Add fuzzing based on serde_json

This test ensures that we can parse anything that serde_json can
produce, which *ought* to ensure reasonable coverage?

fuzz/fuzz_targets/json_only_valid_serde.rs

@@ -0,0 +1,77 @@
+#![no_main]
+
+use arbitrary::{Arbitrary, Error, Unstructured};
+use libfuzzer_sys::fuzz_target;
+use std::collections::HashMap;
+
+extern crate fwd;
+use fwd::server::refresh::docker::JsonValue;
+
+/// InputNumber is a JSON number, i.e., a finite 64-bit floating point value
+/// that is not NaN. We need to define our own little wrapper here so that we
+/// can convince Arbitrary to only make finite f64s.
+///
+/// Ideally we would actually wrap serde_json::Number but there are rules
+/// about mixing 3rd party traits with 3rd party types.
+#[derive(Debug, PartialEq)]
+struct InputNumber(f64);
+
+impl<'a> Arbitrary<'a> for InputNumber {
+    fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self, Error> {
+        let value = f64::arbitrary(u)?;
+        if value.is_finite() {
+            Ok(InputNumber(value))
+        } else {
+            Err(Error::IncorrectFormat) // REJECT
+        }
+    }
+
+    #[inline]
+    fn size_hint(depth: usize) -> (usize, Option<usize>) {
+        f64::size_hint(depth)
+    }
+}
+
+/// TestInput is basically serde_json::Value, except (a) it has a HashMap and
+/// not serde_json's special `Map` structure, and (b) it has `InputNumber`
+/// instead of `json_serde::Number` for reasons described above.
+#[derive(Debug, PartialEq, Arbitrary)]
+enum TestInput {
+    Null,
+    Bool(bool),
+    Number(InputNumber),
+    String(String),
+    Object(HashMap<String, TestInput>),
+    Array(Vec<TestInput>),
+}
+
+fn convert(value: &TestInput) -> serde_json::Value {
+    match value {
+        TestInput::Null => serde_json::Value::Null,
+        TestInput::Bool(b) => serde_json::Value::Bool(*b),
+        TestInput::Number(n) => serde_json::Value::Number(
+            serde_json::Number::from_f64(n.0).expect("Unable to make an f64"),
+        ),
+        TestInput::String(s) => serde_json::Value::String(s.clone()),
+        TestInput::Object(o) => {
+            let mut out = serde_json::map::Map::new();
+            for (k, v) in o.into_iter() {
+                out.insert(k.clone(), convert(v));
+            }
+            serde_json::Value::Object(out)
+        }
+        TestInput::Array(v) => {
+            serde_json::Value::Array(v.into_iter().map(convert).collect())
+        }
+    }
+}
+
+fuzz_target!(|data: TestInput| {
+    // Convert the arbitrary TestInput into an arbitrary serde_json::Value,
+    // then use serde_json to write out arbitrary JSON.
+    let converted = convert(&data).to_string();
+
+    // Parse the JSON that serde_json produced. This fuzz test should ensure
+    // that we can parse anything that serde_json can produce.
+    let _ = JsonValue::parse(converted.as_bytes());
+});