lib/db: Add Badger backend (fixes #5910) (#6250)

go.mod

@@ -10,6 +10,7 @@ require (
 	github.com/certifi/gocertifi v0.0.0-20190905060710-a5e0173ced67 // indirect
 	github.com/chmduquesne/rollinghash v0.0.0-20180912150627-a60f8e7142b5
 	github.com/d4l3k/messagediff v1.2.1
+	github.com/dgraph-io/badger/v2 v2.0.3
 	github.com/flynn-archive/go-shlex v0.0.0-20150515145356-3f9db97f8568
 	github.com/getsentry/raven-go v0.2.0
 	github.com/go-ldap/ldap/v3 v3.1.10
@@ -45,7 +46,6 @@ require (
 	golang.org/x/sys v0.0.0-20200223170610-d5e6a3e2c0ae
 	golang.org/x/text v0.3.2
 	golang.org/x/time v0.0.0-20190308202827-9d24e82272b4
-	gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 // indirect
 )
 
 go 1.13

go.sum

@@ -12,6 +12,9 @@ github.com/AudriusButkevicius/pfilter v0.0.0-20190627213056-c55ef6137fc6/go.mod
 github.com/AudriusButkevicius/recli v0.0.5 h1:xUa55PvWTHBm17T6RvjElRO3y5tALpdceH86vhzQ5wg=
 github.com/AudriusButkevicius/recli v0.0.5/go.mod h1:Q2E26yc6RvWWEz/TJ/goUp6yXvipYdJI096hpoaqsNs=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
+github.com/DataDog/zstd v1.4.1 h1:3oxKN3wbHibqx897utPC2LTQU4J+IHWWJO+glkAkpFM=
+github.com/DataDog/zstd v1.4.1/go.mod h1:1jcaCB/ufaK+sKp1NBhlGmpz41jOoPQ35bpF36t7BBo=
+github.com/OneOfOne/xxhash v1.2.2/go.mod h1:HSdplMjZKSmBqAxg5vPj2TmRDmfkzw+cTzAElWljhcU=
 github.com/StackExchange/wmi v0.0.0-20180116203802-5d049714c4a6 h1:fLjPD/aNc3UIOA6tDi6QXUemppXK3P9BI7mr2hd6gx8=
 github.com/StackExchange/wmi v0.0.0-20180116203802-5d049714c4a6/go.mod h1:3eOhrUMpNV+6aFIbp5/iudMxNCF27Vw2OZgy4xEx0Fg=
 github.com/StackExchange/wmi v0.0.0-20190523213315-cbe66965904d h1:G0m3OIz70MZUWq3EgK3CesDbo8upS2Vm9/P3FtgI+Jk=
@@ -25,6 +28,7 @@ github.com/alecthomas/units v0.0.0-20151022065526-2efee857e7cf/go.mod h1:ybxpYRF
 github.com/alecthomas/units v0.0.0-20190717042225-c3de453c63f4 h1:Hs82Z41s6SdL1CELW+XaDYmOH4hkBN4/N9og/AsOv7E=
 github.com/alecthomas/units v0.0.0-20190717042225-c3de453c63f4/go.mod h1:ybxpYRFXyAe+OPACYpWeL0wqObRcbAqCMya13uyzqw0=
 github.com/anmitsu/go-shlex v0.0.0-20161002113705-648efa622239/go.mod h1:2FmKhYUyUczH0OGQWaF5ceTx0UBShxjsH6f8oGKYe2c=
+github.com/armon/consul-api v0.0.0-20180202201655-eb2c6b5be1b6/go.mod h1:grANhF5doyWs3UAsr3K4I6qtAmlQcZDesFNEHPZAzj8=
 github.com/beorn7/perks v0.0.0-20180321164747-3a771d992973 h1:xJ4a3vCFaGF/jqvzLMYoU8P317H5OQ+Via4RmuPwCS0=
 github.com/beorn7/perks v0.0.0-20180321164747-3a771d992973/go.mod h1:Dwedo/Wpr24TaqPxmxbtue+5NUziq4I4S80YR8gNf3Q=
 github.com/beorn7/perks v1.0.0 h1:HWo1m869IqiPhD389kmkxeTalrjNbbJTC8LXupb+sl0=
@@ -42,6 +46,8 @@ github.com/ccding/go-stun v0.0.0-20180726100737-be486d185f3d h1:As4937T5NVbJ/DmZ
 github.com/ccding/go-stun v0.0.0-20180726100737-be486d185f3d/go.mod h1:3FK1bMar37f7jqVY7q/63k3OMX1c47pGCufzt3X0sYE=
 github.com/certifi/gocertifi v0.0.0-20190905060710-a5e0173ced67 h1:8k9FLYBLKT+9v2HQJ/a95ZemmTx+/ltJcAiRhVushG8=
 github.com/certifi/gocertifi v0.0.0-20190905060710-a5e0173ced67/go.mod h1:GJKEexRPVJrBSOjoqN5VNOIKJ5Q3RViH6eu3puDRwx4=
+github.com/cespare/xxhash v1.1.0 h1:a6HrQnmkObjyL+Gs60czilIUGqrzKutQD6XZog3p+ko=
+github.com/cespare/xxhash v1.1.0/go.mod h1:XrSqR1VqqWfGrhpAt58auRo0WTKS1nRRg3ghfAqPWnc=
 github.com/cespare/xxhash/v2 v2.1.0 h1:yTUvW7Vhb89inJ+8irsUqiWjh8iT6sQPZiQzI6ReGkA=
 github.com/cespare/xxhash/v2 v2.1.0/go.mod h1:dgIUBU3pDso/gPgZ1osOZ0iQf77oPR28Tjxl5dIMyVM=
 github.com/cheekybits/genny v1.0.0 h1:uGGa4nei+j20rOSeDeP5Of12XVm7TGUd4dJA9RDitfE=
@@ -49,7 +55,12 @@ github.com/cheekybits/genny v1.0.0/go.mod h1:+tQajlRqAUrPI7DOSpB0XAqZYtQakVtB7wX
 github.com/chmduquesne/rollinghash v0.0.0-20180912150627-a60f8e7142b5 h1:Wg96Dh0MLTanEaPO0OkGtUIaa2jOnShAIOVUIzRHUxo=
 github.com/chmduquesne/rollinghash v0.0.0-20180912150627-a60f8e7142b5/go.mod h1:Uc2I36RRfTAf7Dge82bi3RU0OQUmXT9iweIcPqvr8A0=
 github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDkc90ppPyw=
