libflux/flog: Support output to stderr

Make flux handle optional in logging functions.  If flux handle
is NULL, output to stderr.

Update documentation and add unit tests appropriately.

Fixes flux-framework#1191

doc/man3/flux_log.adoc

@@ -23,8 +23,9 @@ void flux_log_set_procid (flux_t *h, const char *s);
 DESCRIPTION
 -----------
 
-`flux_log()` creates RFC 5424 format log messages and sends them
-to the Flux message broker on 'h' for handling.
+`flux_log()` creates RFC 5424 format log messages.  The log messages
+are sent to the Flux message broker on 'h' for handling if it is
+specified.  If 'h' is NULL, the log message is output to stderr.
 
 The 'level' parameter should be set to one of the syslog(3) severity
 levels, which are, in order of decreasing importance:
@@ -43,9 +44,9 @@ the 'level' to direct `flux_log()` to wait for a response from the broker
 indicating whether the log request was accepted or not.  Otherwise, log
 requests are "fire and forget".
 
-Log messages are are added to the broker's circular buffer which
-can be accessed with flux-dmesg(3).  From there, a message's disposition
-is up to the broker's log configuration.
+When 'h' is specified, log messages are are added to the broker's
+circular buffer which can be accessed with flux-dmesg(3).  From there,
+a message's disposition is up to the broker's log configuration.
 
 `flux_log_set_procid()` may be used to override the default procid,
 which is initialized to the calling process's PID.
@@ -61,8 +62,9 @@ A Flux log message is formatted as a Flux request with a "raw" payload,
 as defined by Flux RFC 3.  The raw payload is formatted according to
 Internet RFC 5424.
 
-The following Syslog header fields are set in a Flux log messages when it is
-created within `flux_log()`:
+If the Flux handle 'h' is specified, the following Syslog header
+fields are set in a Flux log messages when it is created within
+`flux_log()`:
 
 PRI::
 Set to the user-specified severity level combined with the facility,

src/common/libflux/flog.c

@@ -134,7 +134,7 @@ static int log_rpc (flux_t *h, const char *buf, int len, int flags)
 
 int flux_vlog (flux_t *h, int level, const char *fmt, va_list ap)
 {
-    logctx_t *ctx = getctx (h);
+    logctx_t *ctx;
     int saved_errno = errno;
     uint32_t rank;
     int len;
@@ -143,7 +143,15 @@ int flux_vlog (flux_t *h, int level, const char *fmt, va_list ap)
     struct stdlog_header hdr;
     int rpc_flags = FLUX_RPC_NORESPONSE;
 
-    if (!ctx) {
+    if (!h) {
+        char buf[FLUX_MAX_LOGBUF + 1];
+        const char *lstr = stdlog_severity_to_string (LOG_PRI (level));
+
+        (void)vsnprintf (buf, sizeof (buf), fmt, ap);
+        return fprintf (stderr, "%s: %s\n", lstr, buf);
+    }
+
+    if (!(ctx = getctx (h))) {
         errno = ENOMEM;
         goto fatal;
     }

src/common/libflux/flog.h

@@ -29,6 +29,8 @@ void flux_log_set_appname (flux_t *h, const char *s);
 void flux_log_set_procid (flux_t *h, const char *s);
 
 /* Log a message at the specified level, as defined for syslog(3).
+ *
+ * Flux handle is optional, if set to NULL output to stderr.
  */
 int flux_vlog (flux_t *h, int level, const char *fmt, va_list ap);
 int flux_log (flux_t *h, int level, const char *fmt, ...)
@@ -37,6 +39,8 @@ int flux_log (flux_t *h, int level, const char *fmt, ...)
 /* Log a message at LOG_ERR level, appending a colon, space, and error string.
  * The system 'errno' is assumed to be valid and contain an error code
  * that can be decoded with zmq_strerror(3).
+ *
+ * Flux handle is optional, if set to NULL output to stderr.
  */
 void flux_log_verror (flux_t *h, const char *fmt, va_list ap);
 void flux_log_error (flux_t *h, const char *fmt, ...)

t/Makefile.am

@@ -164,6 +164,7 @@ check_PROGRAMS = \
 	loop/reactor.t \
 	loop/reduce.t \
 	loop/log.t \
+	loop/logstderr \
 	rpc/rpc.t \
 	rpc/mrpc.t \
 	rolemask/loop.t \
@@ -237,6 +238,11 @@ loop_log_t_SOURCES = loop/log.c
 loop_log_t_CPPFLAGS = $(test_cppflags)
 loop_log_t_LDADD = $(test_ldadd) $(LIBDL)
 
+loop_logstderr_SOURCES = loop/logstderr.c
+loop_logstderr_CPPFLAGS = $(test_cppflags)
+loop_logstderr_LDADD = \
+	$(test_ldadd) $(LIBDL) $(LIBUTIL)
+
 loop_reactor_t_SOURCES = loop/reactor.c
 loop_reactor_t_CPPFLAGS = $(test_cppflags)
 loop_reactor_t_LDADD = $(test_ldadd) $(LIBDL)

t/loop/logstderr.c

@@ -0,0 +1,19 @@
+#include <errno.h>
+#include <syslog.h>
+#include <flux/core.h>
+
+int main (int argc, char *argv[])
+{
+    errno = EPERM;
+    flux_log (NULL, LOG_WARNING, "hello");
+
+    errno = ENOENT;
+    flux_log_error (NULL, "world");
+
+    return (0);
+}
+
+/*
+ * vi:tabstop=4 shiftwidth=4 expandtab
+ */
+

t/t0001-basic.t

@@ -355,4 +355,11 @@ test_expect_success 'instance can stop cleanly with subscribers (#1025)' '
 	flux start ${ARGS} -s2 --bootstrap=selfpmi bash -c "nohup flux event sub hb &"
 '
 
+# test for issue #1191
+test_expect_success 'passing NULL to flux_log functions logs to stderr (#1191)' '
+        ${FLUX_BUILD_DIR}/t/loop/logstderr > std.out 2> std.err &&
+        grep "warning: hello" std.err &&
+        grep "err: world: No such file or directory" std.err
+'
+
 test_done