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:
https://git.lix.systems/lix-project/lix/issues/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

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index 6fe1f9a05..34b92148e 100644
--- a/src/libstore/filetransfer.cc
+++ b/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;