Add basic fuzzer of the parser.

promql/fuzz.go

+package promql
+// +build gofuzz
+
+/* PromQL parser fuzzing instrumentation for use with https://github.com/dvyukov/go-fuzz.
+ * 
+ * Fuzz each parser by building appropriately instrumented parser, ex. FuzzParseMetric and execute it with it's 
+ *
+ *     go-fuzz-build -func FuzzParseMetric -o FuzzParseMetric.zip github.com/prometheus/prometheus/promql
+ *
+ * And then run the tests with the appropriate inputs
+ *
+ *     go-fuzz -bin FuzzParseMetric.zip -workdir fuzz-data/ParseMetric
+ *
+ * Further input samples should go in the folders fuzz-data/ParseMetric/corpus.
+ *
+ * Repeat for ParseMetricSeletion, ParseExpr and ParseStmt
+ */
+
+const (
+	fuzz_interesting = 1
+	fuzz_meh         = 0
+	fuzz_discard     = -1
+)
+
+// Fuzz the metric parser
+func FuzzParseMetric(in []byte) int {
+    _, err := ParseMetric(string(in))
+
+    if err == nil {
+        return fuzz_interesting
+    }
+
+    return fuzz_discard
+}
+
+// Fuzz the metric selector parser
+func FuzzParseMetricSelector(in []byte) int {
+    _, err := ParseMetricSelector(string(in))
+
+    if err == nil {
+        return fuzz_interesting
+    }
+
+    return fuzz_discard
+}
+
+// Fuzz the expression parser
+func FuzzParseExpr(in []byte) int {
+    _, err := ParseExpr(string(in))
+
+    if err == nil {
+        return fuzz_interesting
+    }
+
+    return fuzz_discard
+}
+
+// Fuzz the  parser
+func FuzzParseStmts(in []byte) int {
+    _, err := ParseStmts(string(in))
+
+    if err == nil {
+        return fuzz_interesting
+    }
+
+    return fuzz_discard
+}