fix(multipart): bugs revealed by new tests

src/utils/split-multipart.ts

@@ -9,6 +9,13 @@ function isWhitespaceCharCode(charCode: number | undefined): boolean {
   return charCode === linearWhitespace.at(0) || charCode === linearWhitespace.at(1)
 }
 
+function fail(): never {
+  throw new ClientError("Invalid multipart data", {
+    statusCode: 400,
+    code: "ERR_INVALID_MULTIPART_DATA",
+  })
+}
+
 /**
  * Split a buffer's contents by some delimiter.
  *
@@ -19,18 +26,27 @@ export function splitMultipart(toSplit: Buffer, boundary: Buffer): Buffer[]
 export function splitMultipart(toSplit: Buffer, boundary: string, encoding?: BufferEncoding): Buffer[]
 export function splitMultipart(toSplit: Buffer, boundary: string | Buffer, encoding?: BufferEncoding): Buffer[] {
   const parts: Buffer[] = []
+
+  const boundaryWithoutNewline = Buffer.from(`--${boundary}`, encoding)
   boundary = Buffer.from(`\r\n--${boundary}`, encoding)
 
   let currentIndex = 0
   let previousPartBeginIndex = 0
   let seenFinalPartBoundary = false
+
+  if (toSplit.subarray(0, boundaryWithoutNewline.byteLength).equals(boundaryWithoutNewline)) {
+    currentIndex += boundaryWithoutNewline.byteLength
+    while (isWhitespaceCharCode(toSplit.at(currentIndex))) currentIndex++
+    if (!toSplit.subarray(currentIndex, currentIndex + CRLF.byteLength).equals(CRLF)) fail()
+    currentIndex += CRLF.byteLength
+    previousPartBeginIndex = currentIndex
+    parts.push(toSplit.subarray(0, 0))
+  }
+
   while (true) {
     currentIndex = toSplit.indexOf(boundary, currentIndex)
     if (currentIndex < 0) break
-    if (seenFinalPartBoundary) throw new ClientError("Invalid multipart data", {
-      statusCode: 400,
-      code: "ERR_INVALID_MULTIPART_DATA",
-    })
+    if (seenFinalPartBoundary) fail()
     parts.push(toSplit.subarray(previousPartBeginIndex, currentIndex))
     currentIndex += boundary.byteLength
 
@@ -42,15 +58,15 @@ export function splitMultipart(toSplit: Buffer, boundary: string | Buffer, encod
     while (isWhitespaceCharCode(toSplit.at(currentIndex))) currentIndex++
 
     if (!toSplit.subarray(currentIndex, currentIndex + CRLF.byteLength).equals(CRLF)) {
-      throw new ClientError("Invalid multipart data", {
-        statusCode: 400,
-        code: "ERR_INVALID_MULTIPART_DATA",
-      })
+      if (seenFinalPartBoundary && toSplit.at(currentIndex) == null) break
+      fail()
     }
     currentIndex += CRLF.byteLength
     previousPartBeginIndex = currentIndex
   }
 
+  if (!seenFinalPartBoundary) fail()
   parts.push(toSplit.subarray(previousPartBeginIndex))
+  if (parts.length < 3) fail()
   return parts
 }