WIP: Spawn backend plugins v2 (#19835)

* WIP: Spawn backend plugins v2

* Add test for plugin version

* Fix support for SDK plugins

Co-authored-by: Kyle Brandt <kyle@kbrandt.com>
Co-authored-by: Marcus Olsson <olsson.e.marcus@gmail.com>
Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* WIP: Draft PR for fork of V2 sdk / bi-directional support (#19890)

* temporary use export-datasource-plugin branch of grafana-plugin-sdk

* fix failing test

* remove debug (spew) lines

* misc cleanup

* add expressions feature toggle

* use latest grafana-plugin-sdk-go

vendor/github.com/mattetti/filebuffer/filebuffer.go

@@ -0,0 +1,161 @@
+// filebuffer is a package implementing a few file like interfaces
+// backed by a byte buffer.
+// Implemented interfaces:
+//
+// * Reader
+// * ReaderAt
+// * Writer
+// * Seeker
+// * Closer
+package filebuffer
+
+import (
+	"bytes"
+	"errors"
+	"io"
+)
+
+// Buffer implements interfaces implemented by files.
+// The main purpose of this type is to have an in memory replacement for a
+// file.
+type Buffer struct {
+	// Buff is the backing buffer
+	Buff *bytes.Buffer
+	// Index indicates where in the buffer we are at
+	Index    int64
+	isClosed bool
+}
+
+// New returns a new populated Buffer
+func New(b []byte) *Buffer {
+	return &Buffer{Buff: bytes.NewBuffer(b)}
+}
+
+// Bytes returns the bytes available until the end of the buffer.
+func (f *Buffer) Bytes() []byte {
+	if f.isClosed || f.Index >= int64(f.Buff.Len()) {
+		return []byte{}
+	}
+	return f.Buff.Bytes()[f.Index:]
+}
+
+// String implements the Stringer interface
+func (f *Buffer) String() string {
+	return string(f.Buff.Bytes()[f.Index:])
+}
+
+// Read implements io.Reader https://golang.org/pkg/io/#Reader
+// Read reads up to len(p) bytes into p. It returns the number of bytes read (0 <= n <= len(p))
+// and any error encountered. Even if Read returns n < len(p), it may use all of p as scratch
+// space during the call. If some data is available but not len(p) bytes, Read conventionally
+// returns what is available instead of waiting for more.
+
+// When Read encounters an error or end-of-file condition after successfully reading n > 0 bytes,
+// it returns the number of bytes read. It may return the (non-nil) error from the same call or
+// return the error (and n == 0) from a subsequent call. An instance of this general case is
+// that a Reader returning a non-zero number of bytes at the end of the input stream may return
+// either err == EOF or err == nil. The next Read should return 0, EOF.
+func (f *Buffer) Read(b []byte) (n int, err error) {
+	if f.isClosed {
+		return 0, io.EOF
+	}
+	if len(b) == 0 {
+		return 0, nil
+	}
+	if f.Index >= int64(f.Buff.Len()) {
+		return 0, io.EOF
+	}
+	n, err = bytes.NewBuffer(f.Buff.Bytes()[f.Index:]).Read(b)
+	f.Index += int64(n)
+
+	return n, err
+}
+
+// ReadAt implements io.ReaderAt https://golang.org/pkg/io/#ReaderAt
+// ReadAt reads len(p) bytes into p starting at offset off in the underlying input source.
+// It returns the number of bytes read (0 <= n <= len(p)) and any error encountered.
+//
+// When ReadAt returns n < len(p), it returns a non-nil error explaining why more bytes were not returned.
+// In this respect, ReadAt is stricter than Read.
+//
+// Even if ReadAt returns n < len(p), it may use all of p as scratch space during the call.
+// If some data is available but not len(p) bytes, ReadAt blocks until either all the data is available or an error occurs.
+// In this respect ReadAt is different from Read.
+//
+// If the n = len(p) bytes returned by ReadAt are at the end of the input source,
+// ReadAt may return either err == EOF or err == nil.
+//
+// If ReadAt is reading from an input source with a seek offset,
+// ReadAt should not affect nor be affected by the underlying seek offset.
+// Clients of ReadAt can execute parallel ReadAt calls on the same input source.
+func (f *Buffer) ReadAt(p []byte, off int64) (n int, err error) {
+	if f.isClosed {
+		return 0, io.EOF
+	}
+	if off < 0 {
+		return 0, errors.New("filebuffer.ReadAt: negative offset")
+	}
+	reqLen := len(p)
+	buffLen := int64(f.Buff.Len())
+	if off >= buffLen {
+		return 0, io.EOF
+	}
+
+	n = copy(p, f.Buff.Bytes()[off:])
+	if n < reqLen {
+		err = io.EOF
+	}
+	return n, err
+}
+
+// Write implements io.Writer https://golang.org/pkg/io/#Writer
+// by appending the passed bytes to the buffer unless the buffer is closed or index negative.
+func (f *Buffer) Write(p []byte) (n int, err error) {
+	if f.isClosed {
+		return 0, io.EOF
+	}
+	if f.Index < 0 {
+		return 0, io.EOF
+	}
+	// we might have rewinded, let's reset the buffer before appending to it
+	idx := int(f.Index)
+	buffLen := f.Buff.Len()
+	if idx != buffLen && idx <= buffLen {
+		f.Buff = bytes.NewBuffer(f.Bytes()[:f.Index])
+	}
+	n, err = f.Buff.Write(p)
+
+	f.Index += int64(n)
+	return n, err
+}
+
+// Seek implements io.Seeker https://golang.org/pkg/io/#Seeker
+func (f *Buffer) Seek(offset int64, whence int) (idx int64, err error) {
+	if f.isClosed {
+		return 0, io.EOF
+	}
+
+	var abs int64
+	switch whence {
+	case 0:
+		abs = offset
+	case 1:
+		abs = int64(f.Index) + offset
+	case 2:
+		abs = int64(f.Buff.Len()) + offset
+	default:
+		return 0, errors.New("filebuffer.Seek: invalid whence")
+	}
+	if abs < 0 {
+		return 0, errors.New("filebuffer.Seek: negative position")
+	}
+	f.Index = abs
+	return abs, nil
+}
+
+// Close implements io.Closer https://golang.org/pkg/io/#Closer
+// It closes the buffer, rendering it unusable for I/O. It returns an error, if any.
+func (f *Buffer) Close() error {
+	f.isClosed = true
+	return nil
+}