[PATCH v5 17/24] util: fix interleaving of error & trace output

["Daniel P. Berrangé" <berrange@redhat.com>]

The monitor_cur_hmp() function will acquire/release mutex locks, which
will trigger trace probes, which can in turn trigger qemu_log() calls.
vreport() calls monitor_cur() multiple times through its execution
both directly and indirectly via error_vprintf().

The result is that the prefix information printed by vreport() gets
interleaved with qemu_log() output, when run outside the context of
an HMP command dispatcher. This can be seen with:

 $ qemu-system-x86_64 \
     -msg timestamp=on,guest-name=on \
     -display none \
     -object tls-creds-x509,id=f,dir=fish \
     -name fish \
     -d trace:qemu_mutex*
   2025-09-10T16:30:42.514374Z qemu_mutex_unlock released mutex 0x560b0339b4c0 (/var/home/berrange/src/virt/qemu/include/qemu/lockable.h:56)
   2025-09-10T16:30:42.514400Z qemu_mutex_lock waiting on mutex 0x560b033983e0 (/var/home/berrange/src/virt/qemu/include/qemu/lockable.h:56)
   2025-09-10T16:30:42.514402Z qemu_mutex_locked taken mutex 0x560b033983e0 (/var/home/berrange/src/virt/qemu/include/qemu/lockable.h:56)
   2025-09-10T16:30:42.514404Z qemu_mutex_unlock released mutex 0x560b033983e0 (/var/home/berrange/src/virt/qemu/include/qemu/lockable.h:56)
   2025-09-10T16:30:42.516716Z qemu_mutex_lock waiting on mutex 0x560b03398560 (../monitor/monitor.c:91)
   2025-09-10T16:30:42.516723Z qemu_mutex_locked taken mutex 0x560b03398560 (../monitor/monitor.c:91)
   2025-09-10T16:30:42.516726Z qemu_mutex_unlock released mutex 0x560b03398560 (../monitor/monitor.c:96)
   2025-09-10T16:30:42.516728Z qemu_mutex_lock waiting on mutex 0x560b03398560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842057Z qemu_mutex_locked taken mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842058Z qemu_mutex_unlock released mutex 0x564f5e401560 (../monitor/monitor.c:96)
   2025-09-10T16:31:04.842055Z 2025-09-10T16:31:04.842060Z qemu_mutex_lock waiting on mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842061Z qemu_mutex_locked taken mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842062Z qemu_mutex_unlock released mutex 0x564f5e401560 (../monitor/monitor.c:96)
   2025-09-10T16:31:04.842064Z qemu_mutex_lock waiting on mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842065Z qemu_mutex_locked taken mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842066Z qemu_mutex_unlock released mutex 0x564f5e401560 (../monitor/monitor.c:96)
   fish 2025-09-10T16:31:04.842068Z qemu_mutex_lock waiting on mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842069Z qemu_mutex_locked taken mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842070Z qemu_mutex_unlock released mutex 0x564f5e401560 (../monitor/monitor.c:96)
   2025-09-10T16:31:04.842072Z qemu_mutex_lock waiting on mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842097Z qemu_mutex_locked taken mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842099Z qemu_mutex_unlock released mutex 0x564f5e401560 (../monitor/monitor.c:96)
   qemu-system-x86_64:2025-09-10T16:31:04.842100Z qemu_mutex_lock waiting on mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842102Z qemu_mutex_locked taken mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842103Z qemu_mutex_unlock released mutex 0x564f5e401560 (../monitor/monitor.c:96)
    2025-09-10T16:31:04.842105Z qemu_mutex_lock waiting on mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842106Z qemu_mutex_locked taken mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842107Z qemu_mutex_unlock released mutex 0x564f5e401560 (../monitor/monitor.c:96)
   Unable to access credentials fish/ca-cert.pem: No such file or directory2025-09-10T16:31:04.842109Z qemu_mutex_lock waiting on mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842110Z qemu_mutex_locked taken mutex 0x564f5e401560 (../monitor/monitor.c:91)
   2025-09-10T16:31:04.842111Z qemu_mutex_unlock released mutex 0x564f5e401560 (../monitor/monitor.c:96)

To avoid this interleaving (as well as reduce the huge number of
mutex lock/unlock calls) we need to ensure that monitor_cur_is_hmp()
is only called once at the start of vreport(), and if no HMP is
present, no further monitor APIs can be called.

This implies error_[v]printf() cannot be called from vreport().
Instead we must introduce error_[v]printf_mon() which accept a
pre-acquired Monitor object. In some cases, however, fprintf
can be called directly as output will never be directed to the
monitor.

 $ qemu-system-x86_64 \
     -msg timestamp=on,guest-name=on \
     -display none \
     -object tls-creds-x509,id=f,dir=fish \
     -name fish \
     -d trace:qemu_mutex*
   2025-09-10T16:31:22.701691Z qemu_mutex_unlock released mutex 0x5626fd3b84c0 (/var/home/berrange/src/virt/qemu/include/qemu/lockable.h:56)
   2025-09-10T16:31:22.701728Z qemu_mutex_lock waiting on mutex 0x5626fd3b53e0 (/var/home/berrange/src/virt/qemu/include/qemu/lockable.h:56)
   2025-09-10T16:31:22.701730Z qemu_mutex_locked taken mutex 0x5626fd3b53e0 (/var/home/berrange/src/virt/qemu/include/qemu/lockable.h:56)
   2025-09-10T16:31:22.701732Z qemu_mutex_unlock released mutex 0x5626fd3b53e0 (/var/home/berrange/src/virt/qemu/include/qemu/lockable.h:56)
   2025-09-10T16:31:22.703989Z qemu_mutex_lock waiting on mutex 0x5626fd3b5560 (../monitor/monitor.c:91)
   2025-09-10T16:31:22.703996Z qemu_mutex_locked taken mutex 0x5626fd3b5560 (../monitor/monitor.c:91)
   2025-09-10T16:31:22.703999Z qemu_mutex_unlock released mutex 0x5626fd3b5560 (../monitor/monitor.c:96)
   2025-09-10T16:31:22.704000Z fish qemu-system-x86_64: Unable to access credentials fish/ca-cert.pem: No such file or directory

