Add option to watch/tail a single file

README.md

@@ -67,6 +67,7 @@ USAGE:
 
 OPTIONS:
    --rate, -r "3s"  rds log polling rate
+   --file, -f "trace/alert_DATABASE.log.2017-09-27" name of the logfile
 
 ------------------------------------------------------------
 » ./rdstail tail -h
@@ -79,5 +80,6 @@ USAGE:
 
 OPTIONS:
    --lines, -n "20" output the last n lines. use 0 for a full dump of the most recent file
+   --file, -f "trace/alert_DATABASE.log.2017-09-27" name of the logfile
 
 ```

main.go

@@ -12,8 +12,8 @@ import (
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/rds"
-	"github.com/urfave/cli"
 	"github.com/litl/rdstail/src"
+	"github.com/urfave/cli"
 )
 
 func fie(e error) {
@@ -54,14 +54,28 @@ func parseDB(c *cli.Context) string {
 	return db
 }
 
+func parseFile(c *cli.Context) string {
+	file := c.String("file")
+	return file
+}
+
 func watch(c *cli.Context) {
 	r := setupRDS(c)
 	db := parseDB(c)
 	rate := parseRate(c)
-
+	file := parseFile(c)
 	stop := make(chan struct{})
 	go signalListen(stop)
 
+	if file != "" {
+		err := rdstail.WatchFile(r, db, rate, file, func(lines string) error {
+			fmt.Print(lines)
+			return nil
+		}, stop)
+
+		fie(err)
+	}
+
 	err := rdstail.Watch(r, db, rate, func(lines string) error {
 		fmt.Print(lines)
 		return nil
@@ -97,7 +111,12 @@ func papertrail(c *cli.Context) {
 func tail(c *cli.Context) {
 	r := setupRDS(c)
 	db := parseDB(c)
+	file := parseFile(c)
 	numLines := int64(c.Int("lines"))
+	if file != "" {
+		err := rdstail.TailFile(r, db, file, numLines)
+		fie(err)
+	}
 	err := rdstail.Tail(r, db, numLines)
 	fie(err)
 }
@@ -167,6 +186,10 @@ func main() {
 					Value: "3s",
 					Usage: "rds log polling rate",
 				},
+				cli.StringFlag{
+					Name:  "file, f",
+					Usage: "name of the logfile to watch",
+				},
 			},
 		},
 
@@ -180,6 +203,10 @@ func main() {
 					Value: 20,
 					Usage: "output the last n lines. use 0 for a full dump of the most recent file",
 				},
+				cli.StringFlag{
+					Name:  "file, f",
+					Usage: "name of the logfile to tail",
+				},
 			},
 		},
 	}

src/rdstail.go

@@ -126,6 +126,17 @@ func Tail(r *rds.RDS, db string, numLines int64) error {
 	return nil
 }
 
+func TailFile(r *rds.RDS, db string, file string, numLines int64) error {
+	logFile := &rds.DescribeDBLogFilesDetails{LogFileName: aws.String(file)}
+
+	tail, _, err := tailLogFile(r, db, *logFile.LogFileName, numLines, "")
+	if err != nil {
+		return err
+	}
+	fmt.Println(tail)
+	return nil
+}
+
 func Watch(r *rds.RDS, db string, rate time.Duration, callback func(string) error, stop <-chan struct{}) error {
 	// Periodically check for new log files (unless there is a way to detect the file is done being written to)
 	// Poll that log file, retaining the marker
@@ -179,8 +190,40 @@ func Watch(r *rds.RDS, db string, rate time.Duration, callback func(string) erro
 			return nil
 		}
 	}
+}
 
-	return nil
+func WatchFile(r *rds.RDS, db string, rate time.Duration, file string, callback func(string) error, stop <-chan struct{}) error {
+	logFile := &rds.DescribeDBLogFilesDetails{LogFileName: aws.String(file)}
+
+	// Get a marker for the end of the log file by requesting the most recent line
+	lines, marker, err := tailLogFile(r, db, *logFile.LogFileName, 1, "")
+	if err != nil {
+		return err
+	}
+
+	t := time.NewTicker(rate)
+	empty := 0
+	const checkLogfileRate = 4
+	for {
+		select {
+		case <-t.C:
+			lines, marker, err = tailLogFile(r, db, *logFile.LogFileName, 0, marker)
+			if err != nil {
+				return err
+			}
+
+			if lines == "" {
+				empty++
+			} else {
+				empty = 0
+				if err := callback(lines); err != nil {
+					return err
+				}
+			}
+		case <-stop:
+			return nil
+		}
+	}
 }
 
 func FeedPapertrail(r *rds.RDS, db string, rate time.Duration, papertrailHost, app, hostname string, stop <-chan struct{}) error {