Check QEMU process status before periodically checking socket files.

Sometimes, the QEMU process exits early due to various reasons, e.g.,
the missing of kernel binary, providing incorrect qemu commandline
arguments, ... and vmtest didn't check the exit status of QEMU and it
will wait for 5 seconds to detect the error.

This patch teaches vmtest exit earlier when the QEMU process exits
unexpectedly.

Author: higuoxing

src/qemu.rs

@@ -758,11 +758,26 @@ impl Qemu {
     }
 
     /// Waits for QMP and QGA sockets to appear
-    fn wait_for_qemu(&self) -> Result<()> {
+    fn wait_for_qemu(&self, child: &mut Child) -> Result<()> {
         let now = time::Instant::now();
         let timeout = Duration::from_secs(5);
 
         while now.elapsed() < timeout {
+            // Before checking socket files, let's check if the child process fails to start.
+            match child
+                .try_wait()
+                .with_context(|| "Failed to inspect QEMU status".to_string())?
+            {
+                None => { /* QEMU is still running. */ }
+                Some(ec) => {
+                    if ec.success() {
+                        bail!("QEMU exits normally which is not expected")
+                    } else {
+                        bail!("QEMU fails to start: {}", Self::extract_child_stderr(child))
+                    }
+                }
+            }
+
             let qga_ok = self
                 .qga_sock
                 .try_exists()
@@ -1035,7 +1050,7 @@ impl Qemu {
         // Ensure child is cleaned up even if we bail early
         let mut child = scopeguard::guard(child, Self::child_cleanup);
 
-        if let Err(e) = self.wait_for_qemu() {
+        if let Err(e) = self.wait_for_qemu(&mut child) {
             return Err(e).context("Failed waiting for QEMU to be ready");
         }