imap: add option to cache headers

Add option to cache headers for imap accounts. Cache db is located at
$XDG_CACHE_DIR/aerc/{account name}. The cache is cleaned of stale
entries when aerc is first opened.

Two new account level configuration options are introduced:
* cache-headers (Default: false)
* cache-max-age (Default: 30 days (720 hours))

The change in worker/imap/open.go is to set the selected directory. This
is required to access the UIDVALIDITY field, which is used in
combination with the message ID to form the key for use in the cache db.
The key structure is: "header.{UIDVALIDITY}.{UID}"

Where reasonable, cache does not stop aerc from running. In general, if
there is an error in the cache, aerc should continue working as usual.
Errors are either displayed to the user or logged.

All messages are stored without flags, and when retrieved have the flags
set to SEEN. This is to prevent UI flashes. A new method to
FetchMessageFlags is introduced to update flags of cached headers. This
is done asynchronously, and the user will see their messages appear and
then any flags updated. The message will initially show as SEEN, but
will update to unread. I considered updating the cache with the
last-known flag state, however it seems prudent to spare the R/W cycle
and assume that - eventually - all messages will end up read, and if it
isn't the update will occur rather quickly.

Note that leveldb puts a lock on the database, preventing multiple
instances of aerc from accessing the cache at the same time.

Much of this work is based on previous efforts by Vladimír Magyar.

Implements: https://todo.sr.ht/~rjarry/aerc/2
Thanks: Vladimír Magyar <vladimir@mgyar.me>
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Tested-by: inwit <inwit@sindominio.net>
Reviewed-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

9 files changed, 279 insertions, 15 deletions

diff --git a/doc/aerc-imap.5.scd b/doc/aerc-imap.5.scd
index 99640b6..25833a9 100644
--- a/doc/aerc-imap.5.scd
+++ b/doc/aerc-imap.5.scd
@@ -111,6 +111,19 @@ available:
 
 	Default: no folders
 
+*cache-headers*
+	If set to true, headers will be cached. The cached headers will be stored
+	in $XDG_CACHE_HOME/aerc, which defaults to ~/.cache/aerc.
+
+	Default: false
+
+*cache-max-age*
+	Defines the maximum age of cached files. Note: the longest unit of time
+	cache-max-age can be specified in is hours. Set to 0 to disable cleaning
+	the cache
+
+	Default: 720h (30 days)
+
 # SEE ALSO
 
