relay hdf5: use H5F_ACC_RDONLY for is_hdf5_file helper (#518)

* relay hdf5: use H5F_ACC_RDONLY for is_hdf5_file

* update changelog

CHANGELOG.md

@@ -17,6 +17,10 @@ and this project aspires to adhere to [Semantic Versioning](https://semver.org/s
 - Updated to BLT v0.3.0 to resolve BLT/FindMPI issues with rpath linking commands when using OpenMPI.
 
 
+#### Relay
+- Use H5F_ACC_RDONLY in relay::io::is_hdf5_file to avoid errors when checking files that already have open HDF5 handles.
+
+
 ## [0.5.1] - Released 2020-01-18
 
 ### Added

src/libs/relay/conduit_relay_io_hdf5.cpp

@@ -2632,9 +2632,14 @@ is_hdf5_file(const std::string &file_path)
     HDF5ErrorStackSupressor supress_hdf5_errors;
 
     bool res = false;
-    // open the hdf5 file for read + write
+    // open the file for read to check if it is valid hdf5
+    //
+    // don't use H5F_ACC_RDWR, b/c if we already have a file handle open 
+    // that is RDONLY, the open will fail
+    //
+    // use H5F_ACC_RDONLY b/c it will work with open file handles
     hid_t h5_file_id = H5Fopen(file_path.c_str(),
-                               H5F_ACC_RDWR,
+                               H5F_ACC_RDONLY,
                                H5P_DEFAULT);
 
     if( h5_file_id >= 0)

src/tests/relay/t_relay_io_hdf5.cpp

@@ -995,6 +995,15 @@ TEST(conduit_relay_io_hdf5, check_if_file_is_hdf5_file)
     // this should be recoged as hdf5
     EXPECT_TRUE(io::is_hdf5_file(tout));
 
+    // check behavior with files that have open handles
+    hid_t h5_file_id = io::hdf5_open_file_for_read_write(tout);
+    EXPECT_TRUE(io::is_hdf5_file(tout));
+    io::hdf5_close_file(h5_file_id);
+
+    h5_file_id = io::hdf5_open_file_for_read(tout);
+    EXPECT_TRUE(io::is_hdf5_file(tout));
+    io::hdf5_close_file(h5_file_id);
+
     tout = "tout_hdf5_check_non_hdf5_file.json";
 
     if(utils::is_file(tout))
@@ -1010,6 +1019,7 @@ TEST(conduit_relay_io_hdf5, check_if_file_is_hdf5_file)
 
     // check totally bad path
     EXPECT_FALSE(io::is_hdf5_file("/path/to/somewhere/that/cant/exist"));
+
 }