Improve QEMU error messages for boot failure

42a8acc ("Show QEMU error message on boot failure") added QEMU errors
for handshake failure but not for connection failure. Connection failure
should be covered as well.

src/qemu.rs

@@ -625,6 +625,24 @@ impl Qemu {
         });
     }
 
+    /// Extracts stderr out from the child.
+    ///
+    /// Useful for when QEMU has errored out and we want to report the error
+    /// back to the user.
+    ///
+    /// Any failures in extraction will be encoded into the return string.
+    fn extract_child_stderr(child: &mut Child) -> String {
+        let mut err = String::new();
+
+        // unwrap() should never fail b/c we are capturing stdout
+        let mut stderr = child.stderr.take().unwrap();
+        if let Err(e) = stderr.read_to_string(&mut err) {
+            err += &format!("<failed to read child stderr: {}>", e);
+        }
+
+        err
+    }
+
     /// Run the target to completion
     ///
     /// Errors and return status are reported through the `updates` channel passed into the
@@ -654,25 +672,18 @@ impl Qemu {
         let qmp_stream = match self.connect_to_qmp() {
             Ok(s) => s,
             Err(e) => {
-                let _ = self
-                    .updates
-                    .send(Output::BootEnd(Err(e).context("Failed to connect QMP")));
+                let err = Self::extract_child_stderr(&mut child);
+                let _ = self.updates.send(Output::BootEnd(
+                    Err(e).context("Failed to connect QMP").context(err),
+                ));
                 return;
             }
         };
         let mut qmp = Qmp::from_stream(&qmp_stream);
         let qmp_info = match qmp.handshake() {
             Ok(i) => i,
             Err(e) => {
-                // If the handshake failed, grab stderr from qemu and display it
-                // to assist with debugging
-                let mut err = String::new();
-                // unwrap() should never fail b/c we are capturing stdout
-                let mut stderr = child.stderr.take().unwrap();
-                if let Err(e) = stderr.read_to_string(&mut err) {
-                    err += &format!("<failed to read stderr: {}>", e);
-                }
-
+                let err = Self::extract_child_stderr(&mut child);
                 let _ = self.updates.send(Output::BootEnd(
                     Err(e).context("QMP handshake failed").context(err),
                 ));