+github.com/coreos/etcd v3.3.10+incompatible/go.mod h1:uF7uidLiAD3TWHmW31ZFd/JWoc32PjwdhPthX9715RE=
+github.com/coreos/go-etcd v2.0.0+incompatible/go.mod h1:Jez6KQU2B/sWsbdaef3ED8NzMklzPG4d5KIOhIy30Tk=
+github.com/coreos/go-semver v0.2.0/go.mod h1:nnelYz7RCh+5ahJtPPxZlU+153eP4D4r3EedlOD2RNk=
 github.com/coreos/go-systemd v0.0.0-20181012123002-c6f51f82210d/go.mod h1:F5haX7vjVVG0kc13fIWeqUViNPyEJxv/OmvnBo0Yme4=
+github.com/cpuguy83/go-md2man v1.0.10 h1:BSKMNlYxDvnunlTymqtgONjNnaRV1sTpcovwwjF22jk=
+github.com/cpuguy83/go-md2man v1.0.10/go.mod h1:SmD6nW6nTyfqj6ABTjUi3V3JVMnlJmwcJI5acqYI6dE=
 github.com/cpuguy83/go-md2man/v2 v2.0.0-20190314233015-f79a8a8ca69d h1:U+s90UTSYgptZMwQh2aRr3LuazLJIa+Pg3Kc1ylSYVY=
 github.com/cpuguy83/go-md2man/v2 v2.0.0-20190314233015-f79a8a8ca69d/go.mod h1:maD7wRr/U5Z6m/iR4s+kqSMx2CaBsrgA7czyZG/E6dU=
 github.com/d4l3k/messagediff v1.2.1 h1:ZcAIMYsUg0EAp9X+tt8/enBE/Q8Yd5kzPynLyKptt9U=
@@ -57,6 +68,14 @@ github.com/d4l3k/messagediff v1.2.1/go.mod h1:Oozbb1TVXFac9FtSIxHBMnBCq2qeH/2KkE
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/dgraph-io/badger v1.6.1 h1:w9pSFNSdq/JPM1N12Fz/F/bzo993Is1W+Q7HjPzi7yg=
+github.com/dgraph-io/badger/v2 v2.0.3 h1:inzdf6VF/NZ+tJ8RwwYMjJMvsOALTHYdozn0qSl6XJI=
+github.com/dgraph-io/badger/v2 v2.0.3/go.mod h1:3KY8+bsP8wI0OEnQJAKpd4wIJW/Mm32yw2j/9FUVnIM=
+github.com/dgraph-io/ristretto v0.0.2-0.20200115201040-8f368f2f2ab3 h1:MQLRM35Pp0yAyBYksjbj1nZI/w6eyRY/mWoM1sFf4kU=
+github.com/dgraph-io/ristretto v0.0.2-0.20200115201040-8f368f2f2ab3/go.mod h1:KPxhHT9ZxKefz+PCeOGsrHpl1qZ7i70dGTu2u+Ahh6E=
+github.com/dgryski/go-farm v0.0.0-20190423205320-6a90982ecee2 h1:tdlZCpZ/P9DhczCTSixgIKmwPv6+wP5DGjqLYw5SUiA=
+github.com/dgryski/go-farm v0.0.0-20190423205320-6a90982ecee2/go.mod h1:SqUrOPUnsFjfmXRMNPybcSiG0BgUW2AuFH8PAnS2iTw=
+github.com/dustin/go-humanize v1.0.0 h1:VSnTsYCnlFHaM2/igO1h6X3HA71jcobQuxemgkq4zYo=
 github.com/dustin/go-humanize v1.0.0/go.mod h1:HtrtbFcZ19U5GC7JDqmcUSB87Iq5E25KnS6fMYU6eOk=
 github.com/flynn-archive/go-shlex v0.0.0-20150515145356-3f9db97f8568 h1:BMXYYRWTLOJKlh+lOBt6nUQgXAfB7oVIQt5cNreqSLI=
 github.com/flynn-archive/go-shlex v0.0.0-20150515145356-3f9db97f8568/go.mod h1:rZfgFAXFS/z/lEd6LJmf9HVZ1LkgYiHx5pHhV5DR16M=
@@ -132,8 +151,10 @@ github.com/greatroar/blobloom v0.2.1 h1:Ie7+kTQFhcvfFHhzOYSJA2XE5sp8c9iB5iapu2tI
 github.com/greatroar/blobloom v0.2.1/go.mod h1:we9vO6GNYMmsNvCWINtZnQbcGEHUT6hGBAznNHd6RlE=
 github.com/gregjones/httpcache v0.0.0-20180305231024-9cad4c3443a7/go.mod h1:FecbI9+v66THATjSRHfNgh1IVFe/9kFxbXtjV0ctIMA=
 github.com/grpc-ecosystem/grpc-gateway v1.5.0/go.mod h1:RSKVYQBd5MCa4OVpNdGskqpgL2+G+NZTnrVHpWWfpdw=
+github.com/hashicorp/hcl v1.0.0/go.mod h1:E5yfLk+7swimpb2L/Alb/PJmXilQ/rhwaUYs4T20WEQ=
 github.com/hpcloud/tail v1.0.0 h1:nfCOvKYfkgYP8hkirhJocXT2+zOD8yUNjXaWfTlyFKI=
 github.com/hpcloud/tail v1.0.0/go.mod h1:ab1qPbhIpdTxEkNHXyeSf5vhxWSCs/tWer42PpOxQnU=
+github.com/inconshreveable/mousetrap v1.0.0/go.mod h1:PxqpIevigyE2G7u3NXJIT2ANytuPF1OarO4DADm73n8=
 github.com/jackpal/gateway v1.0.6 h1:/MJORKvJEwNVldtGVJC2p2cwCnsSoLn3hl3zxmZT7tk=
 github.com/jackpal/gateway v1.0.6/go.mod h1:lTpwd4ACLXmpyiCTRtfiNyVnUmqT9RivzCDQetPfnjA=
 github.com/jackpal/go-nat-pmp v1.0.2 h1:KzKSgb7qkJvOUTqYl9/Hg/me3pWgBmERKrTGD7BdWus=
@@ -163,6 +184,7 @@ github.com/lucas-clemente/quic-go v0.15.6/go.mod h1:Myi1OyS0FOjL3not4BxT7KN29bRk
 github.com/lucas-clemente/quic-go v0.15.7 h1:Pu7To5/G9JoP1mwlrcIvfV8ByPBlCzif3MCl8+1W83I=
 github.com/lucas-clemente/quic-go v0.15.7/go.mod h1:Myi1OyS0FOjL3not4BxT7KN29bRkcMUV5JVVFLKtDp8=
 github.com/lunixbochs/vtclean v1.0.0/go.mod h1:pHhQNgMf3btfWnGBVipUOjRYhoOsdGqdm/+2c2E2WMI=
+github.com/magiconair/properties v1.8.0/go.mod h1:PppfXfuXeibc/6YijjN8zIbojt8czPbwD3XqdrwzmxQ=
