From d90aac80c718e405e8e2be373781d99e29a5b3fe Mon Sep 17 00:00:00 2001
From: sjmiller609 <7516283+sjmiller609@users.noreply.github.com>
Date: Thu, 7 May 2026 21:21:00 +0000
Subject: [PATCH] qemu: open serial log with O_APPEND via chardev

Use -chardev file,append=on for the serial console instead of -serial
file:. The latter opens the path with plain O_WRONLY|O_CREAT, so QEMU
writes at its internal fd offset. When a copytruncate-style log rotation
truncates the file out from under it, the next write lands at the stale
offset, leaving a sparse hole of NUL bytes from byte 0 onward.

With append=on QEMU uses O_APPEND, so every write atomically seeks to
EOF and post-truncate writes correctly resume at byte 0.
---
 lib/hypervisor/qemu/config.go      | 11 +++++++++--
 lib/hypervisor/qemu/config_test.go |  4 +++-
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/lib/hypervisor/qemu/config.go b/lib/hypervisor/qemu/config.go
index 4b395c40..7c65af29 100644
--- a/lib/hypervisor/qemu/config.go
+++ b/lib/hypervisor/qemu/config.go
@@ -100,9 +100,16 @@ func BuildArgs(cfg hypervisor.VMConfig) []string {
 		args = append(args, "-device", deviceArg)
 	}
 
-	// Serial console output to file
+	// Serial console output to file. Use a chardev with append=on so QEMU
+	// opens the file with O_APPEND. Without it, QEMU writes at its internal
+	// fd offset; if the file is externally truncated (e.g. log rotation via
+	// copytruncate) subsequent writes leave a sparse hole of NUL bytes from
+	// byte 0 to the stale offset, which downstream log readers will pick up.
 	if cfg.SerialLogPath != "" {
-		args = append(args, "-serial", fmt.Sprintf("file:%s", cfg.SerialLogPath))
+		args = append(args,
+			"-chardev", fmt.Sprintf("file,id=serial0,path=%s,append=on", cfg.SerialLogPath),
+			"-serial", "chardev:serial0",
+		)
 	} else {
 		args = append(args, "-serial", "stdio")
 	}
diff --git a/lib/hypervisor/qemu/config_test.go b/lib/hypervisor/qemu/config_test.go
index 1cb64c93..f4a3e452 100644
--- a/lib/hypervisor/qemu/config_test.go
+++ b/lib/hypervisor/qemu/config_test.go
@@ -144,8 +144,10 @@ func TestBuildArgs_SerialLog(t *testing.T) {
 
 	args := BuildArgs(cfg)
 
+	assert.Contains(t, args, "-chardev")
+	assert.Contains(t, args, "file,id=serial0,path=/var/log/app.log,append=on")
 	assert.Contains(t, args, "-serial")
-	assert.Contains(t, args, "file:/var/log/app.log")
+	assert.Contains(t, args, "chardev:serial0")
 }
 
 func TestBuildArgs_NoSerialLog(t *testing.T) {