 *aerc*(1) *aerc-config*(5)
diff --git a/go.mod b/go.mod
index a3b32be..0affccb 100644
--- a/go.mod
+++ b/go.mod
@@ -5,7 +5,7 @@ go 1.13
 require (
 	git.sr.ht/~sircmpwn/getopt v1.0.0
 	github.com/ProtonMail/go-crypto v0.0.0-20211221144345-a4f6767435ab
-	github.com/arran4/golang-ical v0.0.0-20220517104411-fd89fefb0182 // indirect
+	github.com/arran4/golang-ical v0.0.0-20220517104411-fd89fefb0182
 	github.com/creack/pty v1.1.17
 	github.com/danwakefield/fnmatch v0.0.0-20160403171240-cbb64ac3d964
 	github.com/ddevault/go-libvterm v0.0.0-20190526194226-b7d861da3810
@@ -18,7 +18,7 @@ require (
 	github.com/emersion/go-sasl v0.0.0-20211008083017-0b9dcfb154ac
 	github.com/emersion/go-smtp v0.15.0
 	github.com/fsnotify/fsnotify v1.5.1
-	github.com/gatherstars-com/jwz v1.3.0 // indirect
+	github.com/gatherstars-com/jwz v1.3.0
 	github.com/gdamore/tcell/v2 v2.4.0
 	github.com/go-ini/ini v1.63.2
 	github.com/golang/protobuf v1.5.2 // indirect
@@ -35,7 +35,8 @@ require (
 	github.com/pkg/errors v0.9.1
 	github.com/riywo/loginshell v0.0.0-20200815045211-7d26008be1ab
 	github.com/stretchr/testify v1.7.1
-	github.com/xo/terminfo v0.0.0-20210125001918-ca9a967f8778 // indirect
+	github.com/syndtr/goleveldb v1.0.0
+	github.com/xo/terminfo v0.0.0-20210125001918-ca9a967f8778
 	github.com/zenhack/go.notmuch v0.0.0-20211022191430-4d57e8ad2a8b
 	golang.org/x/crypto v0.0.0-20211215153901-e495a2d5b3d3 // indirect
 	golang.org/x/net v0.0.0-20211029224645-99673261e6eb // indirect
diff --git a/go.sum b/go.sum
index 4539610..e6e93f2 100644
--- a/go.sum
+++ b/go.sum
@@ -91,6 +91,7 @@ github.com/envoyproxy/go-control-plane v0.9.0/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymF
 github.com/envoyproxy/go-control-plane v0.9.1-0.20191026205805-5f8ba28d4473/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=
 github.com/envoyproxy/go-control-plane v0.9.4/go.mod h1:6rpuAdCZL397s3pYoYcLgu1mIlRU8Am5FuJP05cCM98=
 github.com/envoyproxy/protoc-gen-validate v0.1.0/go.mod h1:iSmxcyjqTsJpI2R4NaDN7+kN2VEUnK/pcBlmesArF7c=
+github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMoQvtojpjFo=
 github.com/fsnotify/fsnotify v1.5.1 h1:mZcQUHVQUQWoPXXtuf9yuEXKudkV2sx1E06UadKWpgI=
 github.com/fsnotify/fsnotify v1.5.1/go.mod h1:T3375wBYaZdLLcVNkcVbzGHY7f1l/uK5T5Ai1i3InKU=
 github.com/gatherstars-com/jwz v1.3.0 h1:+lVRjWDsLupLL3tJneimJ7VRBCZ6x59R2OW9zB8Wvb4=
@@ -134,6 +135,8 @@ github.com/golang/protobuf v1.4.2/go.mod h1:oDoupMAO8OvCJWAcko0GGGIgR6R6ocIYbsSw
 github.com/golang/protobuf v1.5.0/go.mod h1:FsONVRAS9T7sI+LIUmWTfcYkHO4aIWwzhcaSAoJOfIk=
 github.com/golang/protobuf v1.5.2 h1:ROPKBNFfQgOUMifHyP+KYbvpjbdoFNs+aK7DXlji0Tw=
 github.com/golang/protobuf v1.5.2/go.mod h1:XVQd3VNwM+JqD3oG2Ue2ip4fOMUkwXdXDdiuN0vRsmY=
+github.com/golang/snappy v0.0.0-20180518054509-2e65f85255db h1:woRePGFeVFfLKN/pOkfl+p/TAqKOfFu+7KPlMVpok/w=
+github.com/golang/snappy v0.0.0-20180518054509-2e65f85255db/go.mod h1:/XxbfmMg8lxefKM7IXC3fBNl/7bRcc72aCRzEWrmP2Q=
 github.com/google/btree v0.0.0-20180813153112-4030bb1f1f0c/go.mod h1:lNA+9X1NB3Zf8V7Ke586lFgjr2dZNuvo3lPJSGZ5JPQ=
 github.com/google/btree v1.0.0/go.mod h1:lNA+9X1NB3Zf8V7Ke586lFgjr2dZNuvo3lPJSGZ5JPQ=
 github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
@@ -161,6 +164,7 @@ github.com/googleapis/gax-go/v2 v2.0.4/go.mod h1:0Wqv26UfaUD9n4G6kQubkQ+KchISgw+
 github.com/googleapis/gax-go/v2 v2.0.5/go.mod h1:DWXyrwAJ9X0FpwwEdw+IPEYBICEFu5mhpdKc/us6bOk=
 github.com/hashicorp/golang-lru v0.5.0/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
 github.com/hashicorp/golang-lru v0.5.1/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
+github.com/hpcloud/tail v1.0.0/go.mod h1:ab1qPbhIpdTxEkNHXyeSf5vhxWSCs/tWer42PpOxQnU=
 github.com/ianlancetaylor/demangle v0.0.0-20181102032728-5e5cf60278f6/go.mod h1:aSSvb/t6k1mPoxDqO4vJh6VOCGPwU4O0C2/Eqndh1Sc=
 github.com/imdario/mergo v0.3.12 h1:b6R2BslTbIEToALKP7LxUvijTsNI9TAe80pLWN2g/HU=
 github.com/imdario/mergo v0.3.12/go.mod h1:jmQim1M+e3UYxmgPu/WyfjB3N3VflVyUjjjwH0dnCYA=
@@ -200,6 +204,9 @@ github.com/mitchellh/go-homedir v1.1.0 h1:lukF9ziXFxDFPkA1vsr5zpc1XuPDn/wFntq5mG
 github.com/mitchellh/go-homedir v1.1.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
