[PATCH v3 14/20] 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_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 a decision about whether output will go to monitor_[v]printf()
or fprintf() must be made upfront, and then those functions called
directly.

 $ 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

This change requires adding a stub for monitor_printf() since that
is now called directly by vreport(), where monitor_vprintf() was
only needed previously.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 stubs/monitor-core.c           |  5 +++
 tests/unit/test-util-sockets.c |  1 +
 util/error-report.c            | 60 ++++++++++++++++++++++++----------
 3 files changed, 48 insertions(+), 18 deletions(-)

diff --git a/stubs/monitor-core.c b/stubs/monitor-core.c
index 1e0b11ec29..70b1f7a994 100644
--- a/stubs/monitor-core.c
+++ b/stubs/monitor-core.c
@@ -30,3 +30,8 @@ int monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
 {
     abort();
 }
+
+int monitor_printf(Monitor *mon, const char *fmt, ...)
+{
+    abort();
+}
diff --git a/tests/unit/test-util-sockets.c b/tests/unit/test-util-sockets.c
index d40813c682..53733163dd 100644
--- a/tests/unit/test-util-sockets.c
+++ b/tests/unit/test-util-sockets.c
@@ -76,6 +76,7 @@ Monitor *monitor_cur_hmp(void) { return cur_mon; }
 bool monitor_cur_is_qmp(void) { return false; }
 Monitor *monitor_set_cur(Coroutine *co, Monitor *mon) { abort(); }
 int monitor_vprintf(Monitor *mon, const char *fmt, va_list ap) { abort(); }
+int monitor_printf(Monitor *mon, const char *fmt, ...) { abort(); }
 
 #ifndef _WIN32
 static void test_socket_fd_pass_name_good(void)
diff --git a/util/error-report.c b/util/error-report.c
index c0fcf388de..6c887047e3 100644
--- a/util/error-report.c
+++ b/util/error-report.c
@@ -29,6 +29,9 @@ bool message_with_timestamp;
 bool error_with_guestname;
 const char *error_guest_name;
 
+typedef void (*error_print_func)(void *opaque, const char *fmt, ...);
+typedef void (*error_print_vfunc)(void *opaque, const char *fmt, va_list ap);
+
 /*
  * Print to current monitor if we have one, else to stderr.
  */
@@ -151,34 +154,34 @@ 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(bool cur, error_print_func print_func, void *print_opaque)
 {
     const char *sep = "";
     int i;
     const char *const *argp;
 
-    if (!monitor_cur_hmp() && g_get_prgname()) {
-        error_printf("%s:", g_get_prgname());
+    if (!cur && g_get_prgname()) {
+        print_func(print_opaque, "%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]);
+            print_func(print_opaque, "%s%s", sep, argp[i]);
             sep = " ";
         }
-        error_printf(": ");
+        print_func(print_opaque, ": ");
         break;
     case LOC_FILE:
-        error_printf("%s:", (const char *)cur_loc->ptr);
+        print_func(print_opaque, "%s:", (const char *)cur_loc->ptr);
         if (cur_loc->num) {
-            error_printf("%d:", cur_loc->num);
+            print_func(print_opaque, "%d:", cur_loc->num);
         }
-        error_printf(" ");
+        print_func(print_opaque, " ");
         break;
     default:
-        error_printf("%s", sep);
+        print_func(print_opaque, "%s", sep);
     }
 }
 
@@ -199,34 +202,55 @@ real_time_iso8601(void)
 G_GNUC_PRINTF(2, 0)
 static void vreport(report_type type, const char *fmt, va_list ap)
 {
+    /*
+     * Calling monitor_cur_hmp() will acquire/release mutexes,
+     * which triggers trace probes, which can trigger
+     * qemu_log calls, which would interleave with output
+     * from this. Hence cache the monitor handle upfront
+     * so any tracing appears before we start outputting.
+     */
+    Monitor *cur = monitor_cur_hmp();
+    error_print_func print_func;
+    error_print_vfunc print_vfunc;
+    void *print_opaque;
     gchar *timestr;
 
-    if (message_with_timestamp && !monitor_cur_hmp()) {
+    if (cur) {
+        print_func = (error_print_func)monitor_printf;
+        print_vfunc = (error_print_vfunc)monitor_vprintf;
+        print_opaque = cur;
+    } else {
+        print_func = (error_print_func)fprintf;
+        print_vfunc = (error_print_vfunc)vfprintf;
+        print_opaque = stderr;
+    }
+
+    if (message_with_timestamp && !cur) {
         timestr = real_time_iso8601();
-        error_printf("%s ", timestr);
+        print_func(print_opaque, "%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_hmp()) {
-        error_printf("%s ", error_guest_name);
+    if (error_with_guestname && error_guest_name && !cur) {
+        print_func(print_opaque, "%s ", error_guest_name);
     }
 
-    print_loc();
+    print_loc(!!cur, print_func, print_opaque);
 
     switch (type) {
     case REPORT_TYPE_ERROR:
         break;
     case REPORT_TYPE_WARNING:
-        error_printf("warning: ");
+        print_func(print_opaque, "warning: ");
         break;
     case REPORT_TYPE_INFO:
-        error_printf("info: ");
+        print_func(print_opaque, "info: ");
         break;
     }
 
-    error_vprintf(fmt, ap);
-    error_printf("\n");
+    print_vfunc(print_opaque, fmt, ap);
+    print_func(print_opaque, "\n");
 }
 
 /*
-- 
2.50.1