7 files changed, 76 insertions, 2 deletions

diff --git a/docs/api/channels-messages.md b/docs/api/channels-messages.md
index 9854d22..6391b5a 100644
--- a/docs/api/channels-messages.md
+++ b/docs/api/channels-messages.md
@@ -64,6 +64,14 @@ The request must have the following fields:
 |:-------|:-------|:--|
 | `name` | string | The channel's name. |
 
+The proposed `name` must be valid. The precise definition of valid is still up in the air, but, at minimum:
+
+* It must be non-empty.
+* It must not be "too long." (Currently, 64 characters is too long.)
+* It must begin with a printing character.
+* It must end with a printing character.
+* It must not contain runs of multiple whitespace characters.
+
 ### Success
 
 This endpoint will respond with a status of `202 Accepted` when successful. The body of the response will be a JSON object describing the new channel:
@@ -86,7 +94,11 @@ The returned name may not be identical to the name requested, as the name will b
 
 When completed, the service will emit a [channel created](events.md#channel-created) event with the channel's ID.
 
-### Duplicate channel name
+### Name not valid
+
+This endpoint will respond with a status of `400 Bad Request` if the proposed `name` is not valid.
+
+### Channel name in use
 
 This endpoint will respond with a status of `409 Conflict` if a channel with the requested name already exists.
 
diff --git a/src/channel/app.rs b/src/channel/app.rs
index 8359277..9a19b16 100644
--- a/src/channel/app.rs
+++ b/src/channel/app.rs
@@ -4,7 +4,7 @@ use sqlx::sqlite::SqlitePool;
 
 use super::{
     repo::{LoadError, Provider as _},
-    Channel, Id,
+    validate, Channel, Id,
 };
 use crate::{
     clock::DateTime,
@@ -25,6 +25,10 @@ impl<'a> Channels<'a> {
     }
 
     pub async fn create(&self, name: &Name, created_at: &DateTime) -> Result<Channel, CreateError> {
+        if !validate::name(name) {
+            return Err(CreateError::InvalidName(name.clone()));
+        }
+
         let mut tx = self.db.begin().await?;
         let created = tx.sequence().next(created_at).await?;
         let channel = tx
@@ -149,6 +153,8 @@ impl<'a> Channels<'a> {
 pub enum CreateError {
     #[error("channel named {0} already exists")]
     DuplicateName(Name),
+    #[error("invalid channel name: {0}")]
+    InvalidName(Name),
     #[error(transparent)]
     Database(#[from] sqlx::Error),
     #[error(transparent)]
diff --git a/src/channel/mod.rs b/src/channel/mod.rs
index eb8200b..d5ba828 100644
--- a/src/channel/mod.rs
+++ b/src/channel/mod.rs
@@ -5,5 +5,6 @@ mod id;
 pub mod repo;
 mod routes;
 mod snapshot;
+mod validate;
 
 pub use self::{event::Event, history::History, id::Id, routes::router, snapshot::Channel};
diff --git a/src/channel/routes/post.rs b/src/channel/routes/post.rs
index 810445c..2cf1cc0 100644
--- a/src/channel/routes/post.rs
+++ b/src/channel/routes/post.rs
@@ -54,6 +54,9 @@ impl IntoResponse for Error {
             app::CreateError::DuplicateName(_) => {
                 (StatusCode::CONFLICT, error.to_string()).into_response()
             }
+            app::CreateError::InvalidName(_) => {
+                (StatusCode::BAD_REQUEST, error.to_string()).into_response()
+            }
             other => Internal::from(other).into_response(),
         }
     }
diff --git a/src/channel/routes/test.rs b/src/channel/routes/test.rs
index 10b1e8d..f5369fb 100644
--- a/src/channel/routes/test.rs
+++ b/src/channel/routes/test.rs
@@ -116,6 +116,30 @@ async fn conflicting_canonical_name() {
 }
 
 #[tokio::test]
+async fn invalid_name() {
+    // Set up the environment
+
+    let app = fixtures::scratch_app().await;
+    let creator = fixtures::identity::create(&app, &fixtures::now()).await;
+
+    // Call the endpoint
+
+    let name = fixtures::channel::propose_invalid_name();
+    let request = post::Request { name: name.clone() };
+    let post::Error(error) =
+        post::handler(State(app.clone()), creator, fixtures::now(), Json(request))
+            .await
+            .expect_err("invalid channel name should fail the request");
+
+    // Verify the structure of the response
+
+    assert!(matches!(
+        error,
+        app::CreateError::InvalidName(error_name) if name == error_name
+    ));
+}
+
+#[tokio::test]
 async fn name_reusable_after_delete() {
     // Set up the environment
 
diff --git a/src/channel/validate.rs b/src/channel/validate.rs
new file mode 100644
index 0000000..0c97293
--- /dev/null
+++ b/src/channel/validate.rs
@@ -0,0 +1,23 @@
+use unicode_segmentation::UnicodeSegmentation as _;
+
+use crate::name::Name;
+
+// Picked out of a hat. The power of two is not meaningful.
+const NAME_TOO_LONG: usize = 64;
+
+pub fn name(name: &Name) -> bool {
+    let display = name.display();
+
+    [
+        display.graphemes(true).count() < NAME_TOO_LONG,
+        display.chars().all(|ch| !ch.is_control()),
+        display.chars().next().is_some_and(|c| !c.is_whitespace()),
+        display.chars().last().is_some_and(|c| !c.is_whitespace()),
+        display
+            .chars()
+            .zip(display.chars().skip(1))
+            .all(|(a, b)| !(a.is_whitespace() && b.is_whitespace())),
+    ]
+    .into_iter()
+    .all(|value| value)
+}
diff --git a/src/test/fixtures/channel.rs b/src/test/fixtures/channel.rs
index 0c6480b..98048f2 100644
--- a/src/test/fixtures/channel.rs
+++ b/src/test/fixtures/channel.rs
@@ -1,6 +1,7 @@
 use faker_rand::{
     en_us::{addresses::CityName, names::FullName},
     faker_impl_from_templates,
+    lorem::Paragraphs,
 };
 use rand;
 
@@ -23,6 +24,10 @@ pub fn propose() -> Name {
     rand::random::<NameTemplate>().to_string().into()
 }
 
+pub fn propose_invalid_name() -> Name {
+    rand::random::<Paragraphs>().to_string().into()
+}
+
 struct NameTemplate(String);
 faker_impl_from_templates! {
     NameTemplate; "{} {}", CityName, FullName;