2 files changed, 74 insertions, 9 deletions

diff --git a/db/db.go b/db/db.go
index c13f6d1..c99bbea 100644
--- a/db/db.go
+++ b/db/db.go
@@ -21,7 +21,8 @@ var defaultDBPath string
 //
 // It's stored on disk as a JSON at `$HOME/.config/fern/db.json
 type FernDB struct {
-	mutex *sync.Mutex // For writes to `downloaded`
+	// For locking concurrent read/write access downloaded.
+	mutex *sync.RWMutex
 	// Key: feed-id
 	// Value: feed-id's entries that were downloaded
 	downloaded map[string][]string
@@ -54,7 +55,7 @@ func Open() (*FernDB, error) {
 	if err != nil {
 		// db does not exist yet; create an empty one.
 		db := new(FernDB)
-		db.mutex = new(sync.Mutex)
+		db.mutex = new(sync.RWMutex)
 		db.downloaded = make(map[string][]string)
 		return db, nil
 	}
@@ -71,7 +72,7 @@ func Open() (*FernDB, error) {
 
 	// Unmarshal db into an object.
 	db := new(FernDB)
-	db.mutex = new(sync.Mutex)
+	db.mutex = new(sync.RWMutex)
 	err = json.Unmarshal(bs, &db.downloaded)
 	if err != nil {
 		return nil, err
@@ -79,9 +80,10 @@ func Open() (*FernDB, error) {
 	return db, nil
 }
 
-// Returns true if an `entry` for `feed` exists in the database; false
-// otherwise.
-func (fdb *FernDB) Exists(feed, entry string) bool {
+// Checks if entry exists in feed. Assumes the current go routine
+// already has the mutex lock. Meant for use by the Exists and Add
+// methods.
+func (fdb *FernDB) exists(feed, entry string) bool {
 	if _, ok := fdb.downloaded[feed]; !ok {
 		return false
 	}
@@ -91,7 +93,16 @@ func (fdb *FernDB) Exists(feed, entry string) bool {
 		}
 	}
 	return false
+}
+
+// Returns true if an `entry` for `feed` exists in the database; false
+// otherwise.
+func (fdb *FernDB) Exists(feed, entry string) bool {
+	// Acquire read lock.
+	fdb.mutex.RLock()
+	defer fdb.mutex.RUnlock() // Give up lock before returning.
 
+	return fdb.exists(feed, entry)
 }
 
 // Adds `feed` <-> `entry` to the database.
@@ -100,24 +111,31 @@ func (fdb *FernDB) Exists(feed, entry string) bool {
 // that entry was downloaded and will not try downloading the entry
 // again.
 func (fdb *FernDB) Add(feed, entry string) {
+	// Acquire write lock.
+	fdb.mutex.Lock()
+	defer fdb.mutex.Unlock() // Give up lock before returning.
+
 	// Check if entry already exist for feed.
-	if fdb.Exists(feed, entry) {
+	if fdb.exists(feed, entry) {
 		return
 	}
 
 	// Add entry.
-	fdb.mutex.Lock()
 	if _, ok := fdb.downloaded[feed]; !ok {
 		fdb.downloaded[feed] = make([]string, 0)
 	}
 	fdb.downloaded[feed] = append(fdb.downloaded[feed], entry)
-	fdb.mutex.Unlock()
+
 }
 
 // Writes FernDB to disk in the JSON format.
 //
 // Returns nil on success; error otherwise
 func (fdb *FernDB) Write() error {
+	// Acquire write lock.
+	fdb.mutex.Lock()
+	defer fdb.mutex.Unlock() // Give up lock before returning.
+
 	if len(dbPath) == 0 {
 		return fmt.Errorf("FernDB path not set")
 	}
diff --git a/db/db_test.go b/db/db_test.go
index de27149..966ce85 100644
--- a/db/db_test.go
+++ b/db/db_test.go
@@ -4,6 +4,7 @@
 package db
 
 import (
+	"fmt"
 	"os"
 	"path"
 	"testing"
@@ -370,3 +371,49 @@ func TestWriteExistingDB(t *testing.T) {
 		return
 	}
 }
+
+func TestConcurrentWrites(t *testing.T) {
+	dbPath = path.Join(os.TempDir(), "fern-db.json")
+	defer os.Remove(dbPath)
+	defer resetDBPath()
+
+	db, err := Open()
+	if err != nil {
+		t.Errorf("db open failed: %v", err)
+		return
+	}
+
+	// Randomly create a some entries.
+	numEntries := 1000
+	entries := make([]string, 0)
+	for i := 0; i < numEntries; i++ {
+		entries = append(entries, fmt.Sprintf("entry-%d", i))
+	}
+
+	// Go routine for adding entries to the db.
+	addEntries := func(db *FernDB, feed string, entries []string, donec chan int) {
+		for _, entry := range entries {
+			db.Add(feed, entry)
+		}
+		donec <- 1
+	}
+
+	// Concurrently write entries to a feed.
+	donec := make(chan int)
+	feed := "npr"
+	routines := 5
+	for i := 0; i < routines; i++ {
+		go addEntries(db, feed, entries, donec)
+	}
+	routinesDone := 0
+	for routinesDone != routines {
+		<-donec
+		routinesDone += 1
+	}
+
+	// Check if there are exactly numEntries entries.
+	if len(db.downloaded[feed]) != numEntries {
+		t.Errorf("downloaded entries != %d: %v",
+			numEntries, db.downloaded[feed])
+	}
+}