api: fix PaymentIndex not deserializing from query string correctly

It turns out that the `PaymentIndex` in e.g. `get_new_payments` was
silently getting dropped due to a `#[serde(flatten)]` + query string
limitation.

For query strings, using `#[serde(flatten)]` on a field requires that
all inner fields must be string-ish types (&str, String, Cow<'_, str>,
etc...) OR use `SerializeDisplay` and `DeserializeFromStr`.

This issue is due to a limitation in `serde`. See:
<serde-rs/serde#1183>

The fix here is to remove `#[serde(flatten)]` for querystring
serializeable types (actually just `PaymentIndex`) and instead use
`SerializeDisplay` and `DeserializeFromStr`

common/src/api/qs.rs

@@ -40,7 +40,6 @@ pub struct GetByScid {
 pub struct GetPaymentByIndex {
     /// The index of the payment to be fetched.
     // We use index instead of id so the backend can query by primary key.
-    #[serde(flatten)]
     pub index: PaymentIndex,
 }
 
@@ -50,7 +49,6 @@ pub struct GetPaymentByIndex {
 pub struct GetNewPayments {
     /// Optional [`PaymentIndex`] at which the results should start, exclusive.
     /// Payments with an index less than or equal to this will not be returned.
-    #[serde(flatten)]
     pub start_index: Option<PaymentIndex>,
     /// (Optional) the maximum number of results that can be returned.
     pub limit: Option<u16>,
@@ -73,7 +71,6 @@ pub struct GetPaymentsByIds {
 #[derive(Serialize, Deserialize)]
 pub struct UpdatePaymentNote {
     /// The index of the payment whose note should be updated.
-    #[serde(flatten)]
     pub index: PaymentIndex,
     /// The updated note.
     pub note: Option<String>,

common/src/ln/payments.rs

@@ -37,7 +37,6 @@ use crate::{
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 #[cfg_attr(all(any(test, feature = "test-utils")), derive(Arbitrary))]
 pub struct BasicPayment {
-    #[serde(flatten)]
     pub index: PaymentIndex,
 
     pub kind: PaymentKind,
@@ -174,7 +173,7 @@ pub enum PaymentStatus {
 /// 8776421933930532767-bc_ead3c01be0315dfd4e4c405aaca0f39076cff722a0f680c89c348e3bda9575f3
 /// ```
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
-#[derive(Serialize, Deserialize)]
+#[derive(SerializeDisplay, DeserializeFromStr)]
 #[cfg_attr(any(test, feature = "test-utils"), derive(Arbitrary))]
 pub struct PaymentIndex {
     pub created_at: TimestampMs,

common/src/time.rs

@@ -6,15 +6,16 @@ use std::{
 };
 
 use anyhow::{anyhow, Context};
-use serde::{de, Deserialize, Deserializer, Serialize};
+use serde::{de, Serialize};
 
 /// The number of milliseconds since the [`UNIX_EPOCH`].
 ///
 /// - Internally represented by a non-negative [`i64`] to ease interoperability
 ///   with some platforms we use which don't support unsigned ints.
 /// - Can represent any time from January 1st, 1970 00:00:00.000 UTC to roughly
 ///   292 million years in the future.
-#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd, Serialize)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
+#[derive(Serialize)]
 pub struct TimestampMs(i64);
 
 impl TimestampMs {
@@ -76,24 +77,6 @@ impl From<u32> for TimestampMs {
     }
 }
 
-/// Enforces that the inner [`i64`] is non-negative.
-impl<'de> Deserialize<'de> for TimestampMs {
-    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
-    where
-        D: Deserializer<'de>,
-    {
-        let value = i64::deserialize(deserializer)?;
-        if value >= 0 {
-            Ok(TimestampMs(value))
-        } else {
-            Err(de::Error::invalid_value(
-                de::Unexpected::Signed(value),
-                &"Unix timestamp must be non-negative",
-            ))
-        }
-    }
-}
-
 impl FromStr for TimestampMs {
     type Err = anyhow::Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
@@ -112,6 +95,16 @@ impl Display for TimestampMs {
     }
 }
 
+impl<'de> de::Deserialize<'de> for TimestampMs {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: de::Deserializer<'de>,
+    {
+        i64::deserialize(deserializer)
+            .and_then(|x| Self::try_from(x).map_err(de::Error::custom))
+    }
+}
+
 #[cfg(any(test, feature = "test-utils"))]
 mod arbitrary_impl {
     use proptest::{