fix: download artifacts through the API instead of handing them to Safari

Tapping download opened Artifact.url in the browser, which answered with
"Authorization header is required". That URL is not a web page: git.sr.ht
resolves it to <api origin>/query/artifact/<checksum>/<filename>, which demands
a bearer token. Safari has none and no way to get one, so the download could
never have worked — this predates the upload work.

Fetch it with the client that already holds the token and hand the user the file
through a share sheet. fetchData mirrors fetchText, including its host guard, so
an authenticated request still cannot be aimed anywhere but *.sr.ht over https.

Also guards zero-byte uploads. sr.ht streams into S3, which rejects a zero-part
multipart completion with "MalformedXML: UnknownError" — an error that says
nothing about the cause and cost a round of testing to identify. Empty files are
now refused by name before the request is made.

1 files changed, 31 insertions, 0 deletions

diff --git a/Hutch/Views/Repositories/RepositoryDetailViewModel.swift b/Hutch/Views/Repositories/RepositoryDetailViewModel.swift
index dce39c1..9b6b942 100644
--- a/Hutch/Views/Repositories/RepositoryDetailViewModel.swift
+++ b/Hutch/Views/Repositories/RepositoryDetailViewModel.swift
@@ -542,6 +542,14 @@ final class RepositoryDetailViewModel {
             return false
         }
 
+        // sr.ht streams the upload into S3, which rejects a zero-part multipart
+        // completion with "MalformedXML" — an error that says nothing about the
+        // actual problem. Catch it here where we can name it.
+        guard !fileData.isEmpty else {
+            self.error = "\(fileURL.lastPathComponent) is empty. SourceHut rejects zero-byte artifacts."
+            return false
+        }
+
         do {
             _ = try await client.executeMultipart(
                 service: service,
@@ -567,6 +575,29 @@ final class RepositoryDetailViewModel {
         }
     }
 
+    /// Downloads an artifact and returns a local file URL to share.
+    ///
+    /// `Artifact.url` points at the API origin, not the web one, and returns an
+    /// auth error to anything without a bearer token — so it cannot be opened in
+    /// a browser. Fetch it here and hand the user the file instead.
+    func downloadArtifact(_ artifact: ArtifactInfo) async -> URL? {
+        guard !isMutatingArtifact else { return nil }
+        isMutatingArtifact = true
+        error = nil
+        defer { isMutatingArtifact = false }
+
+        do {
+            let data = try await client.fetchData(url: artifact.url)
+            let destination = FileManager.default.temporaryDirectory
+                .appendingPathComponent(artifact.filename)
+            try data.write(to: destination, options: .atomic)
+            return destination
+        } catch {
+            self.error = "Couldn't download \(artifact.filename). \(error.userFacingMessage)"
+            return nil
+        }
+    }
+
     @discardableResult
     func deleteArtifact(id: Int) async -> Bool {
         guard !isMutatingArtifact else { return false }