bot/src/discord/dynamicCommands.js

@@ -145,6 +145,30 @@ function render(envelope) {
   return { embeds: [embed] }
 }
 
+/**
+ * Deliver the envelope at the privacy the HANDLER asked for, not the privacy the
+ * deferral guessed.
+ *
+ * When the two agree — the ordinary case — this is one `editReply`. When the
+ * handler wants a private answer to a publicly deferred command, the deferred
+ * reply is deleted and the answer arrives as an ephemeral follow-up: the
+ * interaction token stays valid, so this is a supported path rather than a
+ * trick, and the cost is a "thinking…" that appears and vanishes.
+ *
+ * There is no reverse case. A command deferred ephemerally is one whose answers
+ * are all about the caller's own account, and nothing it returns should become
+ * public because a handler forgot a flag.
+ */
+async function reply(interaction, envelope, deferredEphemeral) {
+  const payload = render(envelope)
+  if (!envelope.ephemeral || deferredEphemeral) {
+    await interaction.editReply(payload)
+    return
+  }
+  await interaction.deleteReply()
+  await interaction.followUp({ ...payload, ephemeral: true })
+}
+
 /**
  * Defer, dispatch, edit.
  *
@@ -160,11 +184,17 @@ async function execute(interaction) {
   const definition = pulled.find((c) => c.name === interaction.commandName)
   if (!definition) return false
 
-  // Ephemerality has to be decided BEFORE the answer exists, because it is a
-  // property of the deferral. A command declared for linked users only is
-  // answered privately by default — its answer is about the caller's own
-  // account — and everything else defers publicly; a handler that wants the
-  // opposite says so, and the follow-up carries it.
+  // **Ephemerality is fixed at the DEFERRAL, which happens before the answer
+  // exists.** That is Discord's rule, not a choice here, and it is the whole
+  // reason this needs care: the handler decides privacy per answer — a refusal
+  // is private, a guild summary is not — and by the time it says so the reply is
+  // already public or already not.
+  //
+  // So: defer for the common case (public, or private for a command that only
+  // ever speaks about the caller's own account), and if the envelope disagrees,
+  // reconcile below. Getting this wrong is not cosmetic — the live walk caught it
+  // posting "guild information is not shown to your account" into the channel,
+  // which announces a member's access level to everyone in it.
   const ephemeral = definition.access === 'linked'
   await interaction.deferReply({ ephemeral })
 
@@ -178,18 +208,20 @@ async function execute(interaction) {
   // A transport failure and a handler failure are the same sentence to the
   // member and different lines in the log: one is the app being unreachable,
   // the other is a module's code.
+  // A refusal is ALWAYS private, whatever the command's usual privacy: "you do
+  // not have access to that" is about one member and belongs to one member.
   if (!res.ok) {
     log.warn('command dispatch failed', { command: definition.name, error: res.error })
-    await interaction.editReply({ content: refusal({ reason: 'error' }) })
+    await reply(interaction, { text: refusal({ reason: 'error' }), ephemeral: true }, ephemeral)
     return true
   }
   if (!res.data || !res.data.ok) {
-    await interaction.editReply({ content: refusal(res.data || {}) })
+    await reply(interaction, { text: refusal(res.data || {}), ephemeral: true }, ephemeral)
     return true
   }
 
   const envelope = res.data.response || {}
-  await interaction.editReply(render(envelope))
+  await reply(interaction, envelope, ephemeral)
 
   // The private aside beside a public answer (§9 answer 5). Skipped when the
   // reply was already private — the member would just be told the same thing

bot/test/dynamicCommands.test.js

@@ -53,6 +53,7 @@ function fakeInteraction({ commandName = 'guild', options = {}, userId = '555' }
     },
     deferReply: async (payload) => calls.push(['defer', payload]),
     editReply: async (payload) => calls.push(['edit', payload]),
+    deleteReply: async () => calls.push(['delete']),
     followUp: async (payload) => calls.push(['followUp', payload]),
   }
 }
@@ -195,6 +196,46 @@ test('a notice is not repeated when the answer was already private', async () =>
 
 // Every failure path EDITS. Replying to a deferred interaction throws, so a
 // refusal that used reply() would turn a clean "no" into an unhandled error.
+// Ephemerality is fixed at the DEFERRAL, which happens before the handler has
+// said anything — so honouring a per-answer flag needs the deferred reply
+// withdrawn. The live walk caught the version that ignored it posting "guild
+// information is not shown to your account" into the channel, which announces a
+// member's access level to everyone in it.
+test('a handler asking for privacy gets it, even though the deferral was public', async () => {
+  answers([definition()])
+  await dynamic.pull()
+  appInternal.dispatchCommand = async () => ({
+    ok: true, data: { ok: true, response: { text: 'just for you', ephemeral: true } },
+  })
+  const interaction = fakeInteraction()
+  await dynamic.execute(interaction)
+  assert.deepEqual(interaction.calls.map(([kind]) => kind), ['defer', 'delete', 'followUp'])
+  assert.deepEqual(interaction.calls.at(-1)[1], { content: 'just for you', ephemeral: true })
+})
+
+test('an already-private deferral just edits — no second message', async () => {
+  answers([definition({ access: 'linked' })])
+  await dynamic.pull()
+  appInternal.dispatchCommand = async () => ({
+    ok: true, data: { ok: true, response: { text: 'private', ephemeral: true } },
+  })
+  const interaction = fakeInteraction()
+  await dynamic.execute(interaction)
+  assert.deepEqual(interaction.calls.map(([kind]) => kind), ['defer', 'edit'])
+})
+
+// "You do not have access to that" is about one member and belongs to one
+// member, whatever the command's usual privacy.
+test('a refusal is always private', async () => {
+  answers([definition()])
+  await dynamic.pull()
+  appInternal.dispatchCommand = async () => ({ ok: true, data: { ok: false, reason: 'forbidden' } })
+  const interaction = fakeInteraction()
+  await dynamic.execute(interaction)
+  assert.deepEqual(interaction.calls.map(([kind]) => kind), ['defer', 'delete', 'followUp'])
+  assert.equal(interaction.calls.at(-1)[1].ephemeral, true)
+})
+
 test('a refusal is phrased by the bot and edited into the deferred reply', async () => {
   answers([definition({ access: 'linked' })])
   await dynamic.pull()
@@ -204,7 +245,9 @@ test('a refusal is phrased by the bot and edited into the deferred reply', async
   const interaction = fakeInteraction()
   await dynamic.execute(interaction)
   assert.match(interaction.calls.at(-1)[1].content, /Link your Discord account/)
-  assert.equal(interaction.calls.filter(([kind]) => kind === 'edit').length, 1)
+  // Deferred ephemerally (access: 'linked'), so the refusal is one edit and no
+  // withdrawal — replying twice to a deferred interaction is what throws.
+  assert.deepEqual(interaction.calls.map(([kind]) => kind), ['defer', 'edit'])
 })
 
 test('an unreachable app is the same sentence to the member and a different line in the log', async () => {
@@ -214,6 +257,7 @@ test('an unreachable app is the same sentence to the member and a different line
   const interaction = fakeInteraction()
   await dynamic.execute(interaction)
   assert.match(interaction.calls.at(-1)[1].content, /Something went wrong/)
+  assert.equal(interaction.calls.at(-1)[1].ephemeral, true)
 })
 
 test('an interaction for a command the app no longer serves is left alone', async () => {