On Tue, Sep 23, 2025 at 04:28:42PM +0200, Markus Armbruster wrote: > Daniel P. Berrangé <[email protected]> writes: > > > Some code makes multiple qemu_log calls to incrementally emit > > a single message. Currently timestamps get prepended to all > > qemu_log calls, even those continuing a previous incomplete > > message. > > > > This changes the qemu_log so it skips adding a new line prefix, > > if the previous qemu_log call did NOT end with a newline. > > > > Reported-by: Richard Henderson <[email protected]> > > Signed-off-by: Daniel P. Berrangé <[email protected]> > > This patch has kept nagging me in the back of my brain. So I'm back for > a second look. > > > --- > > util/log.c | 9 ++++++++- > > 1 file changed, 8 insertions(+), 1 deletion(-) > > > > diff --git a/util/log.c b/util/log.c > > index abdcb6b311..2642a55c59 100644 > > --- a/util/log.c > > +++ b/util/log.c > > @@ -143,6 +143,12 @@ void qemu_log_unlock(FILE *logfile) > > } > > } > > > > +/* > > + * 'true' if the previous log message lacked a trailing '\n', > > + * and thus the subsequent call must skip any prefix > > + */ > > +static __thread bool incomplete; > > + > > void qemu_log(const char *fmt, ...) > > { > > FILE *f; > > @@ -154,7 +160,7 @@ void qemu_log(const char *fmt, ...) > > * was emitted if we are delayed acquiring the > > * mutex > > */ > > - if (message_with_timestamp) { > > + if (message_with_timestamp && !incomplete) { > > g_autoptr(GDateTime) dt = g_date_time_new_now_utc(); > > timestr = g_date_time_format_iso8601(dt); > > } > > @@ -170,6 +176,7 @@ void qemu_log(const char *fmt, ...) > f = qemu_log_trylock(); > if (f) { > va_list ap; > > if (timestr) { > fprintf(f, "%s ", timestr); > } > > > va_start(ap, fmt); > > vfprintf(f, fmt, ap); > > va_end(ap); > > + incomplete = fmt[strlen(fmt) - 1] != '\n'; > > qemu_log_unlock(f); > > } > > } > > Two cases: > > (A) Single log > > qemu_log_trylock() returns @global_file, and uses RCU to ensure it > remains valid until qemu_log_unlock(). I think. > > (B) Log split per thread (-d tid) > > qemu_log_trylock() returns thread-local @thread_file. > > In addition, qemu_log_trylock() locks the FILE it returns with > flockfile(), so no other thread can write to it until qemu_log_unlock() > unlocks it with funlockfile(). This ensures the entire output of in > between stays together. > > Let's see how this plays with @incomplete. > > (B) Log split per thread (-d tid) > > @incomplete is thread-local. It records wether the last qemu_log() > in this thread was an incomplete line. If it was, the next > qemu_log() continues the line. Unless something else wrote to > @thread_file in between, but that's not supposed to happen. Good. > > (A) Single log > > All thread log to the same FILE. Consider: > > 1. Thread 1 starts. Its @incomplete is initialized to false. > > 2. Thread 2 starts. Its @incomplete is initialized to false. > > 3. Thread 1 logs "abra". Its @incomplete is set to true. > > 4. Thread 2 logs "interrupt\n". Its @incomplete remains false. > > 5. Thread 2 logs "cadbra\n". Its @incomplete goes back to false. > > Resulting log file contents: > > PREFIX "abra" PREFIX "interrupt\n" > "cadabra\n" > > Not good. > > We could complicate this code further to mitigate. For instance, we > could use a thread-local @incomplete for (B), and a global one for (A). > This ensures log lines start with PREFIX as they should, but does > nothing to avoid mixing up line parts from different threads. My > example would then produce > > PREFIX "abrainterrupt\n" > PREFIX "cababra\n" > > My take: "Doctor, it hurts when I do that!" "Don't do that then." > Logging incomplete lines with qemu_log() can hurt. Don't do that then.
I just took a look at linux-user/syscall.c as that is one place that heavily uses qemu_log() for incomplete lines. What I didn't realize was that the expectation is to call qemu_log_trylock() (which returns the "FILE *" target) and then you can ignore the "FILE *" and just call qemu_log() repeatedly, and finally call qemu_log_unlock(FILE *) when done. https://gitlab.com/qemu-project/qemu/-/blob/master/linux-user/strace.c?ref_type=heads#L4396 This is a slightly wierd API design, but that seems to be the intended way to serailize and in that context, my patch/hack here will be sufficiently good. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
