istream/Length: implement length checks in _FillBucketList()

Fixes the new (failing) unit tests.

src/istream/LengthIstream.cxx

@@ -3,6 +3,7 @@
 // author: Max Kellermann <[email protected]>
 
 #include "LengthIstream.hxx"
+#include "Bucket.hxx"
 
 #include <stdexcept>
 
@@ -21,6 +22,28 @@ LengthIstream::_Skip(off_t length) noexcept
 	return nbytes;
 }
 
+void
+LengthIstream::_FillBucketList(IstreamBucketList &list)
+{
+	IstreamBucketList tmp;
+	FillBucketListFromInput(tmp);
+
+	const bool maybe_more = tmp.HasMore() || tmp.HasNonBuffer();
+	const std::size_t size = tmp.GetTotalBufferSize();
+
+	if ((off_t)size > remaining) {
+		Destroy();
+		throw std::runtime_error{"Too much data in stream"};
+	}
+
+	if (!maybe_more && (off_t)size < remaining) {
+		Destroy();
+		throw std::runtime_error{"Premature end of stream"};
+	}
+
+	list.SpliceBuffersFrom(std::move(tmp));
+}
+
 Istream::ConsumeBucketResult
 LengthIstream::_ConsumeBucketList(std::size_t nbytes) noexcept
 {

src/istream/LengthIstream.hxx

@@ -25,6 +25,7 @@ public:
 
 	off_t _GetAvailable(bool) noexcept override;
 	off_t _Skip(off_t length) noexcept override;
+	void _FillBucketList(IstreamBucketList &list) override;
 	ConsumeBucketResult _ConsumeBucketList(std::size_t nbytes) noexcept override;
 	void _ConsumeDirect(std::size_t nbytes) noexcept override;