lz4 code inside flann

[lepalom]

flann has code from another project lz4. It's not a license problem, but for instance it violates some policy in debian.

Could be possible to activate some flag in cmake that in case that lz4 was not detected use that folder or if detected use the external lz4 library?

See this project blosc
Think that if you use the external lz4 you should add then -lz4 in the pkg-config .pc file

[moriarty]

👍 plus one for this... It also causes problems because
/usr/include/lz4.h and
/usr/include/flann/ext/lz4.h

use #pragma once instead of include gaurds, and then if a user later includes two headers which indirectly include one of each of the lz4.h the pragma once doesn't actually do any gaurding, and both get included and then compile errors occur. despite the files being the same.

amoriarty at SW44 in ~/melodic
$ md5sum /usr/include/lz4.h
f9f3bd3fb79d7a132d8e22c24983193a  /usr/include/lz4.h
amoriarty at SW44 in ~/melodic
$ md5sum /usr/include/flann/ext/lz4.h
f9f3bd3fb79d7a132d8e22c24983193a  /usr/include/flann/ext/lz4.h

For a concrete example, it is impossible to include:
/usr/include/flann/flann.h
/opt/ros/melodic/include/rosbag/bag.h

The only way to fix this, is for both flann and the system version of lz4 to be newer, and include this commit:
lz4/lz4@19e585c

which replaced the #pragma once

-#pragma once
+#ifndef LZ4_H_2983827168210
+#define LZ4_H_2983827168210
...
+#endif /* LZ4_H_2983827168210 */

[mikepurvis]

Resolved by #399, but it has not been released, so packagers will need to patch on that change.

[nh2]

I made a PR to fix lz4 missing from the flann.pc pkg-config file: #481