
+    if (fd[1] != -1) {
+        close(fd[1]);
+    }
+    return ret;
 }
 
 SDL_bool cmd_terminate(pid_t pid) {