validation: Refactor OpenDiskFile into method on FlatFileSeq.

src/flatfile.cpp

@@ -5,6 +5,7 @@
 #include <stdexcept>
 
 #include <flatfile.h>
+#include <logging.h>
 #include <tinyformat.h>
 
 FlatFileSeq::FlatFileSeq(fs::path dir, const char* prefix, size_t chunk_size) :
@@ -21,3 +22,26 @@ fs::path FlatFileSeq::FileName(const CDiskBlockPos& pos) const
 {
     return m_dir / strprintf("%s%05u.dat", m_prefix, pos.nFile);
 }
+
+FILE* FlatFileSeq::Open(const CDiskBlockPos& pos, bool fReadOnly)
+{
+    if (pos.IsNull())
+        return nullptr;
+    fs::path path = FileName(pos);
+    fs::create_directories(path.parent_path());
+    FILE* file = fsbridge::fopen(path, fReadOnly ? "rb": "rb+");
+    if (!file && !fReadOnly)
+        file = fsbridge::fopen(path, "wb+");
+    if (!file) {
+        LogPrintf("Unable to open file %s\n", path.string());
+        return nullptr;
+    }
+    if (pos.nPos) {
+        if (fseek(file, pos.nPos, SEEK_SET)) {
+            LogPrintf("Unable to seek to position %u of %s\n", pos.nPos, path.string());
+            fclose(file);
+            return nullptr;
+        }
+    }
+    return file;
+}