 github.com/mailru/easyjson v0.0.0-20190312143242-1de009706dbe/go.mod h1:C1wdFJiN94OJF2b5HbByQZoLdCWB1Yqtg26g4irojpc=
 github.com/marten-seemann/qpack v0.1.0/go.mod h1:LFt1NU/Ptjip0C2CPkhimBz5CGE3WGDAUWqna+CNTrI=
 github.com/marten-seemann/qtls v0.9.1 h1:O0YKQxNVPaiFgMng0suWEOY2Sb4LT2sRn9Qimq3Z1IQ=
@@ -183,6 +205,8 @@ github.com/mgutz/ansi v0.0.0-20170206155736-9520e82c474b/go.mod h1:01TrycV0kFyex
 github.com/microcosm-cc/bluemonday v1.0.1/go.mod h1:hsXNsILzKxV+sX77C5b8FSuKF00vh2OMYv+xgHpAMF4=
 github.com/minio/sha256-simd v0.1.1 h1:5QHSlgo3nt5yKOJrC7W8w7X+NFl8cMPZm96iu8kKUJU=
 github.com/minio/sha256-simd v0.1.1/go.mod h1:B5e1o+1/KgNmWrSQK08Y6Z1Vb5pwIktudl0J58iy0KM=
+github.com/mitchellh/go-homedir v1.1.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
+github.com/mitchellh/mapstructure v1.1.2/go.mod h1:FVVH3fgwuzCH5S8UJGiWEs2h04kUh9fWfEaFds41c1Y=
 github.com/modern-go/concurrent v0.0.0-20180228061459-e0a39a4cb421/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=
 github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd/go.mod h1:6dJC0mAP4ikYIbvyc7fijjWJddQyLn8Ig3JB5CqoB9Q=
 github.com/modern-go/reflect2 v0.0.0-20180701023420-4b7aa43c6742/go.mod h1:bx2lNnkwVCuqBIxFjflWJWanXIb3RllmbCylyMrvgv0=
@@ -204,6 +228,7 @@ github.com/oschwald/geoip2-golang v1.4.0 h1:5RlrjCgRyIGDz/mBmPfnAF4h8k0IAcRv9Pvr
 github.com/oschwald/geoip2-golang v1.4.0/go.mod h1:8QwxJvRImBH+Zl6Aa6MaIcs5YdlZSTKtzmPGzQqi9ng=
 github.com/oschwald/maxminddb-golang v1.6.0 h1:KAJSjdHQ8Kv45nFIbtoLGrGWqHFajOIm7skTyz/+Dls=
 github.com/oschwald/maxminddb-golang v1.6.0/go.mod h1:DUJFucBg2cvqx42YmDa/+xHvb0elJtOm3o4aFQ/nb/w=
+github.com/pelletier/go-toml v1.2.0/go.mod h1:5z9KED0ma1S8pY6P1sdut58dfprrGBbd/94hg7ilaic=
 github.com/petermattis/goid v0.0.0-20180202154549-b0b1615b78e5 h1:q2e307iGHPdTGp0hoxKjt1H5pDo6utceo3dQVK3I5XQ=
 github.com/petermattis/goid v0.0.0-20180202154549-b0b1615b78e5/go.mod h1:jvVRKCrJTQWu0XVbaOlby/2lO20uSCHEMzzplHXte1o=
 github.com/pkg/errors v0.8.0/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
@@ -276,6 +301,14 @@ github.com/sirupsen/logrus v1.4.2 h1:SPIRibHv4MatM3XXNO2BJeFLZwZ2LvZgfQ5+UNI2im4
 github.com/sirupsen/logrus v1.4.2/go.mod h1:tLMulIdttU9McNUspp0xgXVQah82FyeX6MwdIuYE2rE=
 github.com/sourcegraph/annotate v0.0.0-20160123013949-f4cad6c6324d/go.mod h1:UdhH50NIW0fCiwBSr0co2m7BnFLdv4fQTgdqdJTHFeE=
 github.com/sourcegraph/syntaxhighlight v0.0.0-20170531221838-bd320f5d308e/go.mod h1:HuIsMU8RRBOtsCgI77wP899iHVBQpCmg4ErYMZB+2IA=
+github.com/spaolacci/murmur3 v0.0.0-20180118202830-f09979ecbc72/go.mod h1:JwIasOWyU6f++ZhiEuf87xNszmSA2myDM2Kzu9HwQUA=
+github.com/spaolacci/murmur3 v1.1.0/go.mod h1:JwIasOWyU6f++ZhiEuf87xNszmSA2myDM2Kzu9HwQUA=
+github.com/spf13/afero v1.1.2/go.mod h1:j4pytiNVoe2o6bmDsKpLACNPDBIoEAkihy7loJ1B0CQ=
+github.com/spf13/cast v1.3.0/go.mod h1:Qx5cxh0v+4UWYiBimWS+eyWzqEqokIECu5etghLkUJE=
+github.com/spf13/cobra v0.0.5/go.mod h1:3K3wKZymM7VvHMDS9+Akkh4K60UwM26emMESw8tLCHU=
+github.com/spf13/jwalterweatherman v1.0.0/go.mod h1:cQK4TGJAtQXfYWX+Ddv3mKDzgVb68N+wFjFa4jdeBTo=
+github.com/spf13/pflag v1.0.3/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
+github.com/spf13/viper v1.3.2/go.mod h1:ZiWeW+zYFKm7srdB9IoDzzZXaJaI5eL9QjNiN/DMA2s=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/objx v0.1.1/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.2.2 h1:bSDNvY7ZPG5RlJ8otE/7V6gMiyenm9RtJ7IUVIAoJ1w=
@@ -293,6 +326,7 @@ github.com/syndtr/goleveldb v1.0.1-0.20190923125748-758128399b1d/go.mod h1:9OrXJ
 github.com/tarm/serial v0.0.0-20180830185346-98f6abe2eb07/go.mod h1:kDXzergiv9cbyO7IOYJZWg1U88JhDg3PB6klq9Hg2pA=
 github.com/thejerf/suture v3.0.2+incompatible h1:GtMydYcnK4zBJ0KL6Lx9vLzl6Oozb65wh252FTBxrvM=
 github.com/thejerf/suture v3.0.2+incompatible/go.mod h1:ibKwrVj+Uzf3XZdAiNWUouPaAbSoemxOHLmJmwheEMc=
+github.com/ugorji/go/codec v0.0.0-20181204163529-d75b2dcb6bc8/go.mod h1:VFNgLljTbGfSG7qAOspJ7OScBnGdDN/yBr0sguwnwf0=
