qga: Add `merged` variant to GuestExecCaptureOutputMode

Currently, any captured output (via `capture-output`) is segregated into
separate GuestExecStatus fields (`out-data` and `err-data`). This means
that downstream consumers have no way to reassemble the captured data
back into the original stream.

This is relevant for chatty and semi-interactive (ie. read only) CLI
tools.  Such tools may deliberately interleave stdout and stderr for
visual effect. If segregated, the output becomes harder to visually
understand.

This commit adds a new enum variant to the GuestExecCaptureOutputMode
qapi to merge the output streams such that consumers can have a pristine
view of the original command output.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>

qga/commands.c

@@ -270,12 +270,26 @@ static void guest_exec_child_watch(GPid pid, gint status, gpointer data)
     g_spawn_close_pid(pid);
 }
 
-/** Reset ignored signals back to default. */
 static void guest_exec_task_setup(gpointer data)
 {
 #if !defined(G_OS_WIN32)
+    bool has_merge = *(bool *)data;
     struct sigaction sigact;
 
+    if (has_merge) {
+        /*
+         * FIXME: When `GLIB_VERSION_MIN_REQUIRED` is bumped to 2.58+, use
+         * g_spawn_async_with_fds() to be portable on windows. The current
+         * logic does not work on windows b/c `GSpawnChildSetupFunc` is run
+         * inside the parent, not the child.
+         */
+        if (dup2(STDOUT_FILENO, STDERR_FILENO) != 0) {
+            slog("dup2() failed to merge stderr into stdout: %s",
+                 strerror(errno));
+        }
+    }
+
+    /* Reset ignored signals back to default. */
     memset(&sigact, 0, sizeof(struct sigaction));
     sigact.sa_handler = SIG_DFL;
 
@@ -409,6 +423,7 @@ GuestExec *qmp_guest_exec(const char *path,
     GIOChannel *in_ch, *out_ch, *err_ch;
     GSpawnFlags flags;
     bool has_output = false;
+    bool has_merge = false;
     GuestExecCaptureOutputMode output_mode;
     g_autofree uint8_t *input = NULL;
     size_t ninput = 0;
@@ -445,13 +460,19 @@ GuestExec *qmp_guest_exec(const char *path,
     case GUEST_EXEC_CAPTURE_OUTPUT_MODE_SEPARATED:
         has_output = true;
         break;
+#if !defined(G_OS_WIN32)
+    case GUEST_EXEC_CAPTURE_OUTPUT_MODE_MERGED:
+        has_output = true;
+        has_merge = true;
+        break;
+#endif
     case GUEST_EXEC_CAPTURE_OUTPUT_MODE__MAX:
         /* Silence warning; impossible branch */
         break;
     }
 
     ret = g_spawn_async_with_pipes(NULL, argv, envp, flags,
-            guest_exec_task_setup, NULL, &pid, input_data ? &in_fd : NULL,
+            guest_exec_task_setup, &has_merge, &pid, input_data ? &in_fd : NULL,
             has_output ? &out_fd : NULL, has_output ? &err_fd : NULL, &gerr);
     if (!ret) {
         error_setg(errp, QERR_QGA_COMMAND_FAILED, gerr->message);

qga/qapi-schema.json

@@ -1210,11 +1210,14 @@
 # @stderr: only capture stderr
 # @separated: capture both stdout and stderr, but separated into
 #             GuestExecStatus out-data and err-data, respectively
+# @merged: capture both stdout and stderr, but merge together
+#          into out-data. not effective on windows guests.
 #
 # Since: 8.0
 ##
  { 'enum': 'GuestExecCaptureOutputMode',
-   'data': [ 'none', 'stdout', 'stderr', 'separated' ] }
+   'data': [ 'none', 'stdout', 'stderr', 'separated',
+             { 'name': 'merged', 'if': { 'not': 'CONFIG_WIN32' } } ] }
 
 ##
 # @GuestExecCaptureOutput: