Avoid calling memcpy when len == 0 in filetransfer.cc

There was a bug report about a potential call to `memcpy` with a null
pointer which is not reproducible:
#492

This occurred in `src/libstore/filetransfer.cc` in `InnerSource::read`.

To ensure that this doesn't happen, an early return is added before
calling `memcpy` if the length of the data to be copied is 0.

This change also adds a test that ensures that when `InnerSource::read`
is called with an empty file, it throws an `EndOfFile` exception.

Change-Id: Ia18149bee9a3488576c864f28475a3a0c9eadfbb

src/libstore/filetransfer.cc

@@ -6,6 +6,7 @@
 #include "signals.hh"
 #include "compression.hh"
 #include "strings.hh"
+#include <cstddef>
 
 #if ENABLE_S3
 #include <aws/core/client/ClientConfiguration.h>
@@ -784,8 +785,10 @@ struct curlFileTransfer : public FileTransfer
 
             size_t read(char * data, size_t len) override
             {
-                auto readPartial = [this](char * data, size_t len) {
+                auto readPartial = [this](char * data, size_t len) -> size_t {
                     const auto available = std::min(len, buffered.size());
+                    if (available == 0u) return 0u;
+
                     memcpy(data, buffered.data(), available);
                     buffered.remove_prefix(available);
                     return available;

tests/unit/libstore/filetransfer.cc

@@ -150,6 +150,14 @@ TEST(FileTransfer, exceptionAbortsDownload)
     }
 }
 
+TEST(FileTransfer, exceptionAbortsRead)
+{
+    auto [port, srv] = serveHTTP("200 ok", "content-length: 0\r\n", [] { return ""; });
+    auto ft = makeFileTransfer();
+    char buf[10] = "";
+    ASSERT_THROW(ft->download(FileTransferRequest(fmt("http://[::1]:%d/index", port)))->read(buf, 10), EndOfFile);
+}
+
 TEST(FileTransfer, NOT_ON_DARWIN(reportsSetupErrors))
 {
     auto [port, srv] = serveHTTP("404 not found", "", [] { return ""; });