tests: internal: fuzzers: split cmetrics decoding

This split is because prometheus has calls to abort
https://github.com/fluent/fluent-bit/blob/e8d608b7d7a7c12eae2dfce019f3c263257ed131/lib/cmetrics/src/cmt_decode_prometheus.y#L44
which forces prometheus decoding to cause exits. Consequently, coverage
reports are missing for some of the cmetrics decoding files and the
aborts also mean fuzzing of the other decoders will be limited.

Coverage report:
https://storage.googleapis.com/oss-fuzz-coverage/fluent-bit/reports/20250422/linux/src/fluent-bit/lib/cmetrics/src/report.html

This the above problems by splitting the decoding harness into two
different fuzzers.

Signed-off-by: David Korczynski <david@adalogics.com>

Author: DavidKorczynski

tests/internal/fuzzers/CMakeLists.txt

@@ -4,6 +4,7 @@ set(UNIT_TESTS_FILES
   base64_fuzzer.c
   engine_fuzzer.c
   cmetrics_decode_fuzz.c
+  cmetrics_decode_prometheus.c
   config_fuzzer.c
   config_random_fuzzer.c
   ctrace_fuzzer.c

tests/internal/fuzzers/cmetrics_decode_fuzz.c

@@ -19,7 +19,6 @@
 
 #include <cmetrics/cmt_decode_msgpack.h>
 #include <cmetrics/cmt_decode_opentelemetry.h>
-#include <cmetrics/cmt_decode_prometheus.h>
 
 
 int
@@ -36,7 +35,7 @@ LLVMFuzzerTestOneInput(const uint8_t * data, size_t size)
         return 0;
     }
 
-    decider = data[0] % 3;
+    decider = data[0] % 2;
 
     /* Adjust data pointer since the first byte is used */
     data += 1;
@@ -50,21 +49,12 @@ LLVMFuzzerTestOneInput(const uint8_t * data, size_t size)
             cmt_decode_opentelemetry_destroy (&decoded_contexts);
         }
     }
-    else if (decider == 1) {
+    else {
         result = cmt_decode_msgpack_create(&cmt, (char *) data, size, &off);
         if (result == 0) {
             cmt_destroy(cmt);
         }
     }
-    else if (decider == 2) {
-        if (size == 0) {
-            return 0;
-        }
-        struct cmt_decode_prometheus_parse_opts opts;
-        result = cmt_decode_prometheus_create(&cmt, data, size, &opts);
-        if (result == CMT_DECODE_PROMETHEUS_SUCCESS) {
-            cmt_decode_prometheus_destroy(cmt);
-        }
-    }
+
     return 0;
 }

tests/internal/fuzzers/cmetrics_decode_prometheus.c

@@ -0,0 +1,41 @@
+/* -*- Mode: C; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+
+/*  Fluent Bit
+ *  ==========
+ *  Copyright (C) 2015-2023 The Fluent Bit Authors
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#include <cmetrics/cmt_decode_prometheus.h>
+
+
+int
+LLVMFuzzerTestOneInput(const uint8_t * data, size_t size)
+{
+    struct cmt *cmt = NULL;
+    int result;
+
+    /* At least one byte is needed for deciding which decoder to use */
+    if (size < 1) {
+        return 0;
+    }
+
+    struct cmt_decode_prometheus_parse_opts opts;
+    result = cmt_decode_prometheus_create(&cmt, data, size, &opts);
+    if (result == CMT_DECODE_PROMETHEUS_SUCCESS) {
+        cmt_decode_prometheus_destroy(cmt);
+    }
+
+    return 0;
+}