worker/osbuild: print raw osbuild output if it cannot be parsed

osbuild output should be always JSON-parsable. However, if a user has a weird
installation of osbuild, it can return malformed JSON. In this case, it's
very hard to debug what happened because the worker doesn't provide any
useful information. This commit adds the non-parsable osbuild output to the
error so there's at least some clue in the logs what happened.

Let me reiterate: In 99% these cases, this shouldn't be needed. It will give
us some hints in these 1% cases though.

You may want to ask if using json.Decoder isn't better because it doesn't
buffer the input. However, it does buffer. Firstly, json.Decoder.Decode()
buffers the whole JSON value (it contains kinda cool state machine to do it)
and in the second run, it parses the buffer. Therefore, the only thing that
this commit actually does is that it moves the buffer out of json.Decoder.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

cmd/osbuild-worker/osbuild.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -29,10 +30,8 @@ func RunOSBuild(manifest distro.Manifest, store, outputDirectory string, errorWr
 		return nil, fmt.Errorf("error setting up stdin for osbuild: %v", err)
 	}
 
-	stdout, err := cmd.StdoutPipe()
-	if err != nil {
-		return nil, fmt.Errorf("error setting up stdout for osbuild: %v", err)
-	}
+	var stdoutBuffer bytes.Buffer
+	cmd.Stdout = &stdoutBuffer
 
 	err = cmd.Start()
 	if err != nil {
@@ -49,13 +48,15 @@ func RunOSBuild(manifest distro.Manifest, store, outputDirectory string, errorWr
 		return nil, fmt.Errorf("error closing osbuild's stdin: %v", err)
 	}
 
+	err = cmd.Wait()
+
+	// try to decode the output even though the job could have failed
 	var result osbuild.Result
-	err = json.NewDecoder(stdout).Decode(&result)
-	if err != nil {
-		return nil, fmt.Errorf("error decoding osbuild output: %#v", err)
+	decodeErr := json.Unmarshal(stdoutBuffer.Bytes(), &result)
+	if decodeErr != nil {
+		return nil, fmt.Errorf("error decoding osbuild output: %v\nthe raw output:\n%s", decodeErr, stdoutBuffer.String())
 	}
 
-	err = cmd.Wait()
 	if err != nil {
 		// ignore ExitError if output could be decoded correctly
 		if _, isExitError := err.(*exec.ExitError); !isExitError {