qga: introduce ga_run_command() helper for guest cmd execution

When executing guest commands in *nix environment, we repeat the same
fork/exec pattern multiple times.  Let's just separate it into a single
helper which would also be able to feed input data into the launched
process' stdin.  This way we can avoid code duplication.

To keep the history more bisectable, let's replace qmp commands
implementations one by one.  Also add G_GNUC_UNUSED attribute to the
helper and remove it in the next commit.

Originally-by: Yuri Pudgorodskiy <yur@virtuozzo.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Link: https://lore.kernel.org/r/20240320161648.158226-3-andrey.drobyshev@virtuozzo.com
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>

qga/commands-posix.c

@@ -76,6 +76,156 @@ static void ga_wait_child(pid_t pid, int *status, Error **errp)
     g_assert(rpid == pid);
 }
 
+static ssize_t ga_pipe_read_str(int fd[2], char **str)
+{
+    ssize_t n, len = 0;
+    char buf[1024];
+
+    close(fd[1]);
+    fd[1] = -1;
+    while ((n = read(fd[0], buf, sizeof(buf))) != 0) {
+        if (n < 0) {
+            if (errno == EINTR) {
+                continue;
+            } else {
+                len = -errno;
+                break;
+            }
+        }
+        *str = g_realloc(*str, len + n + 1);
+        memcpy(*str + len, buf, n);
+        len += n;
+        *str[len] = '\0';
+    }
+    close(fd[0]);
+    fd[0] = -1;
+
+    return len;
+}
+
+/*
+ * Helper to run command with input/output redirection,
+ * sending string to stdin and taking error message from
+ * stdout/err.
+ */
+G_GNUC_UNUSED
+static int ga_run_command(const char *argv[], const char *in_str,
+                          const char *action, Error **errp)
+{
+    pid_t pid;
+    int status;
+    int retcode = -1;
+    int infd[2] = { -1, -1 };
+    int outfd[2] = { -1, -1 };
+    char *str = NULL;
+    ssize_t len = 0;
+
+    if ((in_str && !g_unix_open_pipe(infd, FD_CLOEXEC, NULL)) ||
+        !g_unix_open_pipe(outfd, FD_CLOEXEC, NULL)) {
+        error_setg(errp, "cannot create pipe FDs");
+        goto out;
+    }
+
+    pid = fork();
+    if (pid == 0) {
+        char *cherr = NULL;
+
+        setsid();
+
+        if (in_str) {
+            /* Redirect stdin to infd. */
+            close(infd[1]);
+            dup2(infd[0], 0);
+            close(infd[0]);
+        } else {
+            reopen_fd_to_null(0);
+        }
+
+        /* Redirect stdout/stderr to outfd. */
+        close(outfd[0]);
+        dup2(outfd[1], 1);
+        dup2(outfd[1], 2);
+        close(outfd[1]);
+
+        execvp(argv[0], (char *const *)argv);
+
+        /* Write the cause of failed exec to pipe for the parent to read it. */
+        cherr = g_strdup_printf("failed to exec '%s'", argv[0]);
+        perror(cherr);
+        g_free(cherr);
+        _exit(EXIT_FAILURE);
+    } else if (pid < 0) {
+        error_setg_errno(errp, errno, "failed to create child process");
+        goto out;
+    }
+
+    if (in_str) {
+        close(infd[0]);
+        infd[0] = -1;
+        if (qemu_write_full(infd[1], in_str, strlen(in_str)) !=
+                strlen(in_str)) {
+            error_setg_errno(errp, errno, "%s: cannot write to stdin pipe",
+                             action);
+            goto out;
+        }
+        close(infd[1]);
+        infd[1] = -1;
+    }
+
+    len = ga_pipe_read_str(outfd, &str);
+    if (len < 0) {
+        error_setg_errno(errp, -len, "%s: cannot read from stdout/stderr pipe",
+                         action);
+        goto out;
+    }
+
+    ga_wait_child(pid, &status, errp);
+    if (*errp) {
+        goto out;
+    }
+
+    if (!WIFEXITED(status)) {
+        if (len) {
+            error_setg(errp, "child process has terminated abnormally: %s",
+                       str);
+        } else {
+            error_setg(errp, "child process has terminated abnormally");
+        }
+        goto out;
+    }
+
+    retcode = WEXITSTATUS(status);
+
+    if (WEXITSTATUS(status)) {
+        if (len) {
+            error_setg(errp, "child process has failed to %s: %s",
+                       action, str);
+        } else {
+            error_setg(errp, "child process has failed to %s: exit status %d",
+                       action, WEXITSTATUS(status));
+        }
+        goto out;
+    }
+
+out:
+    g_free(str);
+
+    if (infd[0] != -1) {
+        close(infd[0]);
+    }
+    if (infd[1] != -1) {
+        close(infd[1]);
+    }
+    if (outfd[0] != -1) {
+        close(outfd[0]);
+    }
+    if (outfd[1] != -1) {
+        close(outfd[1]);
+    }
+
+    return retcode;
+}
+
 void qmp_guest_shutdown(const char *mode, Error **errp)
 {
     const char *shutdown_flag;