logmon: Refactor fifo access for windows safety

On unix platforms, it is safe to re-open fifo's for reading after the
first creation if the file is already a fifo, however this is not
possible on windows where this triggers a permissions error on the
socket path, as you cannot recreate it.

We can't transparently handle this in the CreateAndRead handle, because
the Access Is Denied error is too generic to reliably be an IO error.
Instead, we add an explict API for opening a reader to an existing FIFO,
and check to see if the fifo already exists inside the calling package
(e.g logmon)

client/lib/fifo/fifo_unix.go

@@ -21,11 +21,13 @@ func CreateAndRead(path string) (func() (io.ReadCloser, error), error) {
 		return nil, fmt.Errorf("error creating fifo %v: %v", path, err)
 	}
 
-	openFn := func() (io.ReadCloser, error) {
+	return func() (io.ReadCloser, error) {
 		return os.OpenFile(path, unix.O_RDONLY, os.ModeNamedPipe)
-	}
+	}, nil
+}
 
-	return openFn, nil
+func OpenReader(path string) (io.ReadCloser, error) {
+	return os.OpenFile(path, unix.O_RDONLY, os.ModeNamedPipe)
 }
 
 // OpenWriter opens a fifo file for writer, assuming it already exists, returns io.WriteCloser

client/lib/fifo/fifo_windows.go

@@ -78,13 +78,20 @@ func CreateAndRead(path string) (func() (io.ReadCloser, error), error) {
 		return nil, err
 	}
 
-	openFn := func() (io.ReadCloser, error) {
+	return func() (io.ReadCloser, error) {
 		return &winFIFO{
 			listener: l,
 		}, nil
+	}, nil
+}
+
+func OpenReader(path string) (io.ReadCloser, error) {
+	conn, err := winio.DialPipe(path, nil)
+	if err != nil {
+		return nil, err
 	}
 
-	return openFn, nil
+	return &winFIFO{conn: conn}, nil
 }
 
 // OpenWriter opens a fifo that already exists and returns an io.WriteCloser for it

client/logmon/logmon.go

@@ -3,6 +3,7 @@ package logmon
 import (
 	"fmt"
 	"io"
+	"os"
 	"strings"
 	"sync"
 	"time"
@@ -199,7 +200,18 @@ func (l *logRotatorWrapper) isRunning() bool {
 // processOutWriter to attach to the stdout or stderr of a process.
 func newLogRotatorWrapper(path string, logger hclog.Logger, rotator *logging.FileRotator) (*logRotatorWrapper, error) {
 	logger.Info("opening fifo", "path", path)
-	fifoOpenFn, err := fifo.CreateAndRead(path)
+
+	var openFn func() (io.ReadCloser, error)
+	var err error
+
+	if _, ferr := os.Stat(path); os.IsNotExist(ferr) {
+		openFn, err = fifo.CreateAndRead(path)
+	} else {
+		openFn = func() (io.ReadCloser, error) {
+			return fifo.OpenReader(path)
+		}
+	}
+
 	if err != nil {
 		return nil, fmt.Errorf("failed to create fifo for extracting logs: %v", err)
 	}
@@ -211,20 +223,20 @@ func newLogRotatorWrapper(path string, logger hclog.Logger, rotator *logging.Fil
 		openCompleted:     make(chan struct{}),
 		logger:            logger,
 	}
-	wrap.start(fifoOpenFn)
+
+	wrap.start(openFn)
 	return wrap, nil
 }
 
 // start starts a goroutine that copies from the pipe into the rotator. This is
 // called by the constructor and not the user of the wrapper.
-func (l *logRotatorWrapper) start(readerOpenFn func() (io.ReadCloser, error)) {
+func (l *logRotatorWrapper) start(openFn func() (io.ReadCloser, error)) {
 	go func() {
 		defer close(l.hasFinishedCopied)
 
-		reader, err := readerOpenFn()
+		reader, err := openFn()
 		if err != nil {
-			close(l.openCompleted)
-			l.logger.Warn("failed to open log fifo", "error", err)
+			l.logger.Warn("failed to open fifo", "error", err)
 			return
 		}
 		l.processOutReader = reader
@@ -284,5 +296,4 @@ func (l *logRotatorWrapper) Close() {
 	}
 
 	l.rotatorWriter.Close()
-	return
 }