 github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e/go.mod h1:zD1mROLANZcx1PVRCS0qkT7pwLkGfwJo4zjcN/Tysno=
 github.com/olekukonko/tablewriter v0.0.5/go.mod h1:hPp6KlRPjbx+hW8ykQs1w3UBbZlj6HuIJcUGPhkA7kY=
+github.com/onsi/ginkgo v1.6.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
+github.com/onsi/ginkgo v1.7.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
+github.com/onsi/gomega v1.4.3/go.mod h1:ex+gbHU/CVuBBDIJjb2X0qEXbFg53c61hWP/1CpauHY=
 github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
 github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
@@ -220,6 +227,8 @@ github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81P
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/stretchr/testify v1.7.1 h1:5TQK59W5E3v0r2duFAb7P95B6hEeOyEnHRa8MjYSMTY=
 github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/syndtr/goleveldb v1.0.0 h1:fBdIW9lB4Iz0n9khmH8w27SJ3QEJ7+IgjPEwGSZiFdE=
+github.com/syndtr/goleveldb v1.0.0/go.mod h1:ZVVdQEZoIme9iO1Ch2Jdy24qqXrMMOU6lpPAyBWyWuQ=
 github.com/xo/terminfo v0.0.0-20210125001918-ca9a967f8778 h1:QldyIu/L63oPpyvQmHgvgickp1Yw510KJOqX7H24mg8=
 github.com/xo/terminfo v0.0.0-20210125001918-ca9a967f8778/go.mod h1:2MuV+tbUrU1zIOPMxZ5EncGwgmMJsa+9ucAQZXxsObs=
 github.com/yuin/goldmark v1.1.25/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
@@ -263,6 +272,7 @@ golang.org/x/mod v0.2.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.3.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/net v0.0.0-20180724234803-3673e40ba225/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20180826012351-8a410e7b638d/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
+golang.org/x/net v0.0.0-20180906233101-161cd47e91fd/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20190108225652-1e06a53dbb7e/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20190213061140-3a22650c66bd/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
@@ -306,6 +316,7 @@ golang.org/x/sync v0.0.0-20190911185100-cd5d95a43a6e/go.mod h1:RxMgew5VJxzue5/jJ
 golang.org/x/sync v0.0.0-20200317015054-43a5402ce75a/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20180909124046-d0be0721c37e/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190312061237-fead79001313/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
@@ -484,6 +495,9 @@ gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20200227125254-8fa46927fb4f/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=
+gopkg.in/fsnotify.v1 v1.4.7/go.mod h1:Tz8NjZHkW78fSQdbUxIjBTcgA1z1m8ZHf0WmKUhAMys=
+gopkg.in/tomb.v1 v1.0.0-20141024135613-dd632973f1e7/go.mod h1:dt/ZhP58zS4L8KSrWDmTeBkI65Dw0HsyUHuEVlX15mw=
+gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.3.0 h1:clyUAQHOM3G0M3f5vQj7LuJrETvjVot3Z5el9nffUtU=
 gopkg.in/yaml.v2 v2.3.0/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
diff --git a/worker/imap/cache.go b/worker/imap/cache.go
new file mode 100644
index 0000000..ecbedd8
--- /dev/null
+++ b/worker/imap/cache.go
@@ -0,0 +1,174 @@
+package imap
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/gob"
+	"fmt"
+	"os"
+	"path"
+	"time"
+
+	"git.sr.ht/~rjarry/aerc/models"
+	"git.sr.ht/~rjarry/aerc/worker/types"
+	"github.com/emersion/go-message"
+	"github.com/emersion/go-message/mail"
+	"github.com/emersion/go-message/textproto"
+	"github.com/mitchellh/go-homedir"
+	"github.com/syndtr/goleveldb/leveldb"
+)
+
+type CachedHeader struct {
+	BodyStructure models.BodyStructure
+	Envelope      models.Envelope
+	InternalDate  time.Time
+	Uid           uint32
+	Header        []byte
+	Created       time.Time
+}
+
+// initCacheDb opens (or creates) the database for the cache. One database is
+// created per account
+func (w *IMAPWorker) initCacheDb(acct string) {
+	cd, err := cacheDir()
+	if err != nil {
+		w.cache = nil
+		w.worker.Logger.Panicf("cache: unable to find cache directory: %v", err)
+		return
+	}
+	p := path.Join(cd, acct)
+	db, err := leveldb.OpenFile(p, nil)
+	if err != nil {
+		w.cache = nil
+		w.worker.Logger.Printf("cache: error opening cache db: %v", err)
+		return
+	}
+	w.cache = db
+	w.worker.Logger.Printf("cache: cache db opened: %s", p)
