Exception with cursor.Get(k, nil, Set)

[slav]

I have db with dup values (not sure if relevant). When I'm trying to do k, v, e = cursor.Get(buf.Bytes(), nil, lmdb.Set) I get exception (see below). With k, v, e = cursor.Get(buf.Bytes(), nil, lmdb.SetKey) it works. Just Set positions cursor at the key but doesn't read value. But cursor.Get still attempts to read it which leads to the exception.

panic: runtime error: cgo argument has Go pointer to Go pointer

goroutine 21 [running]:
panic(0x56f360, 0xc0420b0260)
        C:/Go/src/runtime/panic.go:500 +0x1af
github.com/bmatsuo/lmdb-go/lmdb._cgoCheckPointer0(0xc0420bc024, 0x0, 0x0, 0x0, 0x0)
        ??:0 +0x60
github.com/bmatsuo/lmdb-go/lmdb.getBytesCopy(0xc0420b0170, 0xc0420b0170, 0x23190e0, 0x556ee0)
        D:/Projects/Go/src/github.com/bmatsuo/lmdb-go/lmdb/val.go:120 +0x51
github.com/bmatsuo/lmdb-go/lmdb.(*Txn).bytes(0xc0420b6120, 0xc0420b0170, 0x25, 0x40, 0xf)
        D:/Projects/Go/src/github.com/bmatsuo/lmdb-go/lmdb/txn.go:276 +0x7a
github.com/bmatsuo/lmdb-go/lmdb.(*Cursor).Get(0xc0420b01a0, 0xc0420bc024, 0x25, 0x40, 0x0, 0x0, 0x0, 0xf, 0xa0, 0x61, ...)
        D:/Projects/Go/src/github.com/bmatsuo/lmdb-go/lmdb/cursor.go:155 +0x147
github.com/bmatsuo/lmdb-go/lmdbscan.(*Scanner).Set(0xc0420ba120, 0xc0420bc024, 0x25, 0x40, 0x0, 0x0, 0x0, 0xf, 0x4c)
        D:/Projects/Go/src/github.com/bmatsuo/lmdb-go/lmdbscan/scanner.go:74 +0xc4
github.com/bmatsuo/lmdb-go/lmdbscan.(*Scanner).SetNext(0xc0420ba120, 0xc0420bc024, 0x25, 0x40, 0x0, 0x0, 0x0, 0xf, 0x9, 0x25)
        D:/Projects/Go/src/github.com/bmatsuo/lmdb-go/lmdbscan/scanner.go:85 +0x9b
bitbucket.org/agileharbor/lmdbbenchmark/lmdbLayer.(*Reader).ReadDupRecords.func1(0xc0420b6120, 0x0, 0x0)
        D:/Projects/Go/src/bitbucket.org/agileharbor/lmdbbenchmark/lmdbLayer/reader.go:103 +0x83d
github.com/bmatsuo/lmdb-go/lmdb.(*Env).run(0xc042072040, 0x412c00, 0x20000, 0xc0420ae0f0, 0x0, 0x0)
        D:/Projects/Go/src/github.com/bmatsuo/lmdb-go/lmdb/env.go:461 +0x157
github.com/bmatsuo/lmdb-go/lmdb.(*Env).View(0xc042072040, 0xc0420ae0f0, 0x9, 0xc000000002)
        D:/Projects/Go/src/github.com/bmatsuo/lmdb-go/lmdb/env.go:423 +0x4a

[bmatsuo]

That is a strange error. The function causing that panic is C.GoBytes(), which seems very strange.

The error is probably because lmdb.Set will not actually read values from the database, as you said. So whatever value lmdb-go is trying to extract wouldn't be valid anyway. I will add a test case for this to reproduce.

I will probably end up having the Cursor.Get method check the cursor op so it can short circuit and return nil slices when op is lmdb.Set.

AFAIK the op in question is the only one which does not return data. Are you aware of any other ops like this @slav?

http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127

Thanks for the report.

[bmatsuo]

Interestingly, on my primary Linux machine lmdb.Set and lmdb.SetKey seem to operate the same (they return the same values).

I notice that you are running your program in a Windows environment. Perhaps you could run the tests for me on your computer and confirm that the test case I added does in fact reproduce the panic for you? It has been some time since I compiled lmdb-go on Windows myself, so I will have to try to get that environment running again.

Branch: bmatsuo/cursor-op-set-testing (commit c9757f4)

[slav]

All the tests in branch "bmatsuo/cursor-op-set-testing" pass on windows.

I'm also unaware which ops init key/data and which don't. Documentation is only specific about MDB_SET_KEY to return key + data.

[bmatsuo]

OK. Thanks for checking that, @slav. I understand what is happening now. This is an issue with the bytes.Buffer type that I have run into before in #56. I can reproduce the error now and I will work on a fix.

[bmatsuo]

I have pushed a commit to that branch (bmatsuo/cursor-op-set-testing). I have a working fix that I will upload to another branch and issue a pull request to respect the process. Thanks again for your help, @slav. It is very much appreciated.