fix: rely on OS temp directory

This is an issue to impose a `/tmp` as a default temporary directory, even thought this is the right one.

On some case, you'd end up having a `/tmp` which is not writable (k8s: securityContext -> readOnlyRootFilesystem: true) and you'd prefer to have a way to override this.

cache/redis_cache.go

@@ -34,8 +34,6 @@ const statsTimeout = 500 * time.Millisecond
 // result from redis in a temporary files before sending it to the http response
 const minTTLForRedisStreamingReader = 15 * time.Second
 
-// tmpDir temporary path to store ongoing queries results
-const tmpDir = "/tmp"
 const redisTmpFilePrefix = "chproxyRedisTmp"
 
 func newRedisCache(client redis.UniversalClient, cfg config.Cache) *redisCache {
@@ -155,7 +153,7 @@ func (r *redisCache) readResultsAboveLimit(offset int, stringKey string, metadat
 	// nb: it would be better to retry the flow if such a failure happened but this requires a huge refactoring of proxy.go
 
 	if ttl <= minTTLForRedisStreamingReader {
-		fileStream, err := newFileWriterReader(tmpDir)
+		fileStream, err := newFileWriterReader(os.TempDir())
 		if err != nil {
 			return nil, err
 		}

cache/redis_cache_test.go

@@ -236,7 +236,7 @@ func TestSmallTTLOnBigPayloadAreCacheWithFile(t *testing.T) {
 
 	//simulate a value almost expired
 	redis.SetTTL(key.String(), 2*time.Second)
-	nbFileCacheBeforeGet, err := countFilesWithPrefix(tmpDir, redisTmpFilePrefix)
+	nbFileCacheBeforeGet, err := countFilesWithPrefix(os.TempDir(), redisTmpFilePrefix)
 	if err != nil {
 		t.Fatalf("could not read directory %s", err)
 	}
@@ -245,7 +245,7 @@ func TestSmallTTLOnBigPayloadAreCacheWithFile(t *testing.T) {
 	if err != nil {
 		t.Fatalf("expected cached to have the value")
 	}
-	nbFileCacheAfterGet, err := countFilesWithPrefix(tmpDir, redisTmpFilePrefix)
+	nbFileCacheAfterGet, err := countFilesWithPrefix(os.TempDir(), redisTmpFilePrefix)
 	if err != nil {
 		t.Fatalf("could not read directory %s", err)
 	}
@@ -261,7 +261,7 @@ func TestSmallTTLOnBigPayloadAreCacheWithFile(t *testing.T) {
 		t.Fatalf("got a value different than the expected one len(value)=%d vs len(expectedValue)=%d", len(string(cachedValue)), len(expectedValue))
 	}
 	cachedData.Data.Close()
-	nbFileCacheAfterClose, err := countFilesWithPrefix(tmpDir, redisTmpFilePrefix)
+	nbFileCacheAfterClose, err := countFilesWithPrefix(os.TempDir(), redisTmpFilePrefix)
 	if err != nil {
 		t.Fatalf("could not read directory %s", err)
 	}

proxy.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"net/http/httputil"
 	"net/url"
+	"os"
 	"strconv"
 	"strings"
 	"sync"
@@ -22,9 +23,6 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-// tmpDir temporary path to store ongoing queries results
-const tmpDir = "/tmp"
-
 // failedTransactionPrefix prefix added to the failed reason for concurrent queries registry
 const failedTransactionPrefix = "[concurrent query failed]"
 
@@ -417,7 +415,7 @@ func (rp *reverseProxy) serveFromCache(s *scope, srw *statResponseWriter, req *h
 
 	// The response wasn't found in the cache.
 	// Request it from clickhouse.
-	tmpFileRespWriter, err := cache.NewTmpFileResponseWriter(srw, tmpDir)
+	tmpFileRespWriter, err := cache.NewTmpFileResponseWriter(srw, os.TempDir())
 	if err != nil {
 		err = fmt.Errorf("%s: %w; query: %q", s, err, q)
 		respondWith(srw, err, http.StatusInternalServerError)