worker/imap: use the IMAP connection from a single goroutine

Unfortunately, the IMAP protocol hasn't been designed to be used from multiple
goroutines at the same time. For instance, if you fetch twice the same message
from two different goroutines, it's not possible to tell whether the response
is for one receiver or the other. For this reason, go-imap clients aren't safe
to use from multiple goroutines.

This commit changes the IMAP workers to be synchronous again (a command is
executed only after the previous one has completed). To use IMAP from different
threads, popular clients (e.g. Thunderbird) typically open multiple
connections.

3 files changed, 77 insertions, 86 deletions

diff --git a/worker/imap/fetch.go b/worker/imap/fetch.go
index af9d3b1..a799f2a 100644
--- a/worker/imap/fetch.go
+++ b/worker/imap/fetch.go
@@ -46,50 +46,46 @@ func (imapw *IMAPWorker) handleFetchMessages(
 	msg types.WorkerMessage, uids *imap.SeqSet, items []imap.FetchItem,
 	section *imap.BodySectionName) {
 
+	messages := make(chan *imap.Message)
+
 	go func() {
-		messages := make(chan *imap.Message)
-		done := make(chan error, 1)
-		go func() {
-			done <- imapw.client.UidFetch(uids, items, messages)
-		}()
-		go func() {
-			for _msg := range messages {
-				imapw.seqMap[_msg.SeqNum-1] = _msg.Uid
-				switch msg.(type) {
-				case *types.FetchMessageHeaders:
-					imapw.worker.PostMessage(&types.MessageInfo{
-						Message:       types.RespondTo(msg),
-						BodyStructure: _msg.BodyStructure,
-						Envelope:      _msg.Envelope,
-						Flags:         _msg.Flags,
-						InternalDate:  _msg.InternalDate,
-						Uid:           _msg.Uid,
-					}, nil)
-				case *types.FetchFullMessages:
-					reader := _msg.GetBody(section)
-					imapw.worker.PostMessage(&types.FullMessage{
-						Message: types.RespondTo(msg),
-						Reader:  reader,
-						Uid:     _msg.Uid,
-					}, nil)
-				case *types.FetchMessageBodyPart:
-					reader := _msg.GetBody(section)
-					imapw.worker.PostMessage(&types.MessageBodyPart{
-						Message: types.RespondTo(msg),
-						Reader:  reader,
-						Uid:     _msg.Uid,
-					}, nil)
-				}
-			}
-			if err := <-done; err != nil {
-				imapw.worker.PostMessage(&types.Error{
+		for _msg := range messages {
+			imapw.seqMap[_msg.SeqNum-1] = _msg.Uid
+			switch msg.(type) {
+			case *types.FetchMessageHeaders:
+				imapw.worker.PostMessage(&types.MessageInfo{
+					Message:       types.RespondTo(msg),
+					BodyStructure: _msg.BodyStructure,
+					Envelope:      _msg.Envelope,
+					Flags:         _msg.Flags,
+					InternalDate:  _msg.InternalDate,
+					Uid:           _msg.Uid,
+				}, nil)
+			case *types.FetchFullMessages:
+				reader := _msg.GetBody(section)
+				imapw.worker.PostMessage(&types.FullMessage{
 					Message: types.RespondTo(msg),
-					Error:   err,
+					Reader:  reader,
+					Uid:     _msg.Uid,
+				}, nil)
+			case *types.FetchMessageBodyPart:
+				reader := _msg.GetBody(section)
+				imapw.worker.PostMessage(&types.MessageBodyPart{
+					Message: types.RespondTo(msg),
+					Reader:  reader,
+					Uid:     _msg.Uid,
 				}, nil)
-			} else {
-				imapw.worker.PostMessage(
-					&types.Done{types.RespondTo(msg)}, nil)
 			}
-		}()
+		}
 	}()
+
+	if err := imapw.client.UidFetch(uids, items, messages); err != nil {
+		imapw.worker.PostMessage(&types.Error{
+			Message: types.RespondTo(msg),
+			Error:   err,
+		}, nil)
+	} else {
+		imapw.worker.PostMessage(
+			&types.Done{types.RespondTo(msg)}, nil)
+	}
 }
diff --git a/worker/imap/list.go b/worker/imap/list.go
index eff3d49..22addc3 100644
--- a/worker/imap/list.go
+++ b/worker/imap/list.go
@@ -8,11 +8,8 @@ import (
 
