fix: retry transient ssh download empty blocks

scripts/src/ssh-file-transfer.ts

@@ -53,6 +53,7 @@ class SshFileTransferError extends Error {
 const fileTransferReadBlockBytes = 45_000;
 const fileTransferWriteB64ArgvLimit = 48_000;
 const fileTransferWriteB64ChunkChars = 12_000;
+const fileTransferReadBlockMaxAttempts = 3;
 
 export function isSshFileTransferOperation(args: string[]): boolean {
   const subcommand = args[0] ?? "";
@@ -204,18 +205,8 @@ async function readRemoteFileVerified(
   let actualBytes = 0;
   let chunkCount = 0;
   for (let blockIndex = 0; actualBytes < remote.bytes; blockIndex += 1) {
-    const read = await checkedFileTransfer(invocation, executor, builders, "read-b64-block", [remotePath, String(blockIndex), String(readBlockBytes)]);
-    const encoded = read.stdout.replace(/\s+/gu, "");
+    const encoded = await readRemoteBase64BlockWithRetry(invocation, executor, builders, remotePath, readBlockBytes, blockIndex, remote.bytes, actualBytes);
     const chunk = encoded.length === 0 ? Buffer.alloc(0) : Buffer.from(encoded, "base64");
-    if (chunk.length === 0) {
-      throw new SshFileTransferError("remote download returned an empty block before EOF", {
-        route: invocation.route.raw,
-        remotePath,
-        blockIndex,
-        expectedBytes: remote.bytes,
-        actualBytes,
-      });
-    }
     chunks.push(chunk);
     actualBytes += chunk.length;
     chunkCount += 1;
@@ -226,6 +217,41 @@ async function readRemoteFileVerified(
   return { remote, content, chunks: chunkCount };
 }
 
+async function readRemoteBase64BlockWithRetry(
+  invocation: ParsedSshInvocation,
+  executor: SshRemoteCommandExecutor,
+  builders: SshFileTransferCommandBuilders,
+  remotePath: string,
+  readBlockBytes: number,
+  blockIndex: number,
+  expectedBytes: number,
+  actualBytes: number,
+): Promise<string> {
+  const attemptErrors: Array<Record<string, unknown>> = [];
+  for (let attempt = 1; attempt <= fileTransferReadBlockMaxAttempts; attempt += 1) {
+    try {
+      const read = await checkedFileTransfer(invocation, executor, builders, "read-b64-block", [remotePath, String(blockIndex), String(readBlockBytes)]);
+      const encoded = read.stdout.replace(/\s+/gu, "");
+      if (encoded.length > 0) return encoded;
+      attemptErrors.push({ attempt, exitCode: read.exitCode, stdoutBytes: read.stdout.length, stderrTail: read.stderr.slice(-500) });
+    } catch (error) {
+      attemptErrors.push({
+        attempt,
+        error: error instanceof Error ? error.message : String(error),
+      });
+    }
+  }
+  throw new SshFileTransferError("remote download returned an empty block before EOF after retries", {
+    route: invocation.route.raw,
+    remotePath,
+    blockIndex,
+    attempts: fileTransferReadBlockMaxAttempts,
+    expectedBytes,
+    actualBytes,
+    attemptErrors,
+  });
+}
+
 async function statRemoteFile(
   invocation: ParsedSshInvocation,
   executor: SshRemoteCommandExecutor,