 github.com/urfave/cli v1.20.0 h1:fDqGv3UG/4jbVl/QkFwEdddtEDjh/5Ov6X+0B/3bPaw=
 github.com/urfave/cli v1.20.0/go.mod h1:70zkFmudgCuE/ngEzBv17Jvp/497gISqfk5gWijbERA=
 github.com/urfave/cli v1.22.2 h1:gsqYFH8bb9ekPA12kRo0hfjngWQjkJPlN9R0N78BoUo=
@@ -301,11 +335,13 @@ github.com/viant/assertly v0.4.8/go.mod h1:aGifi++jvCrUaklKEKT0BU95igDNaqkvz+49u
 github.com/viant/toolbox v0.24.0/go.mod h1:OxMCG57V0PXuIP2HNQrtJf2CjqdmbrOx5EkMILuUhzM=
 github.com/vitrun/qart v0.0.0-20160531060029-bf64b92db6b0 h1:okhMind4q9H1OxF44gNegWkiP4H/gsTFLalHFa4OOUI=
 github.com/vitrun/qart v0.0.0-20160531060029-bf64b92db6b0/go.mod h1:TTbGUfE+cXXceWtbTHq6lqcTvYPBKLNejBEbnUsQJtU=
+github.com/xordataexchange/crypt v0.0.3-0.20170626215501-b2862e3d0a77/go.mod h1:aYKd//L2LvnjZzWKhF00oedf4jCCReLcmhLdhm1A27Q=
 go.opencensus.io v0.18.0/go.mod h1:vKdFvxhtzZ9onBp9VKHK8z/sRpBMnKAsufL7wlDrCOA=
 go4.org v0.0.0-20180809161055-417644f6feb5/go.mod h1:MkTOUMDaeVYJUOUsaDXIhWPZYa1yOyC1qaOBpL57BhE=
 golang.org/x/build v0.0.0-20190111050920-041ab4dc3f9d/go.mod h1:OWs+y06UdEOHN4y+MfF/py+xQ/tYqIWW03b70/CG9Rw=
 golang.org/x/crypto v0.0.0-20180904163835-0709b304e793/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20181030102418-4d3f4d9ffa16/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
+golang.org/x/crypto v0.0.0-20181203042331-505ab145d0a9/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2 h1:VklqNMn3ovrHsnt90PveolxSbWFaJdECFbxSq0Mqo2M=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20190313024323-a1f597ede03a/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
@@ -330,6 +366,7 @@ golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn
 golang.org/x/net v0.0.0-20190313220215-9f648a60d977/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190613194153-d28f0bde5980/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
 golang.org/x/net v0.0.0-20190827160401-ba9fcec4b297 h1:k7pJ2yAPLPgbskkFdhRCsA77k2fySZ1zf2zCjvQCiIM=
 golang.org/x/net v0.0.0-20190827160401-ba9fcec4b297/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
 golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAGxoOufVs8QHGRruUQn6yWY3a++T0U=
@@ -349,6 +386,7 @@ golang.org/x/sys v0.0.0-20180909124046-d0be0721c37e/go.mod h1:STP8DvDyc/dI5b8T5h
 golang.org/x/sys v0.0.0-20180926160741-c2ed4eda69e7/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20181029174526-d69651ed3497/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20181116152217-5ac8a444bdc5/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20181205085412-a5c9d58dba9a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a h1:1BGLXjeY4akVXGgbC9HugT3Jv3hCI0z56oJR5vAMgBU=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190222072716-a9d3bda3a223 h1:DH4skfRX4EBpamg7iV4ZlCpblAHI6s6TDM39bFZumv8=
@@ -356,6 +394,7 @@ golang.org/x/sys v0.0.0-20190222072716-a9d3bda3a223/go.mod h1:STP8DvDyc/dI5b8T5h
 golang.org/x/sys v0.0.0-20190316082340-a2f829d7f35f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20190422165155-953cdadca894/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20190626221950-04f50cda93cb/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191010194322-b09406accb47 h1:/XfQ9z7ib8eEJX2hdgFTZJ/ntt0swNk5oYBziWeTCvY=
 golang.org/x/sys v0.0.0-20191010194322-b09406accb47/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191026070338-33540a1f6037 h1:YyJpGZS1sBuBCzLAR1VEpK193GlqGZbnPFnPV/5Rsb4=
@@ -411,6 +450,7 @@ gopkg.in/alecthomas/kingpin.v2 v2.2.6/go.mod h1:FMv+mEhP44yOT+4EoQTLFTRgOQ1FBLks
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/fsnotify.v1 v1.4.7 h1:xOHLXZwVvI9hhs+cLKq5+I5onOuwQLhQwiu63xxlHs4=
 gopkg.in/fsnotify.v1 v1.4.7/go.mod h1:Tz8NjZHkW78fSQdbUxIjBTcgA1z1m8ZHf0WmKUhAMys=
 gopkg.in/inf.v0 v0.9.1/go.mod h1:cWUDdTG/fYaXco+Dcufb5Vnc6Gp2YChqWtbxRZE0mXw=

