logs: support --tail 0

change the default to -1, so that we can change the semantic of
"--tail 0" to not print any existing log line.

Closes: https://github.com/containers/libpod/issues/4396

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

2 files changed, 11 insertions, 4 deletions

diff --git a/libpod/container_log_linux.go b/libpod/container_log_linux.go
index 8a87a8796..c4acc3d4f 100644
--- a/libpod/container_log_linux.go
+++ b/libpod/container_log_linux.go
@@ -6,6 +6,7 @@ package libpod
 import (
 	"fmt"
 	"io"
+	"math"
 	"strings"
 	"time"
 
@@ -30,7 +31,11 @@ const (
 
 func (c *Container) readFromJournal(options *logs.LogOptions, logChannel chan *logs.LogLine) error {
 	var config journal.JournalReaderConfig
-	config.NumFromTail = options.Tail
+	if options.Tail < 0 {
+		config.NumFromTail = math.MaxUint64
+	} else {
+		config.NumFromTail = uint64(options.Tail)
+	}
 	config.Formatter = journalFormatter
 	defaultTime := time.Time{}
 	if options.Since != defaultTime {
@@ -54,7 +59,7 @@ func (c *Container) readFromJournal(options *logs.LogOptions, logChannel chan *l
 	if r == nil {
 		return errors.Errorf("journal reader creation failed")
 	}
-	if options.Tail == 0 {
+	if options.Tail == math.MaxInt64 {
 		r.Rewind()
 	}
 
diff --git a/libpod/logs/log.go b/libpod/logs/log.go
index 0b1703567..0330df06a 100644
--- a/libpod/logs/log.go
+++ b/libpod/logs/log.go
@@ -31,7 +31,7 @@ type LogOptions struct {
 	Details    bool
 	Follow     bool
 	Since      time.Time
-	Tail       uint64
+	Tail       int64
 	Timestamps bool
 	Multi      bool
 	WaitGroup  *sync.WaitGroup
@@ -54,8 +54,10 @@ func GetLogFile(path string, options *LogOptions) (*tail.Tail, []*LogLine, error
 		logTail []*LogLine
 	)
 	// whence 0=origin, 2=end
-	if options.Tail > 0 {
+	if options.Tail >= 0 {
 		whence = 2
+	}
+	if options.Tail > 0 {
 		logTail, err = getTailLog(path, int(options.Tail))
 		if err != nil {
 			return nil, nil, err