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 util/error-report.c | 67 +++++++++++++++++++++++++++++++--------------
 1 file changed, 46 insertions(+), 21 deletions(-)

diff --git a/util/error-report.c b/util/error-report.c
index 872158ad90..14b170ca4c 100644
--- a/util/error-report.c
+++ b/util/error-report.c
@@ -29,9 +29,12 @@ bool message_with_timestamp;
 bool error_with_guestname;
 const char *error_guest_name;
 
-int error_vprintf(const char *fmt, va_list ap)
+/*
+ * Print to current monitor if we have one, else to stderr.
+ */
+static int G_GNUC_PRINTF(2, 0)
+error_vprintf_mon(Monitor *cur_mon, const char *fmt, va_list ap)
 {
-    Monitor *cur_mon = monitor_cur();
     /*
      * This will return -1 if 'cur_mon' is NULL, or is QMP.
      * IOW this will only print if in HMP, otherwise we
@@ -44,13 +47,30 @@ int error_vprintf(const char *fmt, va_list ap)
     return ret;
 }
 
+static int G_GNUC_PRINTF(2, 3)
+error_printf_mon(Monitor *cur_mon, const char *fmt, ...)
+{
+    va_list ap;
+    int ret;
+
+    va_start(ap, fmt);
+    ret = error_vprintf_mon(cur_mon, fmt, ap);
+    va_end(ap);
+    return ret;
+}
+
+int error_vprintf(const char *fmt, va_list ap)
+{
+    return error_vprintf_mon(monitor_cur(), fmt, ap);
+}
+
 int error_printf(const char *fmt, ...)
 {
     va_list ap;
     int ret;
 
     va_start(ap, fmt);
-    ret = error_vprintf(fmt, ap);
+    ret = error_vprintf_mon(monitor_cur(), fmt, ap);
     va_end(ap);
     return ret;
 }
@@ -153,34 +173,34 @@ void loc_set_file(const char *fname, int lno)
 /*
  * Print current location to current monitor if we have one, else to stderr.
  */
-static void print_loc(void)
+static void print_loc(Monitor *cur)
 {
     const char *sep = "";
     int i;
     const char *const *argp;
 
-    if (!monitor_cur_is_hmp() && g_get_prgname()) {
-        error_printf("%s:", g_get_prgname());
+    if (!cur && g_get_prgname()) {
+        fprintf(stderr, "%s:", g_get_prgname());
         sep = " ";
     }
     switch (cur_loc->kind) {
     case LOC_CMDLINE:
         argp = cur_loc->ptr;
         for (i = 0; i < cur_loc->num; i++) {
-            error_printf("%s%s", sep, argp[i]);
+            error_printf_mon(cur, "%s%s", sep, argp[i]);
             sep = " ";
         }
-        error_printf(": ");
+        error_printf_mon(cur, ": ");
         break;
     case LOC_FILE:
-        error_printf("%s:", (const char *)cur_loc->ptr);
+        error_printf_mon(cur, "%s:", (const char *)cur_loc->ptr);
         if (cur_loc->num) {
-            error_printf("%d:", cur_loc->num);
+            error_printf_mon(cur, "%d:", cur_loc->num);
         }
-        error_printf(" ");
+        error_printf_mon(cur, " ");
         break;
     default:
-        error_printf("%s", sep);
+        error_printf_mon(cur, "%s", sep);
     }
 }
 
@@ -201,34 +221,39 @@ real_time_iso8601(void)
 G_GNUC_PRINTF(2, 0)
 static void vreport(report_type type, const char *fmt, va_list ap)
 {
+    Monitor *cur = NULL;
     gchar *timestr;
 
-    if (message_with_timestamp && !monitor_cur_is_hmp()) {
+    if (monitor_cur_is_hmp()) {
+        cur = monitor_cur();
+    }
+
+    if (message_with_timestamp && !cur) {
         timestr = real_time_iso8601();
-        error_printf("%s ", timestr);
+        fprintf(stderr, "%s ", timestr);
         g_free(timestr);
     }
 
     /* Only prepend guest name if -msg guest-name and -name guest=... are set */
-    if (error_with_guestname && error_guest_name && !monitor_cur_is_hmp()) {
-        error_printf("%s ", error_guest_name);
+    if (error_with_guestname && error_guest_name && !cur) {
+        fprintf(stderr, "%s ", error_guest_name);
     }
 
-    print_loc();
+    print_loc(cur);
 
     switch (type) {
     case REPORT_TYPE_ERROR:
         break;
     case REPORT_TYPE_WARNING:
-        error_printf("warning: ");
+        error_printf_mon(cur, "warning: ");
         break;
     case REPORT_TYPE_INFO:
-        error_printf("info: ");
+        error_printf_mon(cur, "info: ");
         break;
     }
 
-    error_vprintf(fmt, ap);
-    error_printf("\n");
+    error_vprintf_mon(cur, fmt, ap);
+    error_printf_mon(cur, "\n");
 }
 
 /*
-- 
2.52.0