lib/db/backend/backend.go

@@ -7,7 +7,12 @@
 package backend
 
 import (
+	"os"
+	"strings"
 	"sync"
+	"time"
+
+	"github.com/syncthing/syncthing/lib/locations"
 )
 
 // The Reader interface specifies the read-only operations available on the
@@ -118,10 +123,24 @@ const (
 )
 
 func Open(path string, tuning Tuning) (Backend, error) {
+	if os.Getenv("USE_BADGER") != "" {
+		l.Warnln("Using experimental badger db")
+		if err := maybeCopyDatabase(path, strings.Replace(path, locations.BadgerDir, locations.LevelDBDir, 1), OpenBadger, OpenLevelDBRO); err != nil {
+			return nil, err
+		}
+		return OpenBadger(path)
+	}
+
+	if err := maybeCopyDatabase(path, strings.Replace(path, locations.LevelDBDir, locations.BadgerDir, 1), OpenLevelDBAuto, OpenBadger); err != nil {
+		return nil, err
+	}
 	return OpenLevelDB(path, tuning)
 }
 
 func OpenMemory() Backend {
+	if os.Getenv("USE_BADGER") != "" {
+		return OpenBadgerMemory()
+	}
 	return OpenLevelDBMemory()
 }
 
@@ -202,3 +221,70 @@ func (cg *closeWaitGroup) CloseWait() {
 	cg.closeMut.Unlock()
 	cg.WaitGroup.Wait()
 }
+
+type opener func(path string) (Backend, error)
+
+// maybeCopyDatabase copies the database if the destination doesn't exist
+// but the source does.
+func maybeCopyDatabase(toPath, fromPath string, toOpen, fromOpen opener) error {
+	if _, err := os.Lstat(toPath); !os.IsNotExist(err) {
+		// Destination database exists (or is otherwise unavailable), do not
+		// attempt to overwrite it.
+		return nil
+	}
+
+	if _, err := os.Lstat(fromPath); err != nil {
+		// Source database is not available, so nothing to copy
+		return nil
+	}
+
+	fromDB, err := fromOpen(fromPath)
+	if err != nil {
+		return err
+	}
+	defer fromDB.Close()
+
+	toDB, err := toOpen(toPath)
+	if err != nil {
+		// That's odd, but it will be handled & reported in the usual path
+		// so we can ignore it here.
+		return err
+	}
+	defer toDB.Close()
+
+	l.Infoln("Copying database for format conversion...")
+	if err := copyBackend(toDB, fromDB); err != nil {
+		return err
+	}
+
+	// Move the old database out of the way to mark it as migrated.
+	fromDB.Close()
+	_ = os.Rename(fromPath, fromPath+".migrated."+time.Now().Format("20060102150405"))
+	return nil
+}
+
+func copyBackend(to, from Backend) error {
+	srcIt, err := from.NewPrefixIterator(nil)
+	if err != nil {
+		return err
+	}
+	defer srcIt.Release()
+
+	dstTx, err := to.NewWriteTransaction()
+	if err != nil {
+		return err
+	}
+	defer dstTx.Release()
+
+	for srcIt.Next() {
+		if err := dstTx.Put(srcIt.Key(), srcIt.Value()); err != nil {
+			return err
+		}
+	}
+	if srcIt.Error() != nil {
+		return err
+	}
+	srcIt.Release()
+
+	return dstTx.Commit()
+}