+	if w.config.cacheMaxAge.Hours() > 0 {
+		go w.cleanCache()
+	}
+}
+
+func (w *IMAPWorker) cacheHeader(mi *models.MessageInfo) {
+	uv := fmt.Sprintf("%d", w.selected.UidValidity)
+	uid := fmt.Sprintf("%d", mi.Uid)
+	w.worker.Logger.Printf("cache: caching header for message %s.%s", uv, uid)
+	hdr := bytes.NewBuffer(nil)
+	err := textproto.WriteHeader(hdr, mi.RFC822Headers.Header.Header)
+	if err != nil {
+		w.worker.Logger.Printf("cache: error writing header %s.%s: %v", uv, uid, err)
+		return
+	}
+	h := &CachedHeader{
+		BodyStructure: *mi.BodyStructure,
+		Envelope:      *mi.Envelope,
+		InternalDate:  mi.InternalDate,
+		Uid:           mi.Uid,
+		Header:        hdr.Bytes(),
+		Created:       time.Now(),
+	}
+	data := bytes.NewBuffer(nil)
+	enc := gob.NewEncoder(data)
+	err = enc.Encode(h)
+	if err != nil {
+		w.worker.Logger.Printf("cache: error encoding message %s.%s: %v", uv, uid, err)
+		return
+	}
+	err = w.cache.Put([]byte("header."+uv+"."+uid), data.Bytes(), nil)
+	if err != nil {
+		w.worker.Logger.Printf("cache: error writing header to database for message %s.%s: %v", uv, uid, err)
+		return
+	}
+}
+
+func (w *IMAPWorker) getCachedHeaders(msg *types.FetchMessageHeaders) []uint32 {
+	w.worker.Logger.Println("Retrieving headers from cache")
+	var need, found []uint32
+	uv := fmt.Sprintf("%d", w.selected.UidValidity)
+	for _, uid := range msg.Uids {
+		u := fmt.Sprintf("%d", uid)
+		data, err := w.cache.Get([]byte("header."+uv+"."+u), nil)
+		if err != nil {
+			need = append(need, uid)
+			continue
+		}
+		ch := &CachedHeader{}
+		dec := gob.NewDecoder(bytes.NewReader(data))
+		err = dec.Decode(ch)
+		if err != nil {
+			w.worker.Logger.Printf("cache: error decoding cached header %s.%s: %v", uv, u, err)
+			need = append(need, uid)
+			continue
+		}
+		hr := bytes.NewReader(ch.Header)
+		textprotoHeader, err := textproto.ReadHeader(bufio.NewReader(hr))
+		if err != nil {
+			w.worker.Logger.Printf("cache: error reading cached header %s.%s: %v", uv, u, err)
+			need = append(need, uid)
+			continue
+		}
+
+		hdr := &mail.Header{Header: message.Header{Header: textprotoHeader}}
+		mi := &models.MessageInfo{
+			BodyStructure: &ch.BodyStructure,
+			Envelope:      &ch.Envelope,
+			Flags:         []models.Flag{models.SeenFlag}, // Always return a SEEN flag
+			Uid:           ch.Uid,
+			RFC822Headers: hdr,
+		}
+		found = append(found, uid)
+		w.worker.Logger.Printf("cache: located cached header %s.%s", uv, u)
+		w.worker.PostMessage(&types.MessageInfo{
+			Message: types.RespondTo(msg),
+			Info:    mi,
+		}, nil)
+	}
+	if len(found) > 0 {
+		w.worker.PostAction(&types.FetchMessageFlags{
+			Uids: found,
+		}, nil)
+	}
+	return need
+}
+
+func cacheDir() (string, error) {
+	dir, err := os.UserCacheDir()
+	if err != nil {
+		dir, err = homedir.Expand("~/.cache")
+		if err != nil {
+			return "", err
+		}
+	}
+	return path.Join(dir, "aerc"), nil
+}
+
+// cleanCache removes stale entries from the selected mailbox cachedb
+func (w *IMAPWorker) cleanCache() {
+	start := time.Now()
+	var scanned, removed int
+	iter := w.cache.NewIterator(nil, nil)
+	for iter.Next() {
+		data := iter.Value()
+		ch := &CachedHeader{}
+		dec := gob.NewDecoder(bytes.NewReader(data))
+		err := dec.Decode(ch)
+		if err != nil {
+			w.worker.Logger.Printf("cache: error cleaning database %d: %v", w.selected.UidValidity, err)
+			continue
+		}
+		exp := ch.Created.Add(w.config.cacheMaxAge)
+		if exp.Before(time.Now()) {
+			err = w.cache.Delete(iter.Key(), nil)
+			if err != nil {
+				w.worker.Logger.Printf("cache: error cleaning database %d: %v", w.selected.UidValidity, err)
+				continue
+			}
+			removed = removed + 1
+		}
+		scanned = scanned + 1
+	}
+	iter.Release()
+	elapsed := time.Since(start)
+	w.worker.Logger.Printf("cache: cleaned cache, removed %d of %d entries in %f seconds", removed, scanned, elapsed.Seconds())
+}
diff --git a/worker/imap/configure.go b/worker/imap/configure.go
index ddaaf93..f515188 100644
--- a/worker/imap/configure.go
+++ b/worker/imap/configure.go
@@ -56,6 +56,9 @@ func (w *IMAPWorker) handleConfigure(msg *types.Configure) error {
 
