server/authorizer: Fix gzip payload handling.

This PR fixes an issue where an OPA running authorization policies would
be unable to handle gzipped request bodies.

Example OPA CLI setup:

    opa run -s --authorization=basic

Example request:

    echo -n '{}' | gzip | curl -H "Content-Encoding: gzip" --data-binary @- http://127.0.0.1:8181/v1/data

This would result in unhelpful error messages, like:

```json
{
  "code": "invalid_parameter",
  "message": "invalid character '\\x1f' looking for beginning of value"
}
```

The cause was that the request body handling system in the
`server/authorizer` package did not take gzipped payloads into
account. The fix was to borrow the gzip request body handling function
from `server/server.go`, to transparently decompress the body when
needed.

Fixes: open-policy-agent#6804

Signed-off-by: Philip Conrad <[email protected]>

server/authorizer/authorizer.go

@@ -6,6 +6,8 @@
 package authorizer
 
 import (
+	"bytes"
+	"compress/gzip"
 	"context"
 	"io"
 	"net/http"
@@ -151,7 +153,6 @@ func (h *Basic) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 }
 
 func makeInput(r *http.Request) (*http.Request, interface{}, error) {
-
 	path, err := parsePath(r.URL.Path)
 	if err != nil {
 		return r, nil, err
@@ -164,7 +165,11 @@ func makeInput(r *http.Request) (*http.Request, interface{}, error) {
 
 	if expectBody(r.Method, path) {
 		var err error
-		rawBody, err = io.ReadAll(r.Body)
+		plaintextBody, err := readPlainBody(r)
+		if err != nil {
+			return r, nil, err
+		}
+		rawBody, err = io.ReadAll(plaintextBody)
 		if err != nil {
 			return r, nil, err
 		}
@@ -277,3 +282,20 @@ func GetBodyOnContext(ctx context.Context) (interface{}, bool) {
 	}
 	return input.parsed, true
 }
+
+// Note(philipc): Copied over from server/server.go
+func readPlainBody(r *http.Request) (io.ReadCloser, error) {
+	if strings.Contains(r.Header.Get("Content-Encoding"), "gzip") {
+		gzReader, err := gzip.NewReader(r.Body)
+		if err != nil {
+			return nil, err
+		}
+		bytesBody, err := io.ReadAll(gzReader)
+		if err != nil {
+			return nil, err
+		}
+		defer gzReader.Close()
+		return io.NopCloser(bytes.NewReader(bytesBody)), err
+	}
+	return r.Body, nil
+}