use sha256 hash as the _id for attachments

README.md

@@ -197,6 +197,7 @@ Shard the following collections by these keys:
 sh.enableSharding('wildduck');
 sh.shardCollection('wildduck.messages', { mailbox: 1, uid: 1 });
 sh.shardCollection('wildduck.threads', { user: 'hashed' });
+// attachment _id is a sha256 hash of attachment contents
 sh.shardCollection('wildduck.attachments.files', { _id: 'hashed' });
 sh.shardCollection('wildduck.attachments.chunks', { files_id: 'hashed' });
 ```

indexes.yaml

@@ -270,12 +270,6 @@ indexes:
     name: attachment_id_hashed
     key:
       _id: hashed
-- collection: attachments.files
-  type: gridfs # index applies to gridfs database
-  index:
-    name: attachment_hash
-    key:
-      metadata.h: hashed
 - collection: attachments.files
   type: gridfs # index applies to gridfs database
   index:

lib/attachments/gridstore-storage.js

@@ -1,6 +1,5 @@
 'use strict';
 
-const ObjectID = require('mongodb').ObjectID;
 const GridFSBucket = require('mongodb').GridFSBucket;
 
 class GridstoreStorage {
@@ -28,69 +27,85 @@ class GridstoreStorage {
                 transferEncoding: attachmentData.metadata.transferEncoding,
                 length: attachmentData.length,
                 count: attachmentData.metadata.c,
-                hash: attachmentData.metadata.h,
+                hash: attachmentData._id,
                 metadata: attachmentData.metadata
             });
         });
     }
 
     create(attachment, hash, callback) {
-        this.gridfs.collection(this.bucketName + '.files').findOneAndUpdate({
+        hash = Buffer.from(hash, 'hex');
-            'metadata.h': hash
+
-        }, {
+        let returned = false;
-            $inc: {
+        let retried = false;
-                'metadata.c': 1,
+
-                'metadata.m': attachment.magic
+        let id = hash;
+        let metadata = {
+            m: attachment.magic,
+            c: 1,
+            transferEncoding: attachment.transferEncoding
+        };
+
+        Object.keys(attachment.metadata || {}).forEach(key => {
+            if (!(key in attachment.metadata)) {
+                metadata[key] = attachment.metadata[key];
             }
-        }, {
-            returnOriginal: false
-        }, (err, result) => {
-            if (err) {
-                return callback(err);
-            }
-
-            if (result && result.value) {
-                return callback(null, result.value._id);
-            }
-
-            let returned = false;
-
-            let id = new ObjectID();
-            let metadata = {
-                h: hash,
-                m: attachment.magic,
-                c: 1,
-                transferEncoding: attachment.transferEncoding
-            };
-            Object.keys(attachment.metadata || {}).forEach(key => {
-                if (!(key in attachment.metadata)) {
-                    metadata[key] = attachment.metadata[key];
-                }
-            });
-
-            let store = this.gridstore.openUploadStreamWithId(id, null, {
-                contentType: attachment.contentType,
-                metadata
-            });
-
-            store.once('error', err => {
-                if (returned) {
-                    return;
-                }
-                returned = true;
-                callback(err);
-            });
-
-            store.once('finish', () => {
-                if (returned) {
-                    return;
-                }
-                returned = true;
-                return callback(null, id);
-            });
-
-            store.end(attachment.body);
         });
+
+        let tryStore = () => {
+            this.gridfs.collection(this.bucketName + '.files').findOneAndUpdate({
+                _id: hash
+            }, {
+                $inc: {
+                    'metadata.c': 1,
+                    'metadata.m': attachment.magic
+                }
+            }, {
+                returnOriginal: false
+            }, (err, result) => {
+                if (err) {
+                    return callback(err);
+                }
+
+                if (result && result.value) {
+                    // already exists
+                    return callback(null, result.value._id);
+                }
+
+                // try to insert it
+                let store = this.gridstore.openUploadStreamWithId(id, null, {
+                    contentType: attachment.contentType,
+                    metadata
+                });
+
+                store.once('error', err => {
+                    if (returned) {
+                        return;
+                    }
+                    if (err.code === 11000) {
+                        // most probably a race condition, try again
+                        if (!retried) {
+                            retried = true;
+                            return setTimeout(tryStore, 10);
+                        }
+                    }
+                    returned = true;
+                    callback(err);
+                });
+
+                store.once('finish', () => {
+                    if (returned) {
+                        return;
+                    }
+                    returned = true;
+                    return callback(null, id);
+                });
+
+                store.end(attachment.body);
+            });
+        };
+
+        tryStore();
     }
 
     createReadStream(id) {