 	w.config.reconnect_maxwait = 30 * time.Second
 
+	w.config.cacheEnabled = false
+	w.config.cacheMaxAge = 30 * 24 * time.Hour // 30 days
+
 	for key, value := range msg.Config.Params {
 		switch key {
 		case "idle-timeout":
@@ -114,9 +117,26 @@ func (w *IMAPWorker) handleConfigure(msg *types.Configure) error {
 					value, err)
 			}
 			w.config.keepalive_interval = int(val.Seconds())
+		case "cache-headers":
+			cache, err := strconv.ParseBool(value)
+			if err != nil {
+				// Return an error here because the user tried to set header
+				// caching, and we want them to know they didn't set it right -
+				// one way or the other
+				return fmt.Errorf("invalid cache-headers value %v: %v", value, err)
+			}
+			w.config.cacheEnabled = cache
+		case "cache-max-age":
+			val, err := time.ParseDuration(value)
+			if err != nil || val < 0 {
+				return fmt.Errorf("invalid cache-max-age value %v: %v", value, err)
+			}
+			w.config.cacheMaxAge = val
 		}
 	}
-
+	if w.config.cacheEnabled {
+		w.initCacheDb(msg.Config.Name)
+	}
 	w.idler = newIdler(w.config, w.worker)
 	w.observer = newObserver(w.config, w.worker)
 
diff --git a/worker/imap/fetch.go b/worker/imap/fetch.go
index 765039b..e8a8251 100644
--- a/worker/imap/fetch.go
+++ b/worker/imap/fetch.go
@@ -17,7 +17,15 @@ import (
 
 func (imapw *IMAPWorker) handleFetchMessageHeaders(
 	msg *types.FetchMessageHeaders) {
-
+	toFetch := msg.Uids
+	if imapw.config.cacheEnabled && imapw.cache != nil {
+		toFetch = imapw.getCachedHeaders(msg)
+	}
+	if len(toFetch) == 0 {
+		imapw.worker.PostMessage(&types.Done{Message: types.RespondTo(msg)},
+			nil)
+		return
+	}
 	imapw.worker.Logger.Printf("Fetching message headers")
 	section := &imap.BodySectionName{
 		BodyPartName: imap.BodyPartName{
@@ -34,7 +42,7 @@ func (imapw *IMAPWorker) handleFetchMessageHeaders(
 		imap.FetchUid,
 		section.FetchItem(),
 	}
-	imapw.handleFetchMessages(msg, msg.Uids, items,
+	imapw.handleFetchMessages(msg, toFetch, items,
 		func(_msg *imap.Message) error {
 			reader := _msg.GetBody(section)
 			textprotoHeader, err := textproto.ReadHeader(bufio.NewReader(reader))
@@ -48,17 +56,21 @@ func (imapw *IMAPWorker) handleFetchMessageHeaders(
 				return nil
 			}
 			header := &mail.Header{Header: message.Header{Header: textprotoHeader}}
+			info := &models.MessageInfo{
+				BodyStructure: translateBodyStructure(_msg.BodyStructure),
+				Envelope:      translateEnvelope(_msg.Envelope),
+				Flags:         translateImapFlags(_msg.Flags),
+				InternalDate:  _msg.InternalDate,
+				RFC822Headers: header,
+				Uid:           _msg.Uid,
+			}
 			imapw.worker.PostMessage(&types.MessageInfo{
 				Message: types.RespondTo(msg),
-				Info: &models.MessageInfo{
-					BodyStructure: translateBodyStructure(_msg.BodyStructure),
-					Envelope:      translateEnvelope(_msg.Envelope),
-					Flags:         translateImapFlags(_msg.Flags),
-					InternalDate:  _msg.InternalDate,
-					RFC822Headers: header,
-					Uid:           _msg.Uid,
-				},
+				Info:    info,
 			}, nil)
+			if imapw.config.cacheEnabled && imapw.cache != nil {
+				imapw.cacheHeader(info)
+			}
 			return nil
 		})
 }
@@ -169,6 +181,24 @@ func (imapw *IMAPWorker) handleFetchFullMessages(
 		})
 }
 