lib/db/backend/badger_backend.go

@@ -0,0 +1,443 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import (
+	"bytes"
+	"errors"
+	"time"
+
+	badger "github.com/dgraph-io/badger/v2"
+)
+
+const (
+	checkpointFlushMinSize = 128 << KiB
+	maxCacheSize           = 64 << MiB
+)
+
+func OpenBadger(path string) (Backend, error) {
+	opts := badger.DefaultOptions(path)
+	opts = opts.WithMaxCacheSize(maxCacheSize).WithCompactL0OnClose(false)
+	opts.Logger = nil
+	return openBadger(opts)
+}
+
+func OpenBadgerMemory() Backend {
+	opts := badger.DefaultOptions("").WithInMemory(true)
+	opts.Logger = nil
+	backend, err := openBadger(opts)
+	if err != nil {
+		// Opening in-memory should never be able to fail, and is anyway
+		// used just by tests.
+		panic(err)
+	}
+	return backend
+}
+
+func openBadger(opts badger.Options) (Backend, error) {
+	// XXX: We should find good values for memory utilization in the "small"
+	// and "large" cases we support for LevelDB. Some notes here:
+	// https://github.com/dgraph-io/badger/tree/v2.0.3#memory-usage
+	bdb, err := badger.Open(opts)
+	if err != nil {
+		return nil, wrapBadgerErr(err)
+	}
+	return &badgerBackend{
+		bdb:     bdb,
+		closeWG: &closeWaitGroup{},
+	}, nil
+}
+
+// badgerBackend implements Backend on top of a badger
+type badgerBackend struct {
+	bdb     *badger.DB
+	closeWG *closeWaitGroup
+}
+
+func (b *badgerBackend) NewReadTransaction() (ReadTransaction, error) {
+	rel, err := newReleaser(b.closeWG)
+	if err != nil {
+		return nil, err
+	}
+	return badgerSnapshot{
+		txn: b.bdb.NewTransaction(false),
+		rel: rel,
+	}, nil
+}
+
+func (b *badgerBackend) NewWriteTransaction() (WriteTransaction, error) {
+	rel1, err := newReleaser(b.closeWG)
+	if err != nil {
+		return nil, err
+	}
+	rel2, err := newReleaser(b.closeWG)
+	if err != nil {
+		rel1.Release()
+		return nil, err
+	}
+
+	// We use two transactions here to preserve the property that our
+	// leveldb wrapper has, that writes in a transaction are completely
+	// invisible until it's committed, even inside that same transaction.
+	rtxn := b.bdb.NewTransaction(false)
+	wtxn := b.bdb.NewTransaction(true)
+	return &badgerTransaction{
+		badgerSnapshot: badgerSnapshot{
+			txn: rtxn,
+			rel: rel1,
+		},
+		txn: wtxn,
+		bdb: b.bdb,
+		rel: rel2,
+	}, nil
+}
+
+func (b *badgerBackend) Close() error {
+	b.closeWG.CloseWait()
+	return wrapBadgerErr(b.bdb.Close())
+}
+
+func (b *badgerBackend) Get(key []byte) ([]byte, error) {
+	if err := b.closeWG.Add(1); err != nil {
+		return nil, err
+	}
+	defer b.closeWG.Done()
+
+	txn := b.bdb.NewTransaction(false)
+	defer txn.Discard()
+	item, err := txn.Get(key)
+	if err != nil {
+		return nil, wrapBadgerErr(err)
+	}
+	val, err := item.ValueCopy(nil)
+	if err != nil {
+		return nil, wrapBadgerErr(err)
+	}
+	return val, nil
+}
+
+func (b *badgerBackend) NewPrefixIterator(prefix []byte) (Iterator, error) {
+	if err := b.closeWG.Add(1); err != nil {
+		return nil, err
+	}
+
+	txn := b.bdb.NewTransaction(false)
+	it := badgerPrefixIterator(txn, prefix)
+	it.releaseFn = func() {
+		defer b.closeWG.Done()
+		txn.Discard()
+	}
+	return it, nil
+}
+
+func (b *badgerBackend) NewRangeIterator(first, last []byte) (Iterator, error) {
+	if err := b.closeWG.Add(1); err != nil {
+		return nil, err
+	}
+
+	txn := b.bdb.NewTransaction(false)
+	it := badgerRangeIterator(txn, first, last)
+	it.releaseFn = func() {
+		defer b.closeWG.Done()
+		txn.Discard()
+	}
+	return it, nil
+}
+
+func (b *badgerBackend) Put(key, val []byte) error {
+	if err := b.closeWG.Add(1); err != nil {
+		return err
+	}
+	defer b.closeWG.Done()
+
+	txn := b.bdb.NewTransaction(true)
+	if err := txn.Set(key, val); err != nil {
+		txn.Discard()
+		return wrapBadgerErr(err)
+	}
+	return wrapBadgerErr(txn.Commit())
+}
+
+func (b *badgerBackend) Delete(key []byte) error {
+	if err := b.closeWG.Add(1); err != nil {
+		return err
+	}
+	defer b.closeWG.Done()
+
+	txn := b.bdb.NewTransaction(true)
+	if err := txn.Delete(key); err != nil {
+		txn.Discard()
+		return wrapBadgerErr(err)
+	}
+	return wrapBadgerErr(txn.Commit())
+}
+
+func (b *badgerBackend) Compact() error {
+	if err := b.closeWG.Add(1); err != nil {
+		return err
+	}
+	defer b.closeWG.Done()
+
+	// This weird looking loop is as recommended in the README
+	// (https://github.com/dgraph-io/badger/tree/v2.0.3#garbage-collection).
+	// Basically, the RunValueLogGC will pick some promising thing to
+	// garbage collect at random and return nil if it improved the
+	// situation, then return ErrNoRewrite when there is nothing more to GC.
+	// The 0.5 is the discard ratio, for which the method docs say they
+	// "recommend setting discardRatio to 0.5, thus indicating that a file
+	// be rewritten if half the space can be discarded".
+	var err error
+	t0 := time.Now()
+	for err == nil {
+		if time.Since(t0) > time.Hour {
+			l.Warnln("Database compaction is taking a long time, performance may be impacted. Consider investigating and/or opening an issue if this warning repeats.")
+			t0 = time.Now()
+		}
+		err = b.bdb.RunValueLogGC(0.5)
+	}
+
+	if errors.Is(err, badger.ErrNoRewrite) {
+		// GC did nothing, because nothing needed to be done
+		return nil
+	}
