Null eventlog writer

8 files changed, 56 insertions, 0 deletions

diff --git a/docs/users_guide/profiling.rst b/docs/users_guide/profiling.rst
index dbe0da06cf..f5a99c82a4 100644
--- a/docs/users_guide/profiling.rst
+++ b/docs/users_guide/profiling.rst
@@ -900,6 +900,14 @@ There are three more options which relate to heap profiling:
     option is enabled, it's expected that the user will manually start heap
     profiling or request specific samples using functions from ``GHC.Profiling``.
 
+
+.. rts-flag:: --null-eventlog-writer
+
+    :since: 9.2.2
+
+    Don't output eventlog to file, only configure tracing events.
+    Meant to be used with customized event log writer.
+
 .. rts-flag:: -L ⟨num⟩
 
     Sets the maximum length of a cost-centre stack name in a heap
diff --git a/rts/RtsFlags.c b/rts/RtsFlags.c
index a775e2986a..0d41108822 100644
--- a/rts/RtsFlags.c
+++ b/rts/RtsFlags.c
@@ -239,6 +239,7 @@ void initRtsFlagsDefaults(void)
     RtsFlags.TraceFlags.ticky         = false;
     RtsFlags.TraceFlags.trace_output  = NULL;
     RtsFlags.TraceFlags.eventlogFlushTime = 0;
+    RtsFlags.TraceFlags.nullWriter = false;
 #endif
 
 #if defined(PROFILING)
@@ -959,6 +960,11 @@ error = true;
                       OPTION_UNSAFE;
                       RtsFlags.MiscFlags.generate_dump_file = true;
                   }
+                  else if (strequal("null-eventlog-writer",
+                               &rts_argv[arg][2])) {
+                      OPTION_UNSAFE;
+                      RtsFlags.TraceFlags.nullWriter = true;
+                  }
                   else if (strequal("machine-readable",
                                &rts_argv[arg][2])) {
                       OPTION_UNSAFE;
diff --git a/rts/Trace.c b/rts/Trace.c
index dace0970e6..9edac3565e 100644
--- a/rts/Trace.c
+++ b/rts/Trace.c
@@ -107,6 +107,10 @@ void initTracing (void)
     initEventLogging();
 
     if (RtsFlags.TraceFlags.tracing == TRACE_EVENTLOG
+            && RtsFlags.TraceFlags.nullWriter) {
+        startEventLogging(&NullEventLogWriter);
+    }
+    else if (RtsFlags.TraceFlags.tracing == TRACE_EVENTLOG
             && rtsConfig.eventlog_writer != NULL) {
         startEventLogging(rtsConfig.eventlog_writer);
     }
diff --git a/rts/eventlog/EventLogWriter.c b/rts/eventlog/EventLogWriter.c
index 06aad2841e..daa6dc3c9d 100644
--- a/rts/eventlog/EventLogWriter.c
+++ b/rts/eventlog/EventLogWriter.c
@@ -147,9 +147,30 @@ stopEventLogFileWriter(void)
 #endif
 }
 
+static void
+initEventLogFileWriterNoop(void) {}
+
+static bool
+writeEventLogFileNoop(void *eventlog STG_UNUSED, size_t eventlog_size STG_UNUSED) {
+    return true; // writes succeed always
+}
+
+static void
+flushEventLogFileNoop(void) {}
+
+static void
+stopEventLogFileWriterNoop(void) {}
+
 const EventLogWriter FileEventLogWriter = {
     .initEventLogWriter = initEventLogFileWriter,
     .writeEventLog = writeEventLogFile,
     .flushEventLog = flushEventLogFile,
     .stopEventLogWriter = stopEventLogFileWriter
 };
+
+const EventLogWriter NullEventLogWriter = {
+    .initEventLogWriter = initEventLogFileWriterNoop,
+    .writeEventLog = writeEventLogFileNoop,
+    .flushEventLog = flushEventLogFileNoop,
+    .stopEventLogWriter = stopEventLogFileWriterNoop
+};
diff --git a/rts/include/rts/EventLogWriter.h b/rts/include/rts/EventLogWriter.h
index 73a2aec64c..1f53d2f58a 100644
--- a/rts/include/rts/EventLogWriter.h
+++ b/rts/include/rts/EventLogWriter.h
@@ -43,6 +43,11 @@ typedef struct {
  */
 extern const EventLogWriter FileEventLogWriter;
 
+/*
+ * An EventLogWriter which does nothing.
+ */
+extern const EventLogWriter NullEventLogWriter;
+
 enum EventLogStatus {
   /* The runtime system wasn't compiled with eventlog support. */
   EVENTLOG_NOT_SUPPORTED,
diff --git a/rts/include/rts/Flags.h b/rts/include/rts/Flags.h
index e8298023de..9291f492cf 100644
--- a/rts/include/rts/Flags.h
+++ b/rts/include/rts/Flags.h
@@ -181,6 +181,7 @@ typedef struct _TRACE_FLAGS {
     Time eventlogFlushTime;  /* Time between force eventlog flushes (or 0 if disabled) */
     int eventlogFlushTicks;
     char *trace_output;  /* output filename for eventlog */
+    bool nullWriter; /* use null writer instead of file writer */
 } TRACE_FLAGS;
 
 /* See Note [Synchronization of flags and base APIs] */
diff --git a/testsuite/tests/rts/Makefile b/testsuite/tests/rts/Makefile
index 2ada2adc75..fe35bd4d2c 100644
--- a/testsuite/tests/rts/Makefile
+++ b/testsuite/tests/rts/Makefile
@@ -131,6 +131,12 @@ EventlogOutput2:
 	./EventlogOutput +RTS -l
 	ls EventlogOutput.eventlog >/dev/null
 
+.PHONY: EventlogOutputNull
+EventlogOutputNull:
+	"$(TEST_HC)" -eventlog -rtsopts -v0 EventlogOutput.hs
+	./EventlogOutput +RTS -l --null-eventlog-writer
+	test ! -e EventlogOutput.eventlog
+
 .PHONY: T20199
 T20199:
 	"$(TEST_HC)" -no-hs-main -optcxx-std=c++11 -v0 T20199.cpp -o T20199
diff --git a/testsuite/tests/rts/all.T b/testsuite/tests/rts/all.T
index ab18a94218..a62585c6f4 100644
--- a/testsuite/tests/rts/all.T
+++ b/testsuite/tests/rts/all.T
@@ -170,6 +170,11 @@ test('EventlogOutput2',
        omit_ways(['dyn', 'ghci'] + prof_ways) ],
      makefile_test, ['EventlogOutput2'])
 
+test('EventlogOutputNull',
+     [ extra_files(["EventlogOutput.hs"]),
+       omit_ways(['dyn', 'ghci'] + prof_ways) ],
+     makefile_test, ['EventlogOutputNull'])
+
 # Test that Info Table Provenance (IPE) events are emitted.
 test('EventlogOutput_IPE',
      [ extra_files(["EventlogOutput.hs"]),