From e02d9fb502f878f8e8d816f1c222b21863b57a43 Mon Sep 17 00:00:00 2001
From: Chris Bradley <chris.bradley@ecmwf.int>
Date: Mon, 19 Feb 2024 16:50:00 +0000
Subject: [PATCH] GribHandle starting from an offset

---
 src/metkit/codes/GribHandle.cc | 23 +++++++++++++++++++++++
 src/metkit/codes/GribHandle.h  |  3 +++
 2 files changed, 26 insertions(+)

diff --git a/src/metkit/codes/GribHandle.cc b/src/metkit/codes/GribHandle.cc
index 65434aa5..f766f011 100644
--- a/src/metkit/codes/GribHandle.cc
+++ b/src/metkit/codes/GribHandle.cc
@@ -82,6 +82,29 @@ GribHandle::GribHandle(eckit::DataHandle& handle):
     handle_ = h;
 }
 
+GribHandle::GribHandle(eckit::DataHandle& handle, eckit::Offset offset):
+    handle_(nullptr),
+    owned_(true) {
+
+    codes_handle* h = nullptr;
+    int err = 0;
+
+    FILE* f = handle.openf();
+    ASSERT(f);
+
+    fseek(f, offset, SEEK_SET);
+    h = codes_handle_new_from_file(0, f, PRODUCT_GRIB, &err);
+
+    CODES_CALL(err);
+    ASSERT(h);
+    handle_ = h;
+
+    // XXX: Part of a workaround to sync handle and f on linux.
+    // XXX: This needs to be investigated further.
+    handle.seek(ftello(f));
+    fclose(f); // XXX: The other constructor does not close the file, why do we?
+}
+
 GribHandle::~GribHandle() noexcept(false) {
     if (handle_ && owned_) {
         CODES_CALL(codes_handle_delete(handle_));
diff --git a/src/metkit/codes/GribHandle.h b/src/metkit/codes/GribHandle.h
index b900bff6..b7e06f1e 100644
--- a/src/metkit/codes/GribHandle.h
+++ b/src/metkit/codes/GribHandle.h
@@ -53,6 +53,9 @@ class GribHandle : private eckit::NonCopyable {
     /// constructor creating a grib_handle from a DataHandle
     explicit GribHandle(eckit::DataHandle&);
 
+    // Constructor creating a grib_handle from a DataHandle starting from a given offset
+    explicit GribHandle(eckit::DataHandle&, eckit::Offset);
+
     /// destructor will delete the grib_handle if we own it
     ~GribHandle() noexcept(false);