+	if errors.Is(err, badger.ErrRejected) {
+		// GC was already running (could possibly happen), or the database
+		// is closed (can't happen).
+		return nil
+	}
+	if errors.Is(err, badger.ErrGCInMemoryMode) {
+		// GC in in-memory mode, which is fine.
+		return nil
+	}
+	return err
+}
+
+// badgerSnapshot implements backend.ReadTransaction
+type badgerSnapshot struct {
+	txn *badger.Txn
+	rel *releaser
+}
+
+func (l badgerSnapshot) Get(key []byte) ([]byte, error) {
+	item, err := l.txn.Get(key)
+	if err != nil {
+		return nil, wrapBadgerErr(err)
+	}
+	val, err := item.ValueCopy(nil)
+	if err != nil {
+		return nil, wrapBadgerErr(err)
+	}
+	return val, nil
+}
+
+func (l badgerSnapshot) NewPrefixIterator(prefix []byte) (Iterator, error) {
+	return badgerPrefixIterator(l.txn, prefix), nil
+}
+
+func (l badgerSnapshot) NewRangeIterator(first, last []byte) (Iterator, error) {
+	return badgerRangeIterator(l.txn, first, last), nil
+}
+
+func (l badgerSnapshot) Release() {
+	defer l.rel.Release()
+	l.txn.Discard()
+}
+
+type badgerTransaction struct {
+	badgerSnapshot
+	txn  *badger.Txn
+	bdb  *badger.DB
+	rel  *releaser
+	size int
+}
+
+func (t *badgerTransaction) Delete(key []byte) error {
+	t.size += len(key)
+	kc := make([]byte, len(key))
+	copy(kc, key)
+	return t.transactionRetried(func(txn *badger.Txn) error {
+		return txn.Delete(kc)
+	})
+}
+
+func (t *badgerTransaction) Put(key, val []byte) error {
+	t.size += len(key) + len(val)
+	kc := make([]byte, len(key))
+	copy(kc, key)
+	vc := make([]byte, len(val))
+	copy(vc, val)
+	return t.transactionRetried(func(txn *badger.Txn) error {
+		return txn.Set(kc, vc)
+	})
+}
+
+// transactionRetried performs the given operation in the current
+// transaction, with commit and retry if Badger says the transaction has
+// grown too large.
+func (t *badgerTransaction) transactionRetried(fn func(*badger.Txn) error) error {
+	if err := fn(t.txn); err == badger.ErrTxnTooBig {
+		if err := t.txn.Commit(); err != nil {
+			return wrapBadgerErr(err)
+		}
+		t.size = 0
+		t.txn = t.bdb.NewTransaction(true)
+		return wrapBadgerErr(fn(t.txn))
+	} else if err != nil {
+		return wrapBadgerErr(err)
+	}
+	return nil
+}
+
+func (t *badgerTransaction) Commit() error {
+	defer t.rel.Release()
+	defer t.badgerSnapshot.Release()
+	return wrapBadgerErr(t.txn.Commit())
+}
+
+func (t *badgerTransaction) Checkpoint(preFlush ...func() error) error {
+	if t.size < checkpointFlushMinSize {
+		return nil
+	}
+	for _, hook := range preFlush {
+		if err := hook(); err != nil {
+			return err
+		}
+	}
+	err := t.txn.Commit()
+	if err == nil {
+		t.size = 0
+		t.txn = t.bdb.NewTransaction(true)
+	}
+	return wrapBadgerErr(err)
+}
+
+func (t *badgerTransaction) Release() {
+	defer t.rel.Release()
+	defer t.badgerSnapshot.Release()
+	t.txn.Discard()
+}
+
+type badgerIterator struct {
+	it        *badger.Iterator
+	prefix    []byte
+	first     []byte
+	last      []byte
+	releaseFn func()
+	didSeek   bool
+	err       error
+}
+
+func (i *badgerIterator) Next() bool {
+	if i.err != nil {
+		return false
+	}
+	for {
+		if !i.didSeek {
+			if i.first != nil {
+				// Range iterator
+				i.it.Seek(i.first)
+			} else {
+				// Prefix iterator
+				i.it.Seek(i.prefix)
+			}
+			i.didSeek = true
+		} else {
+			i.it.Next()
+		}
+
+		if !i.it.ValidForPrefix(i.prefix) {
+			// Done
+			return false
+		}
+		if i.first == nil && i.last == nil {
+			// No range checks required
+			return true
+		}
+
+		key := i.it.Item().Key()
+		if bytes.Compare(key, i.last) > 0 {
+			// Key is after range last
+			return false
+		}
+		return true
+	}
+}
+
+func (i *badgerIterator) Key() []byte {
+	if i.err != nil {
+		return nil
+	}
+	return i.it.Item().Key()
+}
+
+func (i *badgerIterator) Value() []byte {
+	if i.err != nil {
+		return nil
+	}
+	val, err := i.it.Item().ValueCopy(nil)
+	if err != nil {
+		i.err = err
+	}
+	return val
+}
+
+func (i *badgerIterator) Error() error {
+	return wrapBadgerErr(i.err)
+}
+
+func (i *badgerIterator) Release() {
+	i.it.Close()
+	if i.releaseFn != nil {
+		i.releaseFn()
+	}
+}
+
+// wrapBadgerErr wraps errors so that the backend package can recognize them
+func wrapBadgerErr(err error) error {
+	if err == nil {
+		return nil
+	}
+	if err == badger.ErrDiscardedTxn {
+		return errClosed{}
+	}
+	if err == badger.ErrKeyNotFound {
+		return errNotFound{}
+	}
+	return err
+}
+
+func badgerPrefixIterator(txn *badger.Txn, prefix []byte) *badgerIterator {
+	it := iteratorForPrefix(txn, prefix)
+	return &badgerIterator{it: it, prefix: prefix}
+}
+
+func badgerRangeIterator(txn *badger.Txn, first, last []byte) *badgerIterator {
+	prefix := commonPrefix(first, last)
+	it := iteratorForPrefix(txn, prefix)
+	return &badgerIterator{it: it, prefix: prefix, first: first, last: last}
+}
+
+func iteratorForPrefix(txn *badger.Txn, prefix []byte) *badger.Iterator {
+	opts := badger.DefaultIteratorOptions
+	opts.Prefix = prefix
+	return txn.NewIterator(opts)
+}
+
+func commonPrefix(a, b []byte) []byte {
+	minLen := len(a)
+	if len(b) < minLen {
+		minLen = len(b)
+	}
+	prefix := make([]byte, 0, minLen)
+	for i := 0; i < minLen; i++ {
+		if a[i] != b[i] {
+			break
+		}
+		prefix = append(prefix, a[i])
+	}
+	return prefix
+}

