Make P2P file transfer size limit configurable and default to 16 MiB

Author: vdw

README.md

@@ -96,6 +96,16 @@ To allow peers to rejoin (room stays active until TTL expires), edit [`config/in
 DESTROY_ROOM_ON_PEER_LEAVE = false  # Allow rejoin via browser history
 ```
 
+### P2P file transfer size limit
+
+The file transfer gate is configurable via environment variable (bytes):
+
+```bash
+NULLROOM_FILE_TRANSFER_SIZE_LIMIT_BYTES=16777216
+```
+
+Default is `16 MiB` (`16777216` bytes). The same limit is enforced server-side and reflected client-side in the room UI.
+
 ## Key routes
 
 - `GET /` → room landing page

app/channels/rooms_channel.rb

@@ -28,7 +28,13 @@ def subscribed
 
     # Send initialization message with initiator status
     # First person (count=1) is initiator, second person (count=2) is not
-    transmit({ type: "init", initiator: count == 1, connection_id: @connection_id, file_sharing: true })
+    transmit({
+      type: "init",
+      initiator: count == 1,
+      connection_id: @connection_id,
+      file_sharing: true,
+      file_size_limit: Nullroom::Config::FILE_TRANSFER_SIZE_LIMIT_BYTES
+    })
 
     # If we're the second person, notify first person that we're ready
     if count == 2
@@ -85,7 +91,10 @@ def initiate_file_transfer(data)
       transmit({ type: "file_transfer_authorized" })
     else
       # Soft rejection — only the requesting sender receives this
-      transmit({ type: "file_transfer_error", error: "Beta limit exceeded: Files must be under 24 MB." })
+      transmit({
+        type: "file_transfer_error",
+        error: "Beta limit exceeded: Files must be under #{file_limit_label}."
+      })
     end
   end
 
@@ -94,11 +103,17 @@ def initiate_file_transfer(data)
   # Stubbed gate for the Beta phase.
   # The structure is ready for Blind Token (JWT) enforcement in the Pro launch.
   def authorized_for_file_transfer?(metadata)
-    # 1. Enforce the Beta size limit (24 MiB)
-    return false if metadata.to_h["file_size"].to_i > 25_165_824
+    # 1. Enforce the Beta size limit.
+    return false if metadata.to_h["file_size"].to_i > Nullroom::Config::FILE_TRANSFER_SIZE_LIMIT_BYTES
 
     # 2. TODO: Implement BlindSignatureService.verify?(token, sig) for Pro launch.
     #    For now, all P2P file transfers are permitted during the Beta phase.
     true
   end
+
+  def file_limit_label
+    bytes = Nullroom::Config::FILE_TRANSFER_SIZE_LIMIT_BYTES
+    mebibytes = bytes / 1024.0 / 1024.0
+    mebibytes.round == mebibytes ? "#{mebibytes.to_i} MB" : "#{mebibytes.round(1)} MB"
+  end
 end

app/javascript/controllers/room_controller.js

@@ -44,7 +44,8 @@ export default class extends Controller {
       connectionId: null, // Store our connection ID
       // File transfer
       fileSharing: false,
-      pendingFile: null
+      pendingFile: null,
+      fileSizeLimit: FILE_SIZE_LIMIT
     }
     this.sender   = null
     this.receiver = null
@@ -174,6 +175,7 @@ export default class extends Controller {
             // Store our connection ID and initialize peer
             this.state.connectionId = data.connection_id
             this.state.fileSharing  = data.file_sharing === true
+            this.state.fileSizeLimit = Number(data.file_size_limit) > 0 ? Number(data.file_size_limit) : FILE_SIZE_LIMIT
             console.log("[Room] Got init message, initiator:", data.initiator, "connection_id:", data.connection_id, "file_sharing:", this.state.fileSharing)
             this.initializePeer(data.initiator)
           } else if (data.type === "peer_ready") {
@@ -469,8 +471,8 @@ export default class extends Controller {
    */
   _requestFileTransfer(file) {
     if (this.state.roomTerminated || !this.state.p2p) return
-    if (file.size > FILE_SIZE_LIMIT) {
-      this.showError("Files must be under 24 MB.")
+    if (file.size > this.state.fileSizeLimit) {
