app/vlogscli: fallback to stdout when less isn't available

app/vlogscli/less_wrapper.go

@@ -18,15 +18,14 @@ func isTerminal() bool {
 }
 
 func readWithLess(r io.Reader, disableColors, wrapLongLines bool) error {
-	if !isTerminal() {
-		// Just write everything to stdout if no terminal is available.
+	// Write everything to stdout if there is no terminal or 'less' isn't available in $PATH.
+	// The latter happens e.g. inside the distroless Docker image, which doesn't ship 'less'.
+	path, err := exec.LookPath("less")
+	if !isTerminal() || err != nil {
 		_, err := io.Copy(os.Stdout, r)
 		if err != nil && !isErrPipe(err) {
 			return fmt.Errorf("error when forwarding data to stdout: %w", err)
 		}
-		if err := os.Stdout.Sync(); err != nil {
-			return fmt.Errorf("cannot sync data to stdout: %w", err)
-		}
 		return nil
 	}
 
@@ -44,10 +43,6 @@ func readWithLess(r io.Reader, disableColors, wrapLongLines bool) error {
 	defer cancel()
 
 	// Start 'less' process
-	path, err := exec.LookPath("less")
-	if err != nil {
-		return fmt.Errorf("cannot find 'less' command: %w", err)
-	}
 	opts := []string{"less", "-F", "-X"}
 	if !disableColors {
 		opts = append(opts, "-R")

app/vlogscli/less_wrapper_test.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"io"
+	"os"
+	"strings"
+	"testing"
+)
+
+// readWithLess must forward the response to stdout without failing when stdout
+// can't be paged or synced - e.g. when it is a pipe (`vlogscli ... | grep`) or a
+// terminal without the 'less' command (distroless image). os.Stdout.Sync() returns
+// an error for pipes and terminals, and that error must not abort the query.
+func TestReadWithLessForwardsToStdoutWithoutSyncError(t *testing.T) {
+	pr, pw, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("cannot create pipe: %s", err)
+	}
+
+	// Drain the read end so io.Copy doesn't block.
+	type readResult struct {
+		data []byte
+		err  error
+	}
+	resCh := make(chan readResult, 1)
+	go func() {
+		data, err := io.ReadAll(pr)
+		resCh <- readResult{data, err}
+	}()
+
+	origStdout := os.Stdout
+	os.Stdout = pw // a pipe, so os.Stdout.Sync() fails
+
+	const input = "line one\nline two\nline three\n"
+	err = readWithLess(strings.NewReader(input), true, false)
+
+	os.Stdout = origStdout
+	_ = pw.Close()
+	res := <-resCh
+
+	if err != nil {
+		t.Fatalf("unexpected error from readWithLess: %s", err)
+	}
+	if res.err != nil {
+		t.Fatalf("cannot read forwarded data: %s", res.err)
+	}
+	if string(res.data) != input {
+		t.Fatalf("unexpected stdout;\ngot:\n%q\nwant:\n%q", res.data, input)
+	}
+}

docs/victorialogs/querying/vlogscli.md

@@ -109,6 +109,10 @@ thanks to the way how `less` interacts with [`/select/logsql/query`](https://doc
   VictoriaLogs stops query processing and frees up all the associated resources
   after the response stream is closed.
 
+If the `less` command isn't available in `$PATH` (for example, when `vlogscli` runs inside the
+[distroless](https://github.com/GoogleContainerTools/distroless/) Docker image, which doesn't ship `less`),
+`vlogscli` prints the query response directly to stdout without paging.
+
 See also [`less` docs](https://man7.org/linux/man-pages/man1/less.1.html) and
 [command-line integration docs for VictoriaLogs](https://docs.victoriametrics.com/victorialogs/querying/#command-line).