lib/db/backend/badger_test.go

@@ -0,0 +1,38 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import "testing"
+
+func TestCommonPrefix(t *testing.T) {
+	cases := []struct {
+		a      string
+		b      string
+		common string
+	}{
+		{"", "", ""},
+		{"a", "b", ""},
+		{"aa", "ab", "a"},
+		{"aa", "a", "a"},
+		{"a", "aa", "a"},
+		{"aabab", "ab", "a"},
+		{"ab", "aabab", "a"},
+		{"abac", "ababab", "aba"},
+		{"ababab", "abac", "aba"},
+	}
+
+	for _, tc := range cases {
+		pref := string(commonPrefix([]byte(tc.a), []byte(tc.b)))
+		if pref != tc.common {
+			t.Errorf("commonPrefix(%q, %q) => %q, expected %q", tc.a, tc.b, pref, tc.common)
+		}
+	}
+}
+
+func TestBadgerBackendBehavior(t *testing.T) {
+	testBackendBehavior(t, OpenBadgerMemory)
+}

lib/db/backend/leveldb_open.go

@@ -33,7 +33,7 @@ const (
 	MiB = 20
 )
 
-// Open attempts to open the database at the given location, and runs
+// OpenLevelDB attempts to open the database at the given location, and runs
 // recovery on it if opening fails. Worst case, if recovery is not possible,
 // the database is erased and created from scratch.
 func OpenLevelDB(location string, tuning Tuning) (Backend, error) {
@@ -45,7 +45,13 @@ func OpenLevelDB(location string, tuning Tuning) (Backend, error) {
 	return newLeveldbBackend(ldb), nil
 }
 
-// OpenRO attempts to open the database at the given location, read only.
+// OpenLevelDBAuto is OpenLevelDB with TuningAuto tuning.
+func OpenLevelDBAuto(location string) (Backend, error) {
+	return OpenLevelDB(location, TuningAuto)
+}
+
+// OpenLevelDBRO attempts to open the database at the given location, read
+// only.
 func OpenLevelDBRO(location string) (Backend, error) {
 	opts := &opt.Options{
 		OpenFilesCacheCapacity: dbMaxOpenFiles,

lib/db/lowlevel.go

@@ -382,6 +382,8 @@ func (db *Lowlevel) dropDeviceFolder(device, folder []byte, meta *metadataTracke
 	if err != nil {
 		return err
 	}
+	defer dbi.Release()
+
 	var gk, keyBuf []byte
 	for dbi.Next() {
 		name := db.keyer.NameFromDeviceFileKey(dbi.Key())
@@ -400,10 +402,10 @@ func (db *Lowlevel) dropDeviceFolder(device, folder []byte, meta *metadataTracke
 			return err
 		}
 	}
+	dbi.Release()
 	if err := dbi.Error(); err != nil {
 		return err
 	}
-	dbi.Release()
 
 	if bytes.Equal(device, protocol.LocalDeviceID[:]) {
 		key, err := db.keyer.GenerateBlockMapKey(nil, folder, nil, nil)
@@ -491,6 +493,7 @@ func (db *Lowlevel) checkGlobals(folder []byte, meta *metadataTracker) error {
 			}
 		}
 	}
+	dbi.Release()
 	if err := dbi.Error(); err != nil {
 		return err
 	}

lib/db/schemaupdater.go

@@ -212,6 +212,10 @@ func (db *schemaUpdater) updateSchema0to1(_ int) error {
 			return err
 		}
 	}
+	dbi.Release()
+	if err != dbi.Error() {
+		return err
+	}
 
 	for folder := range changedFolders {
 		if err := db.dropFolderMeta([]byte(folder)); err != nil {

lib/db/transactions.go

@@ -509,7 +509,9 @@ func (db *Lowlevel) newReadWriteTransaction() (readWriteTransaction, error) {
 }
 
 func (t readWriteTransaction) Commit() error {
-	t.readOnlyTransaction.close()
+	// The readOnlyTransaction must close after commit, because they may be
+	// backed by the same actual lower level transaction.
+	defer t.readOnlyTransaction.close()
 	return t.WriteTransaction.Commit()
 }
 

lib/locations/locations.go

@@ -44,12 +44,20 @@ const (
 	DataBaseDir   BaseDirEnum = "data"
 	// User's home directory, *not* -home flag
 	UserHomeBaseDir BaseDirEnum = "userHome"
+
+	LevelDBDir = "index-v0.14.0.db"
+	BadgerDir  = "indexdb.badger"
 )
 
 // Platform dependent directories
 var baseDirs = make(map[BaseDirEnum]string, 3)
 
 func init() {
+	if os.Getenv("USE_BADGER") != "" {
+		// XXX: Replace the leveldb name with the badger name.
+		locationTemplates[Database] = strings.Replace(locationTemplates[Database], LevelDBDir, BadgerDir, 1)
+	}
+
 	userHome := userHomeDir()
 	config := defaultConfigDir(userHome)
 	baseDirs[UserHomeBaseDir] = userHome
@@ -80,8 +88,6 @@ func GetBaseDir(baseDir BaseDirEnum) string {
 	return baseDirs[baseDir]
 }
 
-var databaseDirname = "index-v0.14.0.db"
-
 // Use the variables from baseDirs here
 var locationTemplates = map[LocationEnum]string{
 	ConfigFile:    "${config}/config.xml",
@@ -89,7 +95,7 @@ var locationTemplates = map[LocationEnum]string{
 	KeyFile:       "${config}/key.pem",
 	HTTPSCertFile: "${config}/https-cert.pem",
 	HTTPSKeyFile:  "${config}/https-key.pem",
-	Database:      "${data}/" + databaseDirname,
+	Database:      "${data}/" + LevelDBDir,
 	LogFile:       "${data}/syncthing.log", // -logfile on Windows
 	CsrfTokens:    "${data}/csrftokens.txt",
 	PanicLog:      "${data}/panic-${timestamp}.log",
@@ -150,7 +156,14 @@ func defaultDataDir(userHome, config string) string {
 
 	default:
 		// If a database exists at the "normal" location, use that anyway.
-		if _, err := os.Lstat(filepath.Join(config, databaseDirname)); err == nil {
+		// We look for both LevelDB and Badger variants here regardless of
+		// what we're currently configured to use, because we might be
+		// starting up in Badger mode with only a LevelDB database present
+		// (will be converted), or vice versa.
+		if _, err := os.Lstat(filepath.Join(config, LevelDBDir)); err == nil {
+			return config
+		}
+		if _, err := os.Lstat(filepath.Join(config, BadgerDir)); err == nil {
 			return config
 		}
 		// Always use this env var, as it's explicitly set by the user