+      this.showError(`Files must be under ${this._fileSizeLimitLabel()}.`)
       return
     }
     // Store the file and ask the server gate
@@ -493,6 +495,7 @@ export default class extends Controller {
       (name, percent) => this.updateFileProgress(name, percent),
       (msg) => this.showError(msg)
     )
+    this.sender.setFileSizeLimit(this.state.fileSizeLimit)
 
     this.receiver = new FileTransferReceiver(
       decryptFn,
@@ -581,6 +584,11 @@ export default class extends Controller {
     return `${(bytes / 1_048_576).toFixed(1)} MB`
   }
 
+  _fileSizeLimitLabel() {
+    const mebibytes = this.state.fileSizeLimit / (1024 * 1024)
+    return Number.isInteger(mebibytes) ? `${mebibytes} MB` : `${mebibytes.toFixed(1)} MB`
+  }
+
   // ── Utilities ─────────────────────────────────────────────────────────────
 
   // Escape user-provided text before injecting into the DOM.

app/javascript/modules/file_transfer.js

@@ -12,7 +12,12 @@ const CHUNK_SIZE   = 65_536       // 64 KB per spec
 const MAX_BUFFER   = 16_777_216   // 16 MB — pause sending above this (backpressure)
 
 /** Maximum file size allowed in the Beta phase. Mirrors the server-side gate. */
-export const FILE_SIZE_LIMIT = 25_165_824  // 24 MiB (24 × 1024 × 1024)
+export const FILE_SIZE_LIMIT = 16_777_216  // 16 MiB (16 × 1024 × 1024)
+
+function fileSizeLimitLabel(bytes) {
+  const mebibytes = bytes / (1024 * 1024)
+  return Number.isInteger(mebibytes) ? `${mebibytes} MB` : `${mebibytes.toFixed(1)} MB`
+}
 
 // ─────────────────────────────────────────────────────────────────────────────
 // FileTransferSender
@@ -37,9 +42,17 @@ export class FileTransferSender {
     this.encryptFn = encryptFn
     this.onProgress = onProgress
     this.onError    = onError
+    this.fileSizeLimit = FILE_SIZE_LIMIT
     this._sending   = false
   }
 
+  setFileSizeLimit(bytes) {
+    const parsed = Number(bytes)
+    if (Number.isFinite(parsed) && parsed > 0) {
+      this.fileSizeLimit = parsed
+    }
+  }
+
   /**
    * Validate and stream a File object over the file data channel.
    * Caller should already have received server authorisation before calling this.
@@ -52,8 +65,8 @@ export class FileTransferSender {
     }
 
     // Client-side size guard (mirrors server gate — instant UX feedback)
-    if (file.size > FILE_SIZE_LIMIT) {
-      this.onError("Files must be under 24 MB.")
+    if (file.size > this.fileSizeLimit) {
+      this.onError(`Files must be under ${fileSizeLimitLabel(this.fileSizeLimit)}.`)
       return
     }
 

app/views/rooms/show.html.erb

@@ -115,7 +115,7 @@
         <%= render "shared/icon", name: "paperclip", css: "w-4 h-4 shrink-0" %>
         <span class="text-xs font-mono">
           Drop a file or click to select
-          <span class="text-white/30">(max 24 MB)</span>
+          <span class="text-white/30">(max 16 MB)</span>
         </span>
         <%# Invisible native file picker layered over the styled div %>
         <input

config/initializers/nullroom.rb

@@ -7,5 +7,9 @@ module Config
     # When true: room is destroyed immediately when any peer leaves (no rejoin possible via browser history)
     # When false: room remains active until TTL expires (peers can reconnect via back/history)
     DESTROY_ROOM_ON_PEER_LEAVE = false
+
+    # Maximum P2P file transfer size in bytes.
+    # Override with NULLROOM_FILE_TRANSFER_SIZE_LIMIT_BYTES in environment.
+    FILE_TRANSFER_SIZE_LIMIT_BYTES = ENV.fetch("NULLROOM_FILE_TRANSFER_SIZE_LIMIT_BYTES", 16 * 1024 * 1024).to_i
   end
 end

test/channels/rooms_channel_test.rb

@@ -242,7 +242,7 @@ def del(*keys)
 
       response = transmissions.last.deep_symbolize_keys
       assert_equal "file_transfer_error", response[:type]
-      assert_includes response[:error], "24 MB"
+      assert_includes response[:error], "16 MB"
     end
   end