+func (imapw *IMAPWorker) handleFetchMessageFlags(msg *types.FetchMessageFlags) {
+	items := []imap.FetchItem{
+		imap.FetchFlags,
+		imap.FetchUid,
+	}
+	imapw.handleFetchMessages(msg, msg.Uids, items,
+		func(_msg *imap.Message) error {
+			imapw.worker.PostMessage(&types.MessageInfo{
+				Message: types.RespondTo(msg),
+				Info: &models.MessageInfo{
+					Flags: translateImapFlags(_msg.Flags),
+					Uid:   _msg.Uid,
+				},
+			}, nil)
+			return nil
+		})
+}
+
 func (imapw *IMAPWorker) handleFetchMessages(
 	msg types.WorkerMessage, uids []uint32, items []imap.FetchItem,
 	procFunc func(*imap.Message) error) {
diff --git a/worker/imap/open.go b/worker/imap/open.go
index 238f1e2..65060fe 100644
--- a/worker/imap/open.go
+++ b/worker/imap/open.go
@@ -12,13 +12,14 @@ import (
 func (imapw *IMAPWorker) handleOpenDirectory(msg *types.OpenDirectory) {
 	imapw.worker.Logger.Printf("Opening %s", msg.Directory)
 
-	_, err := imapw.client.Select(msg.Directory, false)
+	sel, err := imapw.client.Select(msg.Directory, false)
 	if err != nil {
 		imapw.worker.PostMessage(&types.Error{
 			Message: types.RespondTo(msg),
 			Error:   err,
 		}, nil)
 	} else {
+		imapw.selected = sel
 		imapw.worker.PostMessage(&types.Done{Message: types.RespondTo(msg)}, nil)
 	}
 }
diff --git a/worker/imap/worker.go b/worker/imap/worker.go
index da0716e..e890bb8 100644
--- a/worker/imap/worker.go
+++ b/worker/imap/worker.go
@@ -9,6 +9,7 @@ import (
 	sortthread "github.com/emersion/go-imap-sortthread"
 	"github.com/emersion/go-imap/client"
 	"github.com/pkg/errors"
+	"github.com/syndtr/goleveldb/leveldb"
 
 	"git.sr.ht/~rjarry/aerc/lib"
 	"git.sr.ht/~rjarry/aerc/models"
@@ -49,6 +50,8 @@ type imapConfig struct {
 	keepalive_period   time.Duration
 	keepalive_probes   int
 	keepalive_interval int
+	cacheEnabled       bool
+	cacheMaxAge        time.Duration
 }
 
 type IMAPWorker struct {
@@ -63,6 +66,7 @@ type IMAPWorker struct {
 
 	idler    *idler
 	observer *observer
+	cache    *leveldb.DB
 }
 
 func NewIMAPWorker(worker *types.Worker) (types.Backend, error) {
@@ -178,6 +182,8 @@ func (w *IMAPWorker) handleMessage(msg types.WorkerMessage) error {
 		w.handleFetchMessageBodyPart(msg)
 	case *types.FetchFullMessages:
 		w.handleFetchFullMessages(msg)
+	case *types.FetchMessageFlags:
+		w.handleFetchMessageFlags(msg)
 	case *types.DeleteMessages:
 		w.handleDeleteMessages(msg)
 	case *types.FlagMessages:
diff --git a/worker/types/messages.go b/worker/types/messages.go
index d2d98fd..e303ade 100644
--- a/worker/types/messages.go
+++ b/worker/types/messages.go
@@ -133,6 +133,11 @@ type FetchMessageBodyPart struct {
 	Part []int
 }
 
+type FetchMessageFlags struct {
+	Message
+	Uids []uint32
+}
+
 type DeleteMessages struct {
 	Message
 	Uids []uint32