testing: allow disabling fsync to make tests run faster (#6871)

### Describe Your Changes

fsync() ensures that the data is written to disk. In production this is
needed for data durability. However, during the development, when the
unit tests are run, this level of durability is not needed. Therefore
fsync() can be disabled which will makes test runs two times faster.

The disabling is done by setting the `DISABLE_FSYNC_FOR_TESTING`
environment variable. The valid values for this variable are the same as
the values of the arg of `go doc strconv.ParseBool`:

```
1, t, T, TRUE, true, True, 0, f, F, FALSE, false, False.
```

Any other value means `false`.

The variable is set for all test build targets. Compare running times:

Build Target | DISABLE_FSYNC_FOR_TESTING=0 | DISABLE_FSYNC_FOR_TESTING=1
----------------- | ------------------------------------------------ |
-------------------------------------------------
make test | 1m5s  | 0m22s
make test-race | 3m1s | 1m42s
make test-pure | 1m7s | 0m20s
make test-full | 1m21s | 0m32s
make test-full-386 | 1m42s | 0m36s

When running tests for a given package, fsync can be disabled as
follows:

```shell
DISABLE_FSYNC_FOR_TESTING=1 go test ./lib/storage
```

Disabling fsync() is intended for testing purposes only and the name of
the variables reflects that.

What could also have been done but haven't:

- lib/filestream/filestream.go: `Writer.MustFlush()` also uses f.Sync()
but nothing has been done to it, because the Writer.MustFlush() is not
used anywhere in the VM codebase. A side question: what is the general
policy for the unused code?
- lib/filestream/filestream.go: Writer.Write() calls `adviceDontNeed()`
which calls unix.Fdatasync(). Disabling it could potentially improve
running time, but running tests with this code disabled has shown
otherwise.

### Checklist

The following checks are **mandatory**:

- [ x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

---------

Signed-off-by: Artem Fetishev <wwctrsrx@gmail.com>

.gitignore

@@ -26,3 +26,4 @@ _site
 /docs/.jekyll-metadata
 coverage.txt
 cspell.json
+*~

Makefile

@@ -333,7 +333,7 @@ release-vmutils: \
 
 release-vmutils-linux-386:
 	GOOS=linux GOARCH=386 $(MAKE) release-vmutils-goos-goarch
-	
+
 release-vmutils-linux-amd64:
 	GOOS=linux GOARCH=amd64 $(MAKE) release-vmutils-goos-goarch
 
@@ -444,19 +444,19 @@ check-all: fmt vet golangci-lint govulncheck
 clean-checkers: remove-golangci-lint remove-govulncheck
 
 test:
-	go test ./lib/... ./app/...
+	DISABLE_FSYNC_FOR_TESTING=1 go test ./lib/... ./app/...
 
 test-race:
-	go test -race ./lib/... ./app/...
+	DISABLE_FSYNC_FOR_TESTING=1 go test -race ./lib/... ./app/...
 
 test-pure:
-	CGO_ENABLED=0 go test ./lib/... ./app/...
+	DISABLE_FSYNC_FOR_TESTING=1 CGO_ENABLED=0 go test ./lib/... ./app/...
 
 test-full:
-	go test -coverprofile=coverage.txt -covermode=atomic ./lib/... ./app/...
+	DISABLE_FSYNC_FOR_TESTING=1 go test -coverprofile=coverage.txt -covermode=atomic ./lib/... ./app/...
 
 test-full-386:
-	GOARCH=386 go test -coverprofile=coverage.txt -covermode=atomic ./lib/... ./app/...
+	DISABLE_FSYNC_FOR_TESTING=1 GOARCH=386 go test -coverprofile=coverage.txt -covermode=atomic ./lib/... ./app/...
 
 benchmark:
 	go test -bench=. ./lib/...

