[PATCH v6 22/27] util: introduce common helper for error-report & log code

["Daniel P. Berrangé" <berrange@redhat.com>]

The error-report and log code both have a need to add prefixes
to messages they are printing, with the current example being
a timestamp.

The format and configuration they use should be consistent, so
providing a common helper will ensure this is always the case.
Initially the helper only emits a timestamp, but future patches
will expand this.

This takes the liberty of assigning the new file to the same
maintainer as the existing error-report.c file, given it will
be extracting some functionality from the latter.

While vreport() dynamically changes between reporting to the
monitor vs stderr, depending on whether HMP is active or not,
message prefixes are only ever used in the non-HMP case. Thus
the helper API can take a FILE * object and not have to deal
with the monitor at all.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 MAINTAINERS            |  2 ++
 include/qemu/message.h | 28 ++++++++++++++++++++++++++++
 util/meson.build       |  1 +
 util/message.c         | 23 +++++++++++++++++++++++
 4 files changed, 54 insertions(+)
 create mode 100644 include/qemu/message.h
 create mode 100644 util/message.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 29f88d48f3..96a4ce438e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3261,9 +3261,11 @@ M: Markus Armbruster <armbru@redhat.com>
 S: Supported
 F: include/qapi/error.h
 F: include/qemu/error-report.h
+F: include/qemu/message.h
 F: qapi/error.json
 F: util/error.c
 F: util/error-report.c
+F: util/message.c
 F: scripts/coccinelle/err-bad-newline.cocci
 F: scripts/coccinelle/error-use-after-free.cocci
 F: scripts/coccinelle/error_propagate_null.cocci
diff --git a/include/qemu/message.h b/include/qemu/message.h
new file mode 100644
index 0000000000..0a06421f77
--- /dev/null
+++ b/include/qemu/message.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#ifndef QEMU_MESSAGE_H
+#define QEMU_MESSAGE_H
+
+enum QMessageFormatFlags {
+    QMESSAGE_FORMAT_TIMESTAMP = (1 << 0),
+};
+
+/**
+ * qmessage_set_format:
+ * @flags: the message information to emit
+ *
+ * Select which pieces of information to
+ * emit for messages
+ */
+void qmessage_set_format(int flags);
+
+/**
+ * qmessage_context_print:
+ * @fp: file to emit the prefix on
+ *
+ * Emit a message prefix with the information selected by
+ * an earlier call to qmessage_set_format.
+ */
+void qmessage_context_print(FILE *fp);
+
+#endif /* QEMU_MESSAGE_H */
diff --git a/util/meson.build b/util/meson.build
index 7c9445615d..3035bd4bee 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -38,6 +38,7 @@ util_ss.add(files('host-utils.c'))
 util_ss.add(files('bitmap.c', 'bitops.c'))
 util_ss.add(files('fifo8.c'))
 util_ss.add(files('cacheflush.c'))
+util_ss.add(files('message.c'))
 util_ss.add(files('error.c', 'error-report.c'))
 util_ss.add(files('qemu-print.c'))
 util_ss.add(files('id.c'))
diff --git a/util/message.c b/util/message.c
new file mode 100644
index 0000000000..99a403f9d0
--- /dev/null
+++ b/util/message.c
@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include "qemu/osdep.h"
+
+#include "qemu/message.h"
+#include "monitor/monitor.h"
+
+static int message_format;
+
+void qmessage_set_format(int flags)
+{
+    message_format = flags;
+}
+
+void qmessage_context_print(FILE *fp)
+{
+    if (message_format & QMESSAGE_FORMAT_TIMESTAMP) {
+        g_autoptr(GDateTime) dt = g_date_time_new_now_utc();
+        g_autofree char *timestr = g_date_time_format_iso8601(dt);
+        fputs(timestr, fp);
+        fputc(' ', fp);
+    }
+}
-- 
2.53.0