util/log: Always send errors to logfile when daemonized

When QEMU is started with `-daemonize`, all stdio descriptors get
redirected to `/dev/null`. This basically means that anything
printed with error_report() and friends is lost.

Current logging code allows to redirect to a file with `-D` but
this requires to enable some logging item with `-d` as well to
be functional.

Relax the check on the log flags when QEMU is daemonized, so that
other users of stderr can benefit from the redirection, without the
need to enable unwanted debug logs. Previous behaviour is retained
for the non-daemonized case. The logic is unrolled as an `if` for
better readability. The qemu_log_level and log_per_thread globals
reflect the state we want to transition to at this point : use
them instead of the intermediary locals for correctness.

qemu_set_log_internal() is adapted to open a per-thread log file
when '-d tid' is passed. This is done by hijacking qemu_try_lock()
which seems simpler that refactoring the code.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20221108140032.1460307-3-groug@kaod.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/log.c

@@ -79,13 +79,15 @@ static int log_thread_id(void)
 
 static void qemu_log_thread_cleanup(Notifier *n, void *unused)
 {
-    fclose(thread_file);
-    thread_file = NULL;
+    if (thread_file != stderr) {
+        fclose(thread_file);
+        thread_file = NULL;
+    }
 }
 
 /* Lock/unlock output. */
 
-FILE *qemu_log_trylock(void)
+static FILE *qemu_log_trylock_with_err(Error **errp)
 {
     FILE *logfile;
 
@@ -96,6 +98,9 @@ FILE *qemu_log_trylock(void)
                 = g_strdup_printf(global_filename, log_thread_id());
             logfile = fopen(filename, "w");
             if (!logfile) {
+                error_setg_errno(errp, errno,
+                                 "Error opening logfile %s for thread %d",
+                                 filename, log_thread_id());
                 return NULL;
             }
             thread_file = logfile;
@@ -122,6 +127,11 @@ FILE *qemu_log_trylock(void)
     return logfile;
 }
 
+FILE *qemu_log_trylock(void)
+{
+    return qemu_log_trylock_with_err(NULL);
+}
+
 void qemu_log_unlock(FILE *logfile)
 {
     if (logfile) {
@@ -265,16 +275,21 @@ static bool qemu_set_log_internal(const char *filename, bool changed_name,
 #endif
     qemu_loglevel = log_flags;
 
-    /*
-     * In all cases we only log if qemu_loglevel is set.
-     * Also:
-     *   If per-thread, open the file for each thread in qemu_log_lock.
-     *   If not daemonized we will always log either to stderr
-     *     or to a file (if there is a filename).
-     *   If we are daemonized, we will only log if there is a filename.
-     */
     daemonized = is_daemonized();
-    need_to_open_file = log_flags && !per_thread && (!daemonized || filename);
+    need_to_open_file = false;
+    if (!daemonized) {
+        /*
+         * If not daemonized we only log if qemu_loglevel is set, either to
+         * stderr or to a file (if there is a filename).
+         * If per-thread, open the file for each thread in qemu_log_trylock().
+         */
+        need_to_open_file = qemu_loglevel && !log_per_thread;
+    } else {
+        /*
+         * If we are daemonized, we will only log if there is a filename.
+         */
+        need_to_open_file = filename != NULL;
+    }
 
     if (logfile) {
         fflush(logfile);
@@ -287,19 +302,34 @@ static bool qemu_set_log_internal(const char *filename, bool changed_name,
         }
     }
 
+    if (log_per_thread && daemonized) {
+        logfile = thread_file;
+    }
+
     if (!logfile && need_to_open_file) {
         if (filename) {
-            logfile = fopen(filename, "w");
-            if (!logfile) {
-                error_setg_errno(errp, errno, "Error opening logfile %s",
-                                 filename);
-                return false;
+            if (log_per_thread) {
+                logfile = qemu_log_trylock_with_err(errp);
+                if (!logfile) {
+                    return false;
+                }
+                qemu_log_unlock(logfile);
+            } else {
+                logfile = fopen(filename, "w");
+                if (!logfile) {
+                    error_setg_errno(errp, errno, "Error opening logfile %s",
+                                     filename);
+                    return false;
+                }
             }
             /* In case we are a daemon redirect stderr to logfile */
             if (daemonized) {
                 dup2(fileno(logfile), STDERR_FILENO);
                 fclose(logfile);
-                /* This will skip closing logfile in rcu_close_file. */
+                /*
+                 * This will skip closing logfile in rcu_close_file()
+                 * or qemu_log_thread_cleanup().
+                 */
                 logfile = stderr;
             }
         } else {
@@ -308,7 +338,11 @@ static bool qemu_set_log_internal(const char *filename, bool changed_name,
             logfile = stderr;
         }
 
-        qatomic_rcu_set(&global_file, logfile);
+        if (log_per_thread && daemonized) {
+            thread_file = logfile;
+        } else {
+            qatomic_rcu_set(&global_file, logfile);
+        }
     }
     return true;
 }