fix(signal): wait for attachment file before transcription (#93)

Signal-cli fires SSE events as soon as message metadata arrives, but attachment
files may still be downloading. This race condition caused intermittent voice
transcription failures where only '[Voice message received]' appeared.

Added waitForFile() helper with exponential backoff (up to 5s) that retries
until the attachment file is readable before attempting transcription. Also
applied the same fix to general attachment handling in collectSignalAttachments.

Fixes #92

Co-authored-by: letta-code <248085862+letta-code@users.noreply.github.com>
Co-authored-by: Cameron <cpfiffer@users.noreply.github.com>

src/channels/signal.ts

@@ -17,7 +17,8 @@ import { spawn, type ChildProcess } from 'node:child_process';
 import { randomUUID } from 'node:crypto';
 import { homedir } from 'node:os';
 import { join } from 'node:path';
-import { copyFile, stat } from 'node:fs/promises';
+import { copyFile, stat, access } from 'node:fs/promises';
+import { constants } from 'node:fs';
 
 export interface SignalConfig {
   phoneNumber: string;        // Bot's phone number (E.164 format, e.g., +15551234567)
@@ -89,6 +90,33 @@ type SignalSseEvent = {
   };
 };
 
+/**
+ * Wait for a file to exist on disk with exponential backoff.
+ * Signal-cli may still be downloading attachments when the SSE event fires.
+ *
+ * @param filePath - Path to check
+ * @param maxWaitMs - Maximum time to wait (default: 5000ms)
+ * @param intervalMs - Initial polling interval (default: 100ms)
+ * @returns true if file exists, false if timeout exceeded
+ */
+async function waitForFile(filePath: string, maxWaitMs = 5000, intervalMs = 100): Promise<boolean> {
+  const startTime = Date.now();
+
+  while (Date.now() - startTime < maxWaitMs) {
+    try {
+      await access(filePath, constants.R_OK);
+      return true;
+    } catch {
+      // File not ready yet, wait and retry
+      await new Promise((resolve) => setTimeout(resolve, intervalMs));
+      // Exponential backoff: double the interval, cap at 500ms
+      intervalMs = Math.min(intervalMs * 2, 500);
+    }
+  }
+
+  return false;
+}
+
 export class SignalAdapter implements ChannelAdapter {
   readonly id = 'signal' as const;
   readonly name = 'Signal';
@@ -545,17 +573,21 @@ This code expires in 1 hour.`;
             }
           } else {
             // Read attachment from signal-cli attachments directory
-            const { readFileSync, existsSync } = await import('node:fs');
+            // Note: signal-cli may still be downloading when SSE event fires, so we wait
+            const { readFileSync } = await import('node:fs');
             const { homedir } = await import('node:os');
             const { join } = await import('node:path');
             
             const attachmentPath = join(homedir(), '.local/share/signal-cli/attachments', voiceAttachment.id);
-            console.log(`[Signal] Reading attachment from: ${attachmentPath}`);
+            console.log(`[Signal] Waiting for attachment: ${attachmentPath}`);
             
-            if (!existsSync(attachmentPath)) {
-              console.error(`[Signal] Attachment file not found: ${attachmentPath}`);
-              throw new Error(`Attachment file not found: ${attachmentPath}`);
+            // Wait for file to be available (signal-cli may still be downloading)
+            const fileReady = await waitForFile(attachmentPath, 5000);
+            if (!fileReady) {
+              console.error(`[Signal] Attachment file not found after waiting: ${attachmentPath}`);
+              throw new Error(`Attachment file not found after waiting: ${attachmentPath}`);
             }
+            console.log(`[Signal] Attachment file ready: ${attachmentPath}`);
             
             const buffer = readFileSync(attachmentPath);
             console.log(`[Signal] Read ${buffer.length} bytes`);
@@ -749,6 +781,14 @@ This code expires in 1 hour.`;
         }
       }
       
+      // Wait for file to be available (signal-cli may still be downloading)
+      const fileReady = await waitForFile(sourcePath, 5000);
+      if (!fileReady) {
+        console.warn(`[Signal] Attachment ${name} not found after waiting, skipping.`);
+        results.push(entry);
+        continue;
+      }
+      
       // Copy to our attachments directory
       const target = buildAttachmentPath(this.config.attachmentsDir, 'signal', chatId, name);
       try {