lib/envutil/envutil.go

@@ -0,0 +1,19 @@
+package envutil
+
+import (
+	"os"
+	"strconv"
+)
+
+// GetenvBool retrieves the value of the environment variable named by the key,
+// attempts to convert the value to bool type and returns the result. In order
+// for conversion to succeed, the value must be any value supported by
+// strconv.ParseBool() function, otherwise the function will return false.
+func GetenvBool(key string) bool {
+	s := os.Getenv(key)
+	b, err := strconv.ParseBool(s)
+	if err != nil {
+		return false
+	}
+	return b
+}

lib/envutil/envutil_test.go

@@ -0,0 +1,37 @@
+package envutil
+
+import (
+	"os"
+	"testing"
+)
+
+func TestGetenvBool(t *testing.T) {
+	f := func(value string, want bool) {
+		t.Helper()
+
+		key := "VM_LIB_ENVUTIL_TEST_GETENV_BOOL"
+		os.Setenv(key, value)
+		defer os.Unsetenv(key)
+
+		if got := GetenvBool(key); got != want {
+			t.Errorf("GetenvBool(%s=%s) unexpected return value: got %t, want %t", key, value, got, want)
+		}
+	}
+
+	f("TRUE", true)
+	f("True", true)
+	f("true", true)
+	f("T", true)
+	f("t", true)
+	f("1", true)
+	f("FALSE", false)
+	f("False", false)
+	f("false", false)
+	f("F", false)
+	f("f", false)
+	f("0", false)
+
+	f("", false)
+	f("unsupported", false)
+	f("tRuE", false)
+}

lib/filestream/filestream.go

@@ -9,6 +9,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/envutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/memory"
 	"github.com/VictoriaMetrics/metrics"
@@ -18,6 +19,8 @@ var disableFadvise = flag.Bool("filestream.disableFadvise", false, "Whether to d
 	"The fadvise() syscall prevents from eviction of recently accessed data from OS page cache during background merges and backups. "+
 	"In some rare cases it is better to disable the syscall if it uses too much CPU")
 
+var disableFSyncForTesting = envutil.GetenvBool("DISABLE_FSYNC_FOR_TESTING")
+
 const dontNeedBlockSize = 16 * 1024 * 1024
 
 // ReadCloser is a standard interface for filestream Reader.
@@ -241,8 +244,10 @@ func (w *Writer) MustClose() {
 	putBufioWriter(w.bw)
 	w.bw = nil
 
-	if err := w.f.Sync(); err != nil {
+	if !disableFSyncForTesting {
-		logger.Panicf("FATAL: cannot sync file %q: %d", w.f.Name(), err)
+		if err := w.f.Sync(); err != nil {
+			logger.Panicf("FATAL: cannot sync file %q: %d", w.f.Name(), err)
+		}
 	}
 	if err := w.st.close(); err != nil {
 		logger.Panicf("FATAL: cannot close streamTracker for file %q: %s", w.f.Name(), err)

lib/fs/fs.go

@@ -11,11 +11,14 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/envutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fasttime"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/filestream"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 )
 
+var disableFSyncForTesting = envutil.GetenvBool("DISABLE_FSYNC_FOR_TESTING")
+
 var tmpFileNum atomic.Uint64
 
 // MustSyncPath syncs contents of the given path.

lib/fs/fs_solaris.go

@@ -30,9 +30,11 @@ func mustSyncPath(path string) {
 	if err != nil {
 		logger.Panicf("FATAL: cannot open file for fsync: %s", err)
 	}
-	if err := d.Sync(); err != nil {
+	if !disableFSyncForTesting {
-		_ = d.Close()
+		if err := d.Sync(); err != nil {
-		logger.Panicf("FATAL: cannot flush %q to storage: %s", path, err)
+			_ = d.Close()
+			logger.Panicf("FATAL: cannot flush %q to storage: %s", path, err)
+		}
 	}
 	if err := d.Close(); err != nil {
 		logger.Panicf("FATAL: cannot close %q: %s", path, err)

lib/fs/fs_unix.go

@@ -23,9 +23,11 @@ func mustSyncPath(path string) {
 	if err != nil {
 		logger.Panicf("FATAL: cannot open file for fsync: %s", err)
 	}
-	if err := d.Sync(); err != nil {
+	if !disableFSyncForTesting {
-		_ = d.Close()
+		if err := d.Sync(); err != nil {
-		logger.Panicf("FATAL: cannot flush %q to storage: %s", path, err)
+			_ = d.Close()
+			logger.Panicf("FATAL: cannot flush %q to storage: %s", path, err)
+		}
 	}
 	if err := d.Close(); err != nil {
 		logger.Panicf("FATAL: cannot close %q: %s", path, err)