 func (imapw *IMAPWorker) handleListDirectories(msg *types.ListDirectories) {
 	mailboxes := make(chan *imap.MailboxInfo)
-	done := make(chan error, 1)
 	imapw.worker.Logger.Println("Listing mailboxes")
-	go func() {
-		done <- imapw.client.List("", "*", mailboxes)
-	}()
+
 	go func() {
 		for mbox := range mailboxes {
 			imapw.worker.PostMessage(&types.Directory{
@@ -21,14 +18,15 @@ func (imapw *IMAPWorker) handleListDirectories(msg *types.ListDirectories) {
 				Attributes: mbox.Attributes,
 			}, nil)
 		}
-		if err := <-done; err != nil {
-			imapw.worker.PostMessage(&types.Error{
-				Message: types.RespondTo(msg),
-				Error:   err,
-			}, nil)
-		} else {
-			imapw.worker.PostMessage(
-				&types.Done{types.RespondTo(msg)}, nil)
-		}
 	}()
+
+	if err := imapw.client.List("", "*", mailboxes); err != nil {
+		imapw.worker.PostMessage(&types.Error{
+			Message: types.RespondTo(msg),
+			Error:   err,
+		}, nil)
+	} else {
+		imapw.worker.PostMessage(
+			&types.Done{types.RespondTo(msg)}, nil)
+	}
 }
diff --git a/worker/imap/open.go b/worker/imap/open.go
index 3705bc0..dc5d6d1 100644
--- a/worker/imap/open.go
+++ b/worker/imap/open.go
@@ -8,17 +8,16 @@ import (
 
 func (imapw *IMAPWorker) handleOpenDirectory(msg *types.OpenDirectory) {
 	imapw.worker.Logger.Printf("Opening %s", msg.Directory)
-	go func() {
-		_, err := imapw.client.Select(msg.Directory, false)
-		if err != nil {
-			imapw.worker.PostMessage(&types.Error{
-				Message: types.RespondTo(msg),
-				Error:   err,
-			}, nil)
-		} else {
-			imapw.worker.PostMessage(&types.Done{types.RespondTo(msg)}, nil)
-		}
-	}()
+
+	_, err := imapw.client.Select(msg.Directory, false)
+	if err != nil {
+		imapw.worker.PostMessage(&types.Error{
+			Message: types.RespondTo(msg),
+			Error:   err,
+		}, nil)
+	} else {
+		imapw.worker.PostMessage(&types.Done{types.RespondTo(msg)}, nil)
+	}
 }
 
 func (imapw *IMAPWorker) handleFetchDirectoryContents(
@@ -26,25 +25,23 @@ func (imapw *IMAPWorker) handleFetchDirectoryContents(
 
 	imapw.worker.Logger.Printf("Fetching UID list")
 
-	go func() {
-		seqSet := &imap.SeqSet{}
-		seqSet.AddRange(1, imapw.selected.Messages)
-		uids, err := imapw.client.UidSearch(&imap.SearchCriteria{
-			SeqNum: seqSet,
-		})
-		if err != nil {
-			imapw.worker.PostMessage(&types.Error{
-				Message: types.RespondTo(msg),
-				Error:   err,
-			}, nil)
-		} else {
-			imapw.worker.Logger.Printf("Found %d UIDs", len(uids))
-			imapw.seqMap = make([]uint32, len(uids))
-			imapw.worker.PostMessage(&types.DirectoryContents{
-				Message: types.RespondTo(msg),
-				Uids:    uids,
-			}, nil)
-			imapw.worker.PostMessage(&types.Done{types.RespondTo(msg)}, nil)
-		}
-	}()
+	seqSet := &imap.SeqSet{}
+	seqSet.AddRange(1, imapw.selected.Messages)
+	uids, err := imapw.client.UidSearch(&imap.SearchCriteria{
+		SeqNum: seqSet,
+	})
+	if err != nil {
+		imapw.worker.PostMessage(&types.Error{
+			Message: types.RespondTo(msg),
+			Error:   err,
+		}, nil)
+	} else {
+		imapw.worker.Logger.Printf("Found %d UIDs", len(uids))
+		imapw.seqMap = make([]uint32, len(uids))
+		imapw.worker.PostMessage(&types.DirectoryContents{
+			Message: types.RespondTo(msg),
+			Uids:    uids,
+		}, nil)
+		imapw.worker.PostMessage(&types.Done{types.RespondTo(msg)}, nil)
+	}
 }