There are three general patterns to QEMU log output
1. Single complete message calls
qemu_log("Some message\n");
2. Direct use of fprintf
FILE *f = qemu_log_trylock()
fprintf(f, "...");
fprintf(f, "...");
fprintf(f, "...\n");
qemu_log_unlock(f)
3. Mixed use of qemu_log_trylock/qemu_log()
FILE *f = qemu_log_trylock()
qemu_log("....");
qemu_log("....");
qemu_log("....\n");
qemu_log_unlock(f)
When message prefixes are enabled, the timestamp will be
unconditionally emitted for all qemu_log() calls. This
works fine in the 1st case, and has no effect in the 2nd
case. In the 3rd case, however, we get the timestamp
printed over & over in each fragment.
One can suggest that pattern (3) is pointless as it is
functionally identical to (2) but with extra indirection
and overhead. None the less we have a fair bit of code
that does this.
The qemu_log() call itself is nothing more than a wrapper
which does pattern (2) with a single fprintf() call.
One might question whether (2) should include the message
prefix in the same way that (1), but there are scenarios
where this could be inappropriate / unhelpful such as the
CPU register dumps or linux-user strace output.
This patch fixes the problem in pattern (3) by keeping
track of the call depth of qemu_log_trylock() and then
only emitting the the prefix when the starting depth
was zero. In doing this qemu_log_trylock_context() is
also introduced as a variant of qemu_log_trylock()
that emits the prefix. Callers doing to batch output
can thus choose whether a prefix is appropriate or
not.
Fixes: 012842c07552 (log: make '-msg timestamp=on' apply to all qemu_log usage)
Reported-by: Richard Henderson <[email protected]>
Signed-off-by: Daniel P. Berrangé <[email protected]>
---
include/qemu/log.h | 7 +++++++
util/log.c | 49 ++++++++++++++++++++++++++--------------------
2 files changed, 35 insertions(+), 21 deletions(-)
diff --git a/include/qemu/log.h b/include/qemu/log.h
index e9d3c6806b..95f417c2b7 100644
--- a/include/qemu/log.h
+++ b/include/qemu/log.h
@@ -69,6 +69,13 @@ bool qemu_log_separate(void);
*/
FILE *qemu_log_trylock(void) G_GNUC_WARN_UNUSED_RESULT;
+/**
+ * As qemu_log_trylock(), but will also print the message
+ * context, if any is configured and this caused the
+ * acquisition of the FILE lock
+ */
+FILE *qemu_log_trylock_context(void) G_GNUC_WARN_UNUSED_RESULT;
+
/**
* Releases the lock on the log output, previously
* acquired by qemu_log_trylock().
diff --git a/util/log.c b/util/log.c
index c44d66b5ce..2ce7286f31 100644
--- a/util/log.c
+++ b/util/log.c
@@ -127,13 +127,39 @@ static FILE *qemu_log_trylock_with_err(Error **errp)
return logfile;
}
+/*
+ * Zero if there's been no opening qemu_log_trylock call,
+ * indicating the need for message context to be emitted
+ *
+ * Non-zero if we're in the middle of printing a message,
+ * possibly over multiple lines and must skip further
+ * message context
+ */
+static __thread uint log_depth;
+
FILE *qemu_log_trylock(void)
{
- return qemu_log_trylock_with_err(NULL);
+ FILE *f = qemu_log_trylock_with_err(NULL);
+ log_depth++;
+ return f;
+}
+
+FILE *qemu_log_trylock_context(void)
+{
+ FILE *f = qemu_log_trylock();
+ if (log_depth == 1 && message_with_timestamp) {
+ g_autofree const char *timestr = NULL;
+ g_autoptr(GDateTime) dt = g_date_time_new_now_utc();
+ timestr = g_date_time_format_iso8601(dt);
+ fprintf(f, "%s ", timestr);
+ }
+ return f;
}
void qemu_log_unlock(FILE *logfile)
{
+ assert(log_depth);
+ log_depth--;
if (logfile) {
fflush(logfile);
qemu_funlockfile(logfile);
@@ -145,28 +171,9 @@ void qemu_log_unlock(FILE *logfile)
void qemu_log(const char *fmt, ...)
{
- FILE *f;
- g_autofree const char *timestr = NULL;
-
- /*
- * Prepare the timestamp *outside* the logging
- * lock so it better reflects when the message
- * was emitted if we are delayed acquiring the
- * mutex
- */
- if (message_with_timestamp) {
- g_autoptr(GDateTime) dt = g_date_time_new_now_utc();
- timestr = g_date_time_format_iso8601(dt);
- }
-
- f = qemu_log_trylock();
+ FILE *f = qemu_log_trylock_context();
if (f) {
va_list ap;
-
- if (timestr) {
- fprintf(f, "%s ", timestr);
- }
-
va_start(ap, fmt);
vfprintf(f, fmt, ap);
va_